Hydraulic Hose ID and OD Chart

Hydraulic hose size is one of the most important factors in the design, installation, and maintenance of a hydraulic system. Two dimensions are commonly used when describing hose size: inside diameter (ID) and outside diameter (OD). The inside diameter determines the available flow area for hydraulic fluid, while the outside diameter represents the overall physical size of the hose, including the inner tube, reinforcement layers, and outer cover.

In most hydraulic applications, hose size is primarily specified by its inside diameter. The ID directly affects fluid velocity, pressure drop, heat generation, and overall system efficiency. A hose that is too small can cause excessive fluid velocity and pressure loss, while an unnecessarily large hose may increase cost, weight, and installation space. Hydraulic hose sizes are also commonly identified using dash sizes, where the dash number generally represents the nominal inside diameter in sixteenths of an inch. For example, a -8 hose typically corresponds to a 1/2-inch (12.7 mm) nominal ID.

Unlike the inside diameter, the outside diameter is not necessarily the same for hoses with the same nominal size. Two 1/2-inch hydraulic hoses may have similar IDs but significantly different ODs because of differences in reinforcement construction, working-pressure rating, cover thickness, and hose standard. A compact one- or two-wire braid hose, for example, may have a smaller OD than a heavy-duty four- or six-spiral hose with the same nominal ID.

This Hydraulic Hose ID and OD Chart provides a practical reference for comparing common hose dash sizes, nominal inside diameters, and outside dimensions. It also explains how hydraulic hose dimensions are measured, why OD varies between hose types, how dash sizing works, and how to select the correct hose diameter based on flow rate, pressure, and application requirements.

1. What Are Hydraulic Hose ID and OD?

Hydraulic hose dimensions are commonly described using two basic measurements: inside diameter (ID) and outside diameter (OD). Although both dimensions are important, they serve different purposes when selecting and installing a hydraulic hose.

The inside diameter (ID) is the diameter of the open passage through which hydraulic fluid flows. It is one of the most important dimensions used to identify hydraulic hose size because it determines the cross-sectional flow area available inside the hose.

A larger ID provides a larger flow area and generally allows a higher flow rate at a lower fluid velocity. A smaller ID restricts the flow area, increasing fluid velocity for the same flow rate. If the hose is undersized, this can contribute to excessive pressure drop, heat generation, noise, and reduced hydraulic system efficiency.

For this reason, hydraulic hoses are generally identified by their nominal inside diameter rather than their outside diameter.

For example:

• A -4 hydraulic hose generally has a nominal ID of 1/4 in.
• A -6 hydraulic hose generally has a nominal ID of 3/8 in.
• A -8 hydraulic hose generally has a nominal ID of 1/2 in.
• A -12 hydraulic hose generally has a nominal ID of 3/4 in.
• A -16 hydraulic hose generally has a nominal ID of 1 in.

The outside diameter (OD) is measured across the complete outside of the hose. It includes the inner tube, reinforcement layers, and outer cover.

In simplified form:

OD = ID + 2t

where:

• OD = outside diameter
• ID = inside diameter
• t = total hose wall thickness

However, hydraulic hose wall thickness is not standardized solely by nominal ID. It varies according to hose construction and performance requirements.

A typical hydraulic hose may consist of:

Inner Tube → Reinforcement Layer(s) → Outer Cover

The reinforcement may consist of textile braid, steel-wire braid, spiral steel wire, or multiple reinforcement layers. As reinforcement increases, the hose OD will generally increase even when the ID remains approximately the same.

For example, two hoses may both have a nominal 1/2-inch ID, but a compact braided hose and a high-pressure spiral hose can have noticeably different outside diameters.

This distinction is important because ID is primarily associated with hydraulic flow capacity, while OD is especially important for physical installation.

Hose OD affects:

• Clamp selection
• Routing clearance
• Hose spacing
• Protective sleeve selection
• Bulkhead and panel openings
• Hose carrier installation
• Minimum space around equipment
• Compatibility with certain hose-end attachment systems

Therefore, both dimensions should be checked when designing a hydraulic hose assembly. ID should not be selected from installation space alone, and OD should not normally be used to identify an unknown hose size without additional information.

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2. Hydraulic Hose ID and OD Chart

The following chart provides a quick reference for common hydraulic hose sizes. Hydraulic hose nominal ID is closely associated with the dash-size system, with each dash increment generally representing 1/16 inch of nominal inside diameter.

Dash Size	Nominal ID (in.)	Nominal ID (mm)
-2	1/8	3.2
-3	3/16	4.8
-4	1/4	6.4
-5	5/16	7.9
-6	3/8	9.5
-8	1/2	12.7
-10	5/8	15.9
-12	3/4	19.1
-16	1	25.4
-20	1-1/4	31.8
-24	1-1/2	38.1
-32	2	50.8

Why Is OD Not Fixed in the Chart?

Unlike nominal ID, hydraulic hose OD cannot be accurately determined from dash size alone.

For example, a -8 hose identifies a nominal 1/2-inch ID, but it does not define one universal outside diameter.

The actual OD depends on several factors:

• Hose standard
• Inner tube thickness
• Reinforcement material
• Number of reinforcement layers
• Steel-wire braid diameter
• Spiral-wire construction
• Outer cover thickness
• Working-pressure rating
• Manufacturer design
• Dimensional tolerances

Therefore:

Same Dash Size ≠ Same Outside Diameter

Consider two -8 hoses. Both may have a nominal ID of approximately 1/2 inch, but one may use a relatively compact wire-braid construction while another uses multiple spiral-wire reinforcement layers for higher-pressure service.

Their flow passages are similar in size, but their overall outside dimensions can be quite different.

For this reason, a more detailed ID/OD chart should identify the hose standard or specific hose series before listing an exact outside diameter.

A practical chart format is:

Dash Size	Nominal ID	Actual ID Range	Hose Type/Standard	OD Range
-4	1/4 in.	Manufacturer/standard dependent	SAE/EN hose type	Varies
-6	3/8 in.	Manufacturer/standard dependent	SAE/EN hose type	Varies
-8	1/2 in.	Manufacturer/standard dependent	SAE/EN hose type	Varies
-10	5/8 in.	Manufacturer/standard dependent	SAE/EN hose type	Varies
-12	3/4 in.	Manufacturer/standard dependent	SAE/EN hose type	Varies
-16	1 in.	Manufacturer/standard dependent	SAE/EN hose type	Varies

For engineering and hose assembly work, the manufacturer’s datasheet should therefore be consulted whenever the exact OD, ID tolerance, minimum bend radius, working pressure, or fitting compatibility is required.

ID vs. OD: Which Dimension Should You Use?

The correct dimension depends on what you are trying to determine.

Use ID when evaluating:

• Hydraulic flow capacity
• Fluid velocity
• Pressure drop
• Hose dash size
• System performance
• Basic hose sizing

Use OD when evaluating:

• Installation clearance
• Hose clamps
• Protective sleeves
• Hose routing
• Bundled hose assemblies
• Panel openings
• Hose guides and supports

In most cases, the first step in hydraulic hose selection is to determine the required ID from flow requirements, then verify the OD and other physical dimensions using the selected hose manufacturer’s technical data.

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3. Hydraulic Hose Dash Size vs. Inside Diameter

Hydraulic hose dash size is a common way to identify nominal hose size. For many standard hydraulic hoses, the dash number represents the hose inside diameter in sixteenths of an inch.

The basic relationship is:

Hose ID (inch) = Dash Number / 16

For example, a -8 hose means:

8 / 16 = 1 / 2 inch

Therefore:

-8 hose = 1/2-inch nominal ID

In metric units:

1/2 inch × 25.4 = 12.7 mm

So:

-8 hose = approximately 12.7 mm nominal ID

Hydraulic Hose Dash Size Conversion Chart

Dash Size	Sixteenths of an Inch	Nominal ID (in.)	Decimal ID (in.)	Nominal ID (mm)
-2	2/16	1/8	0.125	3.2
-3	3/16	3/16	0.188	4.8
-4	4/16	1/4	0.250	6.4
-5	5/16	5/16	0.313	7.9
-6	6/16	3/8	0.375	9.5
-8	8/16	1/2	0.500	12.7
-10	10/16	5/8	0.625	15.9
-12	12/16	3/4	0.750	19.1
-16	16/16	1	1.000	25.4
-20	20/16	1-1/4	1.250	31.8
-24	24/16	1-1/2	1.500	38.1
-32	32/16	2	2.000	50.8

How to Convert Dash Size to Hose ID

To convert a dash size to inches:

Hose ID (inch) = Dash Number / 16

Examples:

-4 hose

4 / 16 = 1/4 inch

Therefore:

-4 = 1/4-inch ID = approximately 6.4 mm

-6 hose

6 / 16 = 3/8 inch

Therefore:

-6 = 3/8-inch ID = approximately 9.5 mm

-12 hose

12 / 16 = 3/4 inch

Therefore:

-12 = 3/4-inch ID = approximately 19.1 mm

-16 hose

16 / 16 = 1 inch

Therefore:

-16 = 1-inch ID = 25.4 mm

How to Convert Hose ID to Dash Size

If the hose ID is already known in inches, the approximate dash number can be calculated using:

Dash Number = Hose ID (inch) × 16

For example, for a 1/2-inch hose:

0.5 × 16 = 8

Therefore:

1/2-inch ID = -8

For a 3/4-inch hose:

0.75 × 16 = 12

Therefore:

3/4-inch ID = -12

For a 1-inch hose:

1 × 16 = 16

Therefore:

1-inch ID = -16

Converting Hose ID Between Inches and Millimeters

To convert inches to millimeters:

ID (mm) = ID (inch) × 25.4

Example:

0.5 × 25.4 = 12.7 mm

Therefore:

1/2 inch = 12.7 mm

To convert millimeters to inches:

ID (inch) = ID (mm) / 25.4

Example:

12.7 / 25.4 = 0.5 inch

Therefore:

12.7 mm = 1/2 inch

Dash Size Refers to Nominal ID, Not OD

One of the most common mistakes is assuming that the dash number represents the outside diameter of the hose.

It does not.

For example:

-8 does not mean the hose OD is 8/16 inch.

Instead:

-8 = nominal 8/16-inch ID = 1/2-inch ID

The actual outside diameter is larger because it includes:

Inner Tube + Reinforcement + Outer Cover

This can be summarized as:

Dash Size → Nominal ID → Internal Flow Size

while:

Hose Construction → Wall Thickness → Outside Diameter

Therefore, two -8 hoses can have approximately the same nominal 1/2-inch ID while having very different ODs.

Why Dash Size Is Useful

The dash-size system provides a quick and convenient way to identify common hydraulic hose sizes.

Instead of writing:

1/2-inch hydraulic hose

it can simply be identified as:

-8 hose

Similarly:

• -4 = 1/4 inch
• -6 = 3/8 inch
• -8 = 1/2 inch
• -10 = 5/8 inch
• -12 = 3/4 inch
• -16 = 1 inch
• -20 = 1-1/4 inch
• -24 = 1-1/2 inch
• -32 = 2 inch

This makes dash sizing especially convenient when working with hydraulic hoses, fittings, assembly drawings, BOMs, and replacement parts.

Important Note About Actual Hose ID

Dash size represents a nominal hose size, not necessarily the exact measured inside diameter.

The actual ID may vary slightly due to:

• Applicable hose standard
• Manufacturing tolerances
• Hose construction
• Manufacturer design
• Hose series

Therefore, when an exact dimension is required, always check the technical datasheet for the specific hose product.

The dash-size system should be used as a nominal identification method, while the manufacturer’s dimensional data should be used for detailed engineering and hose assembly design.

4. Why Hydraulic Hose OD Varies

Unlike inside diameter, hydraulic hose outside diameter is not determined by dash size alone. Two hoses can have the same nominal ID and dash size but noticeably different outside diameters.

The main reason is that hydraulic hoses are designed for different pressure levels, operating conditions, flexibility requirements, and industry standards. These requirements determine the thickness and construction of the hose wall.

A typical hydraulic hose consists of three basic sections:

Inner Tube → Reinforcement → Outer Cover

Each section contributes to the final OD of the hose.

Inner Tube Thickness

The inner tube is the layer that comes into direct contact with the hydraulic fluid. It must resist the fluid chemically while maintaining flexibility and dimensional stability.

Inner tube thickness can vary according to:

• Hose design
• Fluid compatibility
• Temperature rating
• Pressure rating
• Manufacturer specification

Even when two hoses have the same nominal ID, different inner tube thicknesses can contribute to different overall outside diameters.

Reinforcement Layers

Reinforcement generally has the greatest influence on the difference in OD between hydraulic hose types.

Common reinforcement constructions include:

• Textile braid
• Single steel-wire braid
• Double steel-wire braid
• Four-spiral steel wire
• Six-spiral steel wire

A relatively light-duty hose may use textile reinforcement, while a high-pressure hydraulic hose may require several layers of high-tensile steel wire.

As reinforcement layers are added, the hose wall becomes thicker.

For example, consider several hoses with the same nominal 1/2-inch ID:

1/2″ Textile Hose

Inner Tube → Textile Reinforcement → Cover

1/2″ One-Wire Braid Hose

Inner Tube → Steel-Wire Braid → Cover

1/2″ Two-Wire Braid Hose

Inner Tube → Wire Braid → Wire Braid → Cover

1/2″ Spiral Hose

Inner Tube → Multiple Spiral-Wire Layers → Cover

Although all of these hoses may provide approximately the same internal flow passage, their outside diameters can be substantially different.

Outer Cover Thickness

The outer cover protects the reinforcement from the surrounding environment.

Depending on the application, manufacturers may design covers with increased resistance to:

• Abrasion
• Ozone
• Weather
• Oil
• Chemicals
• Heat
• Flame
• Mechanical damage

Heavy-duty abrasion-resistant covers can increase the OD compared with compact hoses using thinner covers.

Pressure Rating and Hose OD

Higher working pressure often requires stronger reinforcement.

For example, a high-pressure spiral hose may use four or six layers of steel reinforcement. This construction is typically thicker than a one- or two-wire braided hose of the same nominal ID.

However, pressure rating should not be determined from OD alone.

A thicker hose is not automatically suitable for higher pressure. Working pressure must always be verified from the hose specification and applicable standard.

Why OD Matters During Installation

Although ID is critical for hydraulic performance, OD becomes particularly important when designing the physical installation.

A larger OD may require:

• Larger hose clamps
• More routing space
• Larger protective sleeves
• Greater spacing between parallel hoses
• Larger openings through panels
• Different hose guides
• More clearance around moving components

This becomes particularly important in compact hydraulic equipment where multiple hoses must pass through limited spaces.

Therefore, when replacing a hose, checking only the nominal ID is not always sufficient. The replacement should also be checked for OD, pressure rating, bend radius, fitting compatibility, temperature rating, and construction.

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5. Hydraulic Hose ID and OD by SAE Hose Type

 

Hydraulic hose dimensions can be better understood by comparing different SAE hose constructions. Many hydraulic hoses are manufactured according to specifications historically associated with SAE J517 hose types, such as SAE 100R1, 100R2, 100R12, 100R13, 100R15, 100R16, and 100R17.

These hose types may share the same nominal ID while using very different reinforcement structures.

Common SAE Hydraulic Hose Types

Hose Type	Typical Reinforcement	General Characteristics
SAE 100R1	One steel-wire braid	Medium-pressure hydraulic service
SAE 100R2	Two steel-wire braids	Higher-pressure service
SAE 100R12	Four spiral-wire plies	High-pressure heavy-duty service
SAE 100R13	Multiple spiral-wire plies	High-pressure severe service
SAE 100R15	Multiple spiral-wire plies	Very high-pressure service
SAE 100R16	One or two wire braids depending on size/design	Compact high-pressure hose
SAE 100R17	One or two wire braids depending on size/design	Compact constant-pressure applications

The exact construction and dimensional limits should always be confirmed against the applicable standard and manufacturer’s datasheet.

SAE 100R1 Hose ID and OD

SAE 100R1-type hoses typically use one steel-wire braid reinforcement layer.

Simplified construction:

Oil-Resistant Inner Tube → One Steel-Wire Braid → Outer Cover

Because only one primary wire-braid reinforcement layer is required, the overall hose construction can be relatively compact compared with multi-spiral hoses.

Typical applications include:

• General hydraulic systems
• Industrial machinery
• Agricultural equipment
• Return and pressure lines within the hose rating
• Mobile hydraulic equipment

SAE 100R2 Hose ID and OD

SAE 100R2-type hoses commonly use two steel-wire braid reinforcement layers.

Construction:

Inner Tube → First Wire Braid → Second Wire Braid → Outer Cover

For the same nominal ID, a 100R2-type hose will commonly have a different wall construction and OD from a 100R1-type hose.

The additional reinforcement allows the hose to handle demanding hydraulic pressure conditions, subject to the size-specific pressure rating.

SAE 100R12 Hose ID and OD

SAE 100R12-type hose is commonly associated with four layers of spiral steel-wire reinforcement.

Simplified construction:

Inner Tube → Four Spiral-Wire Reinforcement Layers → Outer Cover

This construction is intended for demanding high-pressure hydraulic applications.

Because of its reinforcement structure, a 100R12 hose with a 1/2-inch ID should not be expected to have the same OD as a 1/2-inch one-wire braided hose.

SAE 100R13 and SAE 100R15

SAE 100R13 and 100R15-type hoses are designed for demanding high-pressure applications and typically use heavy multi-spiral reinforcement.

Typical applications can include:

• Heavy construction machinery
• Mining equipment
• Large mobile hydraulic systems
• High-pressure hydrostatic systems
• Severe-duty hydraulic circuits

The relatively heavy reinforcement construction can result in larger wall thicknesses and different minimum bend-radius requirements compared with lighter braided hoses.

SAE 100R16 and SAE 100R17

SAE 100R16 and 100R17-type hoses are commonly associated with relatively compact constructions.

A compact hose can provide a smaller outside diameter and/or improved bend characteristics compared with some conventional hose constructions serving similar applications.

This can be useful where:

• Installation space is limited
• Hose bundles are dense
• Tight routing is required
• Reduced assembly envelope is important

However, the term compact does not mean that every hose of a particular dash size will have the same OD.

Comparing Hoses with the Same ID

Suppose several hoses all have a nominal size of -8, corresponding to approximately 1/2-inch ID.

They could include:

Hose	Nominal ID	Reinforcement	Relative Construction
SAE 100R1 type	1/2 in.	1 wire braid	Relatively light
SAE 100R2 type	1/2 in.	2 wire braids	Heavier
SAE 100R12 type	1/2 in.	4 spiral plies	Heavy-duty
SAE 100R13/R15 type	1/2 in. where applicable	Multi-spiral	Very heavy-duty
Compact hose	1/2 in.	Design dependent	Space-saving design

The important point is:

Same ID does not mean same OD.

The nominal ID primarily identifies the internal flow passage, while the OD depends strongly on the hose construction.

Therefore, when an exact OD is required, use a dimensional table for the specific SAE type and manufacturer hose series rather than a generic dash-size chart.

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6. How to Measure Hydraulic Hose ID and OD

Correctly measuring a hydraulic hose can help identify an unknown hose, verify installation clearance, select clamps and protective sleeves, and compare an existing hose with a replacement.

However, ID and OD should be measured carefully because hydraulic hoses are flexible components and can deform under measuring pressure.

Tools for Measuring Hydraulic Hose Size

Common measuring tools include:

• Vernier caliper
• Digital caliper
• Inside caliper
• Steel rule for large hoses
• Hose-size identification gauge

For most workshop applications, a digital or vernier caliper provides a convenient method of measuring hose OD and approximate ID.

How to Measure Hydraulic Hose OD

To measure the outside diameter:

1. Make sure the hose is clean and not under pressure.
2. Select a straight, undamaged section of hose.
3. Position the caliper jaws across the outside of the hose.
4. Close the jaws gently without compressing the rubber cover.
5. Record the measured diameter.
6. Repeat the measurement at another orientation if necessary.

Because flexible hose may not be perfectly circular, measuring in more than one direction can help identify deformation.

For example:

Measured OD 1 = 22.1 mm

Measured OD 2 = 22.5 mm

A difference may indicate that the hose has become slightly oval or has been compressed during service.

How to Measure Hydraulic Hose ID

Measuring ID is easiest when a clean hose end is accessible.

Place the inside-measuring jaws of a caliper against the internal walls of the hose and expand them gently until they contact the inner tube.

Do not stretch or deform the hose while measuring.

The measured value can then be compared with common nominal hydraulic hose sizes.

For example:

Approximate Measured ID	Likely Nominal Size
6.4 mm	1/4 in. (-4)
9.5 mm	3/8 in. (-6)
12.7 mm	1/2 in. (-8)
15.9 mm	5/8 in. (-10)
19.1 mm	3/4 in. (-12)
25.4 mm	1 in. (-16)

These values should be treated as nominal references rather than exact manufacturing dimensions.

Relationship Between Hydraulic Hose ID and OD

Hydraulic hose ID and OD are related through the total thickness of the hose wall.

The simplified relationship is:

OD = ID + (2 × Total Wall Thickness)

Therefore:

ID = OD − (2 × Total Wall Thickness)

And:

Total Wall Thickness = (OD − ID) / 2

Where:

• ID = Inside Diameter
• OD = Outside Diameter
• Total Wall Thickness = thickness measured from the inner surface of the hose to the outer surface on one side

For a hydraulic hose, the total wall thickness generally includes:

Inner Tube + Reinforcement Layer(s) + Outer Cover

For example, if a hose has:

• ID = 12.7 mm
• Total wall thickness = 5 mm

Then:

OD = 12.7 + (2 × 5)

OD = 22.7 mm

However, this formula should mainly be used to explain the geometric relationship between ID and OD. In practice, the wall thickness of a hydraulic hose cannot normally be assumed because it varies with hose construction, reinforcement, pressure rating, standard, and manufacturer.

Therefore:

Do not calculate an unknown hose ID from OD unless the actual wall thickness is known.

For hose identification, it is more reliable to measure the ID directly and then compare it with the nominal dash size and manufacturer specifications.

ID and OD Measurement Best Practices

For accurate hose identification:

• Measure a straight and undamaged section.
• Avoid squeezing the hose with the caliper.
• Measure more than once.
• Check whether the hose is oval.
• Use ID to estimate nominal hose size.
• Do not identify pressure rating from OD.
• Check hose markings whenever available.
• Verify final dimensions using manufacturer data.

Physical measurement is particularly useful for initial identification, but the replacement hose should ultimately be selected based on its complete technical specification—not ID and OD alone.

7. How Hose ID Affects Flow and Pressure Drop

Hydraulic hose inside diameter has a direct effect on fluid velocity, pressure drop, heat generation, and overall hydraulic system efficiency. Selecting the correct hose ID is therefore essential when designing pressure, return, and suction lines.

For a given flow rate, a smaller hose ID creates a higher fluid velocity, while a larger hose ID reduces fluid velocity.

The basic relationship is:

Q = A × V

Where:

• Q = flow rate
• A = internal cross-sectional area of the hose
• V = average fluid velocity

For a circular hydraulic hose, the internal flow area can be calculated as:

A = π × D² / 4

Where:

• A = internal flow area
• D = hose inside diameter
• π = approximately 3.1416

Therefore, fluid velocity can also be expressed as:

V = Q / A

This relationship explains why a relatively small change in hose ID can produce a significant change in fluid velocity.

Example: Comparing 1/2-Inch and 3/4-Inch Hose

For a 1/2-inch ID hose:

A = 3.1416 × 0.5² / 4

A ≈ 0.196 in²

For a 3/4-inch ID hose:

A = 3.1416 × 0.75² / 4

A ≈ 0.442 in²

Although the diameter increases by only 50%, the internal flow area increases from approximately 0.196 in² to 0.442 in².

In other words, the 3/4-inch hose has more than twice the internal flow area of the 1/2-inch hose.

This happens because flow area increases with the square of the inside diameter.

Smaller Hose ID Means Higher Velocity

When the same amount of hydraulic oil passes through two hoses:

Smaller ID → Smaller Flow Area → Higher Velocity

Larger ID → Larger Flow Area → Lower Velocity

Excessively high fluid velocity can contribute to:

• Higher pressure drop
• Increased heat generation
• Increased turbulence
• Hydraulic noise
• Reduced system efficiency
• Greater stress on fittings and hose assemblies

This is why simply choosing the smallest hose that can physically connect to the equipment is not good hydraulic design practice.

Hose ID and Pressure Drop

Pressure drop occurs because hydraulic fluid experiences friction as it flows through the hose.

Pressure loss is affected by:

• Hose ID
• Hose length
• Flow rate
• Oil viscosity
• Oil temperature
• Internal hose condition
• Hose bends
• Fittings
• Adapters
• Quick-connect couplings
• Other restrictions

For the same flow rate:

Smaller ID → Higher Velocity → Higher Friction → Higher Pressure Drop

A larger hose generally produces a lower pressure drop.

Pressure loss also represents wasted hydraulic energy because part of the pump’s output is converted into heat instead of useful work.

Hydraulic power loss can be represented simply as:

Power Loss = Pressure Drop × Flow Rate

The exact conversion factor depends on the units being used.

For example, when using US hydraulic units:

Power Loss (hp) = Pressure Drop (psi) × Flow (GPM) / 1714

When using SI units:

Power Loss (kW) = Pressure Drop (bar) × Flow (L/min) / 600

For example, if a hydraulic line has:

• Pressure drop = 10 bar
• Flow rate = 60 L/min

Then:

Power Loss = 10 × 60 / 600

Power Loss = 1 kW

That energy is effectively being converted into heat in the hydraulic system.

Hose Length Also Affects Pressure Drop

Pressure loss increases as hose length increases.

A short hose may operate satisfactorily at a certain flow rate, while a much longer hose with the same ID can produce excessive pressure drop.

In general:

Longer Hose → Greater Friction → Higher Pressure Drop

Therefore, long hydraulic hose runs may require a larger ID than short hose assemblies carrying the same flow rate.

Suction, Return, and Pressure Lines

Different hydraulic lines have different sizing requirements.

Suction lines generally require relatively large IDs to keep fluid velocity and inlet pressure losses low. Excessive restriction on the pump inlet can contribute to cavitation and poor pump performance.

Return lines should also have sufficient flow area to prevent excessive backpressure as oil returns to the reservoir.

Pressure lines can normally tolerate higher fluid velocities than suction or return lines, but excessive velocity still increases pressure loss and heat generation.

The correct hose ID should therefore be selected according to:

Flow Rate + Line Type + Hose Length + Acceptable Velocity + Acceptable Pressure Drop

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8. How to Select the Correct Hydraulic Hose Size

Selecting the correct hydraulic hose size involves more than matching the hose to the thread size of a hydraulic fitting.

The hose must provide sufficient internal flow area while also satisfying requirements for pressure, temperature, fluid compatibility, bend radius, installation space, and fitting compatibility.

A commonly used hose selection method is STAMPED:

• S – Size
• T – Temperature
• A – Application
• M – Material or Media
• P – Pressure
• E – Ends
• D – Delivery

For hydraulic hose ID and OD selection, the first step is determining the correct Size.

Step 1: Determine the Required Flow Rate

Identify the maximum flow that must pass through the hose.

Hydraulic flow is commonly expressed in:

• L/min
• GPM
• m³/h

The hose should normally be sized according to the maximum expected flow rate, rather than only the average operating flow.

Step 2: Determine the Required Hose ID

Once the required flow rate and acceptable fluid velocity are known, the necessary internal area can be estimated using:

A = Q / V

Where:

• A = required internal flow area
• Q = flow rate
• V = selected fluid velocity

Once the required area is known, the corresponding hose ID can be determined from:

D = √(4 × A / π)

Where:

• D = required hose inside diameter
• A = internal flow area
• π = approximately 3.1416

In practical hydraulic design, the calculated diameter is normally compared with the next available standard hose size.

For example, if the calculated required ID is approximately 11 mm, common available sizes might include:

• -6 hose = approximately 9.5 mm ID
• -8 hose = approximately 12.7 mm ID

In this situation, the -8 hose would generally provide the required flow area, subject to all other design requirements.

Step 3: Convert Hose ID to Dash Size

For many common hydraulic hose sizes:

Dash Size = Hose ID in sixteenths of an inch

For example:

1/4 inch = 4/16 inch = -4

3/8 inch = 6/16 inch = -6

1/2 inch = 8/16 inch = -8

3/4 inch = 12/16 inch = -12

1 inch = 16/16 inch = -16

This makes the dash-size system convenient for identifying nominal hydraulic hose ID.

Step 4: Check Pressure Drop

After choosing an initial hose size, check whether the expected pressure loss is acceptable.

Pressure drop becomes particularly important when the system includes:

• Long hose runs
• High flow rates
• High-viscosity fluids
• Low oil temperatures
• Numerous fittings
• Tight bends
• Quick-connect couplings
• Flow-control devices

If pressure drop is too high, increasing the hose ID can significantly improve hydraulic efficiency.

Step 5: Verify Working Pressure

Hose diameter does not determine its pressure rating.

Two -8 hoses can have the same nominal 1/2-inch ID but very different maximum working pressures because their reinforcement constructions are different.

Always verify the selected hose for:

• Maximum working pressure
• Pressure impulse performance
• Temperature rating
• Applicable safety factor
• Manufacturer specifications
• Applicable hose standard

Never estimate working pressure from hose OD or wall thickness alone.

Step 6: Check Hose OD

After selecting the hose ID and hose series, check the actual outside diameter.

OD is especially important when the hose must fit through:

• Machine frames
• Bulkheads
• Hose clamps
• Protective sleeves
• Hose carriers
• Hose bundles
• Restricted installation spaces

Remember:

ID determines internal flow capacity.

OD determines the physical installation envelope.

Two hoses with the same ID may therefore require different amounts of installation space.

Step 7: Check Minimum Bend Radius

Every hydraulic hose has a specified minimum bend radius.

Installing the hose below this radius can:

• Flatten the hose
• Reduce the effective ID
• Stress the reinforcement
• Cause kinking
• Reduce service life
• Increase the risk of premature failure

The manufacturer’s specified minimum bend radius should always be followed.

Step 8: Verify Hose and Fitting Compatibility

The selected fitting must be compatible with the specific hose type and construction.

The same dash-size fitting is not automatically compatible with every hose having that nominal ID.

Always verify:

• Hose series
• Fitting series
• Crimp specification
• Crimp diameter
• Hose OD
• Applicable assembly procedure

This is particularly important for high-pressure wire-braid and spiral-wire hoses.

Hydraulic Hose Size Selection Checklist

Parameter	What to Check
Hose ID	Adequate internal flow area
Flow Rate	Maximum expected flow
Fluid Velocity	Appropriate for line type
Pressure Drop	Within acceptable limits
Working Pressure	Meets system pressure
Hose OD	Fits available installation space
Bend Radius	Suitable for hose routing
Temperature	Within hose operating range
Fluid Compatibility	Suitable for hydraulic media
Fittings	Compatible with hose series
Standards	Meets applicable requirements

The correct hydraulic hose size is therefore a balance between hydraulic performance and physical installation requirements.

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9. Hydraulic Hose ID and OD FAQs

Is Hydraulic Hose Measured by ID or OD?

Hydraulic hose size is generally specified by its inside diameter (ID).

ID determines the internal flow passage and is the primary dimension associated with hydraulic hose dash size.

OD is mainly important for physical installation, including hose clamps, routing clearance, protective sleeves, and machine openings.

What Is the ID of a -6 Hydraulic Hose?

A -6 hose represents six sixteenths of an inch:

6 / 16 = 3 / 8 inch

Therefore:

-6 hose = 3/8-inch nominal ID

In metric units:

3/8 inch × 25.4 = approximately 9.5 mm

So:

-6 = approximately 9.5 mm ID

What Is the ID of a -8 Hydraulic Hose?

A -8 hose represents eight sixteenths of an inch:

8 / 16 = 1 / 2 inch

Therefore:

-8 hose = 1/2-inch nominal ID

Metric conversion:

1/2 inch × 25.4 = 12.7 mm

So:

-8 = approximately 12.7 mm ID

What Size Hydraulic Hose Is 1/4 Inch ID?

Convert the ID into sixteenths:

1/4 inch = 4/16 inch

Therefore:

1/4-inch hydraulic hose = -4

Metric equivalent:

1/4 inch = approximately 6.4 mm

What Size Hydraulic Hose Is 1/2 Inch ID?

Convert the ID into sixteenths:

1/2 inch = 8/16 inch

Therefore:

1/2-inch hydraulic hose = -8

Metric equivalent:

1/2 inch = 12.7 mm

What Size Hydraulic Hose Is 3/4 Inch ID?

Convert the ID into sixteenths:

3/4 inch = 12/16 inch

Therefore:

3/4-inch hydraulic hose = -12

Metric equivalent:

3/4 inch = approximately 19.1 mm

What Size Hydraulic Hose Is 1 Inch ID?

Convert the ID into sixteenths:

1 inch = 16/16 inch

Therefore:

1-inch hydraulic hose = -16

Metric equivalent:

1 inch = 25.4 mm

How Do You Calculate Hydraulic Hose Dash Size?

For standard dash sizing based on sixteenths of an inch:

Dash Number = Hose ID (inch) × 16

For example, for a 1/2-inch hose:

0.5 × 16 = 8

Therefore:

1/2-inch ID = -8

For a 3/4-inch hose:

0.75 × 16 = 12

Therefore:

3/4-inch ID = -12

How Do You Convert Hydraulic Hose ID from Inches to Millimeters?

Use:

ID (mm) = ID (inch) × 25.4

For example:

1/2 inch × 25.4 = 12.7 mm

Therefore:

1/2-inch hose = approximately 12.7 mm ID

How Do You Convert Millimeters to Inches?

Use:

ID (inch) = ID (mm) / 25.4

For example:

12.7 mm / 25.4 = 0.5 inch

Therefore:

12.7 mm = 1/2 inch

Can I Determine Hydraulic Hose Size from OD?

Not reliably.

Hose OD includes:

Inner Tube + Reinforcement + Outer Cover

Different hose constructions can have the same nominal ID but significantly different ODs.

Therefore:

Same ID does not mean same OD.

The best method is to identify the hose using:

• Hose markings
• Measured ID
• Dash size
• Hose construction
• Manufacturer part number

Why Do Two Hoses with the Same ID Have Different ODs?

Different hoses may use different:

• Inner tube thicknesses
• Reinforcement layers
• Wire sizes
• Spiral constructions
• Outer cover thicknesses

For example, a -8 one-wire braided hose and a -8 four-spiral hose may both have approximately 1/2-inch ID, while their ODs can be very different.

Does a Larger Hose Reduce Pressure Drop?

Generally, yes.

For the same flow rate:

Larger ID → Larger Flow Area → Lower Fluid Velocity → Lower Pressure Drop

However, selecting an excessively large hose can increase:

• Cost
• Weight
• Space requirements
• Fitting size
• Installation difficulty

The hose should therefore be sized according to the required flow rate and acceptable pressure loss rather than simply selecting the largest available size.

Is Dash Size the Same as Hose OD?

No.

Dash size generally refers to the nominal hose ID, not its outside diameter.

For example:

-4 = 1/4-inch ID

-6 = 3/8-inch ID

-8 = 1/2-inch ID

-12 = 3/4-inch ID

-16 = 1-inch ID

The actual OD must be obtained from the manufacturer’s hose datasheet.

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Conclusion

Understanding hydraulic hose ID and OD is essential for selecting the correct hose, designing efficient hydraulic circuits, and ensuring proper installation. The inside diameter determines the available flow area and has a direct effect on fluid velocity and pressure drop, while the outside diameter determines the physical space required for routing, clamps, protective sleeves, and other installation components.

Hydraulic hoses are commonly identified using dash sizes, with the dash number generally corresponding to the nominal ID in sixteenths of an inch. For example, a -8 hose corresponds to a nominal 1/2-inch ID, while a -16 hose corresponds to a nominal 1-inch ID.

However, hoses with the same ID do not necessarily have the same OD. Reinforcement construction, cover thickness, pressure requirements, hose standards, and manufacturer design can all change the outside diameter. This is particularly important when comparing braided, compact, and multi-spiral hydraulic hoses.

A hydraulic hose ID and OD chart is therefore an excellent starting point for hose identification and sizing, but final selection should always be based on the specifications of the actual hose series. Verify the required ID, OD, working pressure, temperature range, fluid compatibility, minimum bend radius, fittings, and applicable standards before specifying or replacing a hydraulic hose.

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