Lesson 2 of 5 · Sizes
Hydraulic hose sizes explained
A hydraulic hose is sized by its inside diameter. The same size can be written in inches, as a dash number, or as a metric DN.
Inside diameter is the size
The size of a hydraulic hose is its inside diameter, also called the bore or ID. This is the dimension that decides how much oil can flow and how fast it moves. A hose described as "half-inch" has a 1/2 in bore, whatever its outside diameter.
Too small a bore makes the oil move too fast, which wastes power as heat and causes pressure loss. Too large a bore adds cost, weight and a bigger bend radius. A replacement hose should always have the same bore as the original.
Three ways to write the same size
Inches: the bore as a fraction, such as 1/2 in.
Dash size: the bore in sixteenths of an inch, written with a dash. 1/2 in is 8/16 in, so it is -08. This is the usual system in catalogues and part numbers.
DN: the metric nominal diameter in millimetres, rounded. 1/2 in is 12.7 mm, written DN12. This is what European and Asian hoses carry on the layline.
| Dash size | Nominal ID (inch) | Exact sixteenths in mm | Common DN label |
|---|---|---|---|
| -02 | 1/8 in | 3.175 mm | — |
| -03 | 3/16 in | 4.763 mm | DN5 |
| -04 | 1/4 in | 6.35 mm | DN6 |
| -05 | 5/16 in | 7.938 mm | DN8 |
| -06 | 3/8 in | 9.525 mm | DN10 |
| -08 | 1/2 in | 12.7 mm | DN12 |
| -10 | 5/8 in | 15.875 mm | DN16 |
| -12 | 3/4 in | 19.05 mm | DN19 |
| -16 | 1 in | 25.4 mm | DN25 |
| -20 | 1-1/4 in | 31.75 mm | DN31 |
| -24 | 1-1/2 in | 38.1 mm | DN38 |
| -32 | 2 in | 50.8 mm | DN51 |
The millimetre column is the dash number divided by 16, times 25.4. The DN label is the rounded metric name used on hose lay lines and in ISO/EN hose standards.
Outside diameter
The outside diameter, or OD, depends on how many reinforcement layers the hose has. A two-wire hose and a four-spiral hose of the same bore have different outside diameters.
OD does not size the hose, but it matters for three things: choosing the fitting shell, choosing clamps and protective sleeves, and checking that the hose fits through a guide or bracket. Compact hose types exist specifically to give the same bore in a smaller OD.
How to measure a hose
- Read the layline first. The size is normally printed as a fraction, a dash size or a DN.
- If the layline is worn off, measure the bore of a cleanly cut end with calipers.
- Do not size a hose from its outside diameter alone.
- Measure length on the finished assembly, between the reference points of the two fittings.
The exception: hoses sized by tube OD
A few hose types, such as SAE 100R5 and some air-conditioning and truck hoses, use a dash number that refers to the tube they replace rather than their own bore. On these a -06 hose has a 5/16 in bore, not 3/8 in. Check the printed inside diameter.
Frequently asked questions
How is hydraulic hose size measured?
By inside diameter. The size is written in inches, as a dash number in sixteenths of an inch, or as a metric DN.
What size is -06 hydraulic hose?
A -06 hose has a 6/16 in bore, which is 3/8 in or about 9.5 mm, labelled DN10.
What does DN mean on a hose?
DN is the nominal inside diameter in millimetres, rounded to a standard value. DN12 is a 1/2 in hose and DN25 is a 1 in hose.
Do I size a hose by inside or outside diameter?
Inside diameter. Outside diameter varies with the number of reinforcement layers and is used only for choosing fittings, clamps and sleeves.
Useful with this lesson
- Unit and dash-size converter
- Find hose length and fittings by machine model
- Hydraulic fitting types and how to identify them
- How to make a hydraulic hose assembly
Ratings and dimensions here are the nominal values set by the standards. The layline and data sheet of the actual hose take precedence.