Hydraulic Hose Dash Sizes Explained (-4, -6, -8 and Beyond)
If you have ever seen a hose called “-8” or “dash 8” and were not sure what it meant, this guide clears it up in one read. Dash sizes are a simple, standardised shorthand for hose bore, and once you know the rule you can convert any of them in your head. Getting the bore right matters: too small and you starve the circuit and overheat the fluid; too large and you waste money and space.
What does a hydraulic hose dash size mean?
The short answer: a dash size is the hose’s nominal inside diameter (ID) measured in sixteenths of an inch. So a -8 hose has an ID of 8/16 inch, which is 1/2 inch. The rule is simply dash number divided by 16 = ID in inches. It always refers to the inside diameter, never the outside diameter or the fitting thread.
This convention is standardised in SAE J517 and used by all major hose manufacturers, which is why “-8” means the same bore whoever makes the hose.
Dash size conversion chart
| Dash size | ID (inch) | ID (mm, approx) |
|---|---|---|
| -4 | 1/4″ | 6.4 mm |
| -6 | 3/8″ | 9.5 mm |
| -8 | 1/2″ | 12.7 mm |
| -10 | 5/8″ | 15.9 mm |
| -12 | 3/4″ | 19.1 mm |
| -16 | 1″ | 25.4 mm |
| -20 | 1-1/4″ | 31.8 mm |
| -24 | 1-1/2″ | 38.1 mm |
| -32 | 2″ | 50.8 mm |
Quick check: -6 is 6/16 = 3/8 inch, -12 is 12/16 = 3/4 inch, -16 is a full 1 inch bore.
The one common exception to know
The sixteenths rule holds for the great majority of hydraulic hoses, but there is a well-known exception: SAE 100R5 hose uses a dash size based on the equivalent tube outside diameter rather than the hose ID. If you are working with R5, do not assume the standard conversion, check the manufacturer’s chart for the actual bore. For everyday R1, R2, R7, R9 and similar hoses, the dash-in-sixteenths rule applies.
Do not confuse hose dash size with fitting dash size
Hose dash size describes the hose bore. Fitting dash sizes describe the fitting’s connection size and do not always match the hose they attach to. A hose and its end fitting are selected together, so specify both rather than assuming the fitting dash equals the hose dash.
How to measure and choose the right bore
- Measure the ID, not the OD, when identifying an existing hose, then convert to a dash size using the chart above.
- Size for flow, not habit. Bore should suit the required flow rate and recommended fluid velocity for the line type (pressure, return or suction), so the circuit is not starved and the fluid does not overheat.
- Match the assembly. Confirm the hose construction, pressure rating and end fittings together, because the right bore in the wrong construction still fails.
The goal is the right bore, whether you buy from us or not
We wrote this so you can read a dash size, convert it, and size the bore for your flow, wherever you source the hose. The rule is simple and it is yours to use.
If you do want it sourced to spec, here is how we work: Velqor supplies hose and cut, coupled and pressure-tested assemblies matched to your bore, pressure, temperature and fluid. Give the dash size or ID with the pressure rating, length and both end fittings, and the assembly is confirmed, tested and documented before dispatch. If a standard size serves you better than a special, we will tell you.
Frequently Asked Questions
What does dash 8 (-8) mean on a hydraulic hose?
A -8 hose has a nominal inside diameter of 8/16 inch, which is 1/2 inch. The dash number is the ID in sixteenths of an inch, so dividing it by 16 gives the bore in inches.
Does the dash size refer to inside or outside diameter?
For most hydraulic hoses the dash size refers to the inside diameter (ID). The main exception is SAE 100R5, which bases its dash size on the equivalent tube outside diameter, so always check the chart for R5.
How do I convert a dash size to millimetres?
Convert the dash to inches first (dash divided by 16), then multiply by 25.4. For example, -8 is 1/2 inch, which is about 12.7 mm.
Is the fitting dash size the same as the hose dash size?
Not always. Hose dash size describes the hose bore, while fitting dash size describes the connection size. Select the hose and fitting together and specify both rather than assuming they match.
How do I pick the right hose bore?
Size the bore for the required flow rate and the recommended fluid velocity for that line type (pressure, return or suction), not just to match the old hose. Too small starves the circuit and overheats the oil; too large wastes cost and space.