Undersize a hydraulic hose and you burn horsepower as heat, starve the actuator, and cook the fluid. Oversize it and you pay for bulk, weight, and routing headaches you didn’t need. Hose selection comes down to three velocity limits and a pressure-drop check — this guide shows how to size a line from flow rate and pressure, and how to read the SAE dash-size system so you order the right part.
The three velocities that set hose size
Hydraulic hose is sized by keeping fluid velocity inside accepted bands. Too fast and you get turbulence, noise, erosion, and pressure loss; too slow and you waste material. The standard targets:
| Line type | Target velocity | Why |
|---|---|---|
| Pressure (working) | 15–25 ft/s (4.5–7.6 m/s) | Balances loss against hose size |
| Return | 10–15 ft/s (3–4.5 m/s) | Lower pressure, keep it laminar |
| Suction (pump inlet) | 2–4 ft/s (0.6–1.2 m/s) | Avoid cavitation — the critical one |
Sizing from flow rate
The required inside diameter follows directly from continuity — area equals flow divided by velocity. In convenient units:
d (in) = √( 0.3208 × Qgpm / Vft/s )
For a 12 gpm pressure line targeting 20 ft/s: d = √(0.3208 × 12 / 20) = √0.1925 = 0.44 in. The next standard size up is a -8 hose (1/2 in ID), which lands the actual velocity right in the target band.
Reading SAE dash sizes
Hydraulic hose and fittings are specified in “dash” sizes — sixteenths of an inch of nominal inside diameter. A -8 hose is 8/16 = 1/2 in ID; a -16 is 1 in.
| Dash | Nominal ID | Typical flow @ 20 ft/s |
|---|---|---|
| -6 | 3/8 in | ~7 gpm |
| -8 | 1/2 in | ~12 gpm |
| -10 | 5/8 in | ~19 gpm |
| -12 | 3/4 in | ~28 gpm |
| -16 | 1 in | ~49 gpm |
Dash size describes the hose ID, but the same dash number on a fitting refers to its own thread series — always confirm the thread standard (JIC 37°, ORB, NPT, BSP) separately.
Checking pressure drop
Velocity gets you the size; pressure drop tells you the energy cost. For a straight run, the Darcy–Weisbach relation gives the loss:
ΔP = f × (L/d) × (ρV² / 2)
where f is the friction factor (from the Reynolds number and the flow regime), L the length, d the diameter, ρ the fluid density, and V the velocity. Two things dominate the result in real systems: fluid viscosity (which swings hard with temperature) and the fittings. Each elbow, tee, and quick-coupler adds an equivalent length that can dwarf the straight run — a single 90° elbow can be worth several feet of hose.
Putting it together
The workflow for any line is: pick the velocity target for the line type → size the ID from flow → round to the nearest dash size → verify the real velocity and pressure drop at your hottest and coldest operating viscosity. If either limit is violated, step the dash size and repeat.
Skip the arithmetic
The Hydraulic Hose Selector sizes the line from your flow and pressure and returns the dash size plus actual velocity. Check the energy cost of the run with the Pressure Drop Calculator.
Not sure what fitting you’re holding? The Hydraulic Thread Identifier tells you the thread standard from a few measurements, and the Fitting Torque Calculator gives you the assembly spec.
Assembly notes that save warranty claims
- Match the fitting thread standard to the port — JIC and ORB are not interchangeable even when they thread together.
- Respect the minimum bend radius; a tight bend collapses the ID and spikes local velocity.
- Leave slack for pressure “growth” — hose shortens under pressure and can pull fittings loose.
- Torque fittings to spec; over-torqued JIC seats crack and under-torqued ones weep.
A quality assembly — the right Parker or Gates hose crimped to the correct fitting — sized with these limits will run cool and quiet for its rated life. Get the size wrong and no fitting quality can save it.
Derate for temperature and fluid type
The velocity bands assume a mineral-oil hydraulic fluid at normal operating temperature. Two conditions push you toward a larger size than the base sizing suggests:
- Cold starts. Viscosity can climb ten-fold on a cold morning. A line that’s fine at 120°F can starve the pump at 20°F because the pressure drop scales with viscosity. Size the suction line for the coldest expected start, not the steady-state temperature.
- Water-glycol and high-water fluids. These are denser and less lubricious than mineral oil; keep suction velocity at the low end of the band and confirm the hose liner is rated for the fluid.
More fluid-power tools on the Fluid & Force page, including the Hydraulic Cylinder Calculator.
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