An undersized hydraulic hose runs hot, wastes horsepower, and fails early. An oversized hose costs money and adds weight for no benefit. Getting hose inside diameter right is a two-step discipline: pick the ID from flow rate and a recommended velocity, then confirm the resulting pressure drop is acceptable. This article shows both steps and points you to the tools that automate them.
Step 1: size from flow and velocity
The governing relationship is simply continuity — flow equals area times velocity:
The catch is choosing v. Fluid velocity that is fine for a pressure line is far too fast for a suction line, where it would cavitate the pump. Industry practice uses different velocity bands for each line type:
| Line type | Recommended velocity | Why |
|---|---|---|
| Pressure | 15–25 ft/s (4.6–7.6 m/s) | Highest band; pressure tolerates turbulence |
| Return | 10–15 ft/s (3–4.6 m/s) | Lower to limit back-pressure |
| Suction | 2–4 ft/s (0.6–1.2 m/s) | Lowest; prevents pump cavitation |
The hydraulic hose selector applies these bands automatically: enter the flow rate and line type, and it returns the minimum ID and the nearest standard dash size.
A word on dash sizes
Hydraulic hose is specified in dash sizes, where each dash is 1/16 inch of nominal ID. A -8 hose is 8/16 = 1/2 inch ID. Round up to the next dash size — never down — so you stay within the velocity limit.
Step 2: verify pressure drop
Sizing for velocity gets you close, but a long run can still drop unacceptable pressure. The Darcy-Weisbach equation gives the loss:
Where f is the Darcy friction factor (a function of Reynolds number and roughness), L is length, d is ID, ρ is fluid density, and v is velocity. Hydraulic oil viscosity changes sharply with temperature, so always run the calculation at your cold-start viscosity — that is the worst case. The pipe pressure drop calculator solves Darcy-Weisbach with the Colebrook friction factor so you do not have to iterate by hand.
Do not forget the ends
A correctly sized hose with the wrong fittings is still a leak waiting to happen. Two tools close the loop:
- Identify the thread — JIC, ORFS, NPT, BSPP, and SAE O-ring boss all look similar at a glance. Use the hydraulic thread identifier with a thread pitch gauge and caliper to confirm before you order adapters.
- Torque the fitting — under-torqued fittings weep, over-torqued fittings crack flares. The fitting torque calculator gives the spec by size and standard.
Worked example
Convert 10 GPM to 0.0223 ft³/s. Required area A = Q/v = 0.0223 / 20 = 0.00111 ft² = 0.160 in². Diameter d = √(4A/π) = 0.451 in. The nearest standard size rounding up is -8 (1/2 in ID), which gives an actual velocity near 16 ft/s — comfortably inside the pressure band. A -6 (3/8 in) would push velocity to ~28 ft/s, above the limit, so -8 is correct.
Frequently asked questions
Can I size suction and pressure lines the same?
No. Suction lines must use a much lower velocity (2–4 ft/s) to avoid cavitation, so they are always a larger ID than the pressure line for the same flow.
Does hose ID match the fitting size?
Usually, but not always. Some fittings neck down internally and create a hidden restriction. Verify the fitting’s true bore, not just its dash label.
Why size for cold-start viscosity?
Cold oil is thicker, so friction factor and pressure drop are highest at start-up. If the system works cold, it works warm.
Three hose-sizing mistakes to avoid
The math is forgiving; the habits are not. The most common error is sizing every line to the pressure-line velocity band — do that on a suction line and you cavitate the pump within hours. The second is forgetting bend radius: a hose crimped tighter than its minimum bend radius restricts flow and fatigues the reinforcement, no matter how generous the ID. The third is ignoring temperature when picking the pressure-drop input — hydraulic oil can triple in viscosity from a cold start, so always design to the cold case. Size for velocity, verify pressure drop warm and cold, and respect the bend radius, and the hose will outlast the machine around it.
Size your hose in the Hydraulic Hose Selector →
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