Calculators · Hydraulics and pneumatics

Hydraulic hose size, fluid velocity and pressure drop calculator

Hydraulic hoses are sized by flow velocity. Too small a hose makes the oil flow fast, wasting pressure as heat and causing noise; too large wastes money and space. These calculators find the inside diameter needed for a flow at the recommended velocity, check the velocity in an existing hose, and estimate the pressure lost along its length.

Hose inside diameter for a flow

Calculates the minimum inside diameter that keeps oil velocity at or below a chosen value. Suction lines need the lowest velocity to avoid pump cavitation; pressure lines can run fastest.

Results
Minimum inside diameter—Choose the next hose size up.

Formula
d = √(4 × Q / (π × v))

Oil velocity in a hose

Divides flow by the hose bore area to check an existing line. Velocities well above the recommended ranges point to a hose that is too small for the flow.

Results
Velocity—m/sAverage oil speed in the hose.

Formula
v = Q / (π/4 × d²)

Pressure drop along a hose

Estimates friction pressure loss in a straight hose using the Darcy–Weisbach equation, with the laminar or turbulent friction factor chosen from the Reynolds number. Fittings, bends and valves add further losses.

Results
Pressure drop—Pressure lost to friction over the length.
Reynolds number—Below about 2,300 the flow is laminar.
Flow regime—Laminar flow is quieter and loses less pressure.

Formula
Re = v × d / ν
f = 64 / Re (laminar) or 0.316 / Re^0.25 (turbulent)
Δp = f × (L / d) × ρ × v² / 2

Worked example: a 60 L/min pressure line

  1. At 5 m/s a 60 L/min pressure line needs at least 16 mm bore, so a −12 (19 mm) hose is chosen.
  2. In that hose the oil moves at 3.5 m/s; in a −8 (12.7 mm) hose it would reach 7.9 m/s.
  3. With ISO VG 46 oil the Reynolds number is about 1,450 — laminar flow.
  4. Over 10 m of straight hose the pressure drop is about 1.2 bar.

Recommended oil velocities

LineVelocity
Suction (pump inlet)about 0.6 – 1.2 m/s
Returnabout 2 – 4 m/s
Pressure, up to about 100 barabout 3 – 4.5 m/s
Pressure, above about 200 barabout 5 – 6 m/s

Related

Hydraulic pump and motor calculatorHydraulic cylinder force, speed and volume calculatorAir compressor sizing, tank fill time and air leak cost calculator

About these results

Results are theoretical estimates from standard engineering formulas. Real-world figures depend on conditions these formulas do not capture. Always follow the manufacturer's specifications for maintenance, loading and safety decisions.

Related pages