Hydraulic Pump Size Calculator

Size an oil pump from flow and pressure: hydraulic HP, input HP and kW with efficiency, plus the displacement your pump needs at its RPM.

Calculator

Required flow (gpm) What your actuators need. Example: 10 gpm for a 3-inch cylinder at 327 in/min.

Working pressure (psi) Pressure at the pump outlet while working, not the relief valve setting.

Pump efficiency (η) Overall efficiency, 0 to 1. Example: 0.85 for a healthy gear or piston pump.

Pump speed (rpm) Shaft speed. Example: 1800 rpm from a 4-pole motor at 60 Hz.

About this calculator

This hydraulic pump size calculator converts actuator demand into pump power and displacement. It is an oil-pump tool — for water pumps working against total dynamic head, use the water pump sizing calculator. Cylinder-side math is in the hydraulic cylinder calculator.

Calculation Results

13.73 HP

Input power · hydraulic 11.67 HP · input kW 10.24 · displacement 1.283 in³/rev

Formula: HP = Q × PSI / 1714; input HP = HP / η; D = Q × 231 / RPM

1.283 in³/rev ≈ 21.0 cc/rev · flow 37.9 L/min. Size the electric motor on input HP.

How to use this result

Input HP is the shaft power the electric motor must supply — pick the next standard motor size up, and check the torque demand at cold-start oil viscosity. Displacement is what the pump's data sheet must state at that RPM. If your duty cycle holds at relief pressure for long periods, re-run the numbers at the relief setting because that heat goes straight into the oil.

Continue Your Calculation

Pump input power does not set cylinder force. Open the cylinder calculator when the next step is actuator sizing.

Cylinder force →

Pump input power chart (HP, η = 0.85)

Input HP = GPM × PSI / 1714 / 0.85 — the motor size you need. Read hydraulic HP by multiplying back by 0.85.

Flow (gpm)	1,000 psi	1,500 psi	2,000 psi	2,500 psi	3,000 psi
5	3.43	5.15	6.86	8.58	10.30
10	6.86	10.30	13.73	17.16	20.59
15	10.30	15.44	20.59	25.74	30.89
20	13.73	20.59	27.46	34.32	41.18
25	17.16	25.74	34.32	42.90	51.48

Hydraulic pump formula

Hydraulic power: HP = Q × PSI / 1714. For 10 gpm at 2,000 psi: 20,000 / 1714 = 11.67 HP in the oil.

Input power: input HP = HP / η. At η = 0.85: 11.67 / 0.85 = 13.73 HP = 10.24 kW on the motor shaft.

Displacement: D = Q × 231 / RPM (in³/rev). At 1,800 rpm: 10 × 231 / 1800 = 1.283 in³/rev ≈ 21.0 cc/rev.

Metric row: flow 10 gpm = 37.9 L/min; input power 13.73 HP × 0.7457 = 10.24 kW; displacement 1 in³/rev = 16.3871 cc/rev.

Working out the cylinder on the other end? Use the hydraulic cylinder calculator. Motor torque at this speed: motor torque calculator.

Pump torque and motor matching

Input torque: T = 63,025 × input HP / RPM (lb·in). At the default operating point — 13.73 input HP at 1,800 rpm — the coupling sees 63,025 × 13.73 / 1800 = 481 lb·in = 40.1 lbf·ft (54.3 N·m). An electric motor for this pump needs at least that breakdown torque with margin.

You can cross-check from displacement: ideal mechanical torque is p × D / 2π — 2,000 psi × 1.283 in³/rev / 6.2832 = 408 lb·in. The 73 lb·in gap to 481 is the mechanical share of the losses that the 0.85 overall efficiency already covers, so never divide by mechanical efficiency again on top of the input-HP figure — that double-counts.

Input torque by flow and pressure (lbf·ft, rpm = 1,800, η = 0.85)
Flow	1,000 psi	1,500 psi	2,000 psi	2,500 psi	3,000 psi
5 gpm	10.0	15.0	20.0	25.0	30.0
10 gpm	20.0	30.0	40.1	50.1	60.1
15 gpm	30.0	45.1	60.1	75.1	90.1
20 gpm	40.1	60.1	80.1	100.1	120.2
25 gpm	50.1	75.1	100.1	125.2	150.2

Torque scales with flow × pressure, exactly like power — the rpm only divides it. If the duty cycle holds high pressure at low flow (clamping, pressing), size the motor on peak torque and check service factor; a motor chosen on average power alone will trip on the peaks.

Reservoir sizing rule of thumb

The tank does three jobs: it holds reserve oil, it gives return flow somewhere to slow down and de-aerate, and it dissipates heat through its walls. The classic rule is 3× pump flow for fixed industrial power units, down to 1.5× for mobile equipment where every litre of oil is weight you carry.

Reservoir volume by pump flow
Pump flow	Fixed industrial (3×)	Mobile (1.5×)
5 gpm	15 gal (57 L)	7.5 gal (28 L)
10 gpm	30 gal (114 L)	15 gal (57 L)
15 gpm	45 gal (170 L)	22.5 gal (85 L)
20 gpm	60 gal (227 L)	30 gal (114 L)
25 gpm	75 gal (284 L)	37.5 gal (142 L)

Treat these as floors, not targets. A high-pressure circuit running near-continuous duty needs a cooler more than a bigger tank; a circuit with large cylinders that park and extend all day needs enough volume above the pump inlet so the return surge never exposes the suction line. Verify the tank stays between 110°F and 140°F at end of shift — that check catches undersizing faster than any multiplier.

Which pump task this is

Question	Use
Oil pump HP, kW, or displacement	This page
Cylinder push/pull force or piston speed	Hydraulic cylinder calculator
Water pump against pipe friction and head	Water pump sizing calculator
Electric motor torque at pump RPM	Motor torque calculator
Motor horsepower in kilowatts	HP to kW

Frequently Asked Questions

How do you calculate hydraulic pump horsepower?

Hydraulic HP = GPM × PSI / 1714. Example: 10 gpm at 2,000 psi = 11.67 HP. Input HP = 11.67 / 0.85 = 13.73 HP on the motor shaft.

What is pump displacement?

The oil volume moved per revolution. D = GPM × 231 / RPM. At 1,800 rpm and 10 gpm the pump needs 1.283 in³/rev, about 21 cc/rev.

What pump efficiency should I use?

0.85 is a sound default for overall efficiency. Healthy gear pumps run 0.80–0.90, piston pumps 0.90+ when new. Drop it for hot oil, high RPM, or a worn pump.

Why is input horsepower higher than hydraulic horsepower?

Leakage and friction eat the difference. At η 0.85 the motor supplies 13.73 HP to deliver 11.67 HP into the oil — the 2.06 HP becomes heat in the reservoir.

How does pump speed change the required displacement?

Displacement = flow × 231 / RPM, so speed and displacement trade off. A 3,600 rpm motor pairs with half the displacement of a 1,800 rpm motor for the same gpm.

Is this the same as sizing a water pump?

No. Oil pumps are sized from flow, pressure, and displacement. Water pumps work against total dynamic head and pipe friction — different inputs. Use the water pump sizing calculator for TDH work.

How do I convert displacement to cc/rev?

1 in³ = 16.3871 cc. The example: 1.283 in³/rev × 16.3871 = 21.0 cc/rev — the size class of many small gear pumps.

What happens if the pump cannot build system pressure?

Actuators move slowly first — flow starves before pressure peaks. The relief valve then dumps flow to tank and oil heats quickly. Check displacement, RPM, and suction restrictions before blaming the relief valve.

What size reservoir does a 10 GPM pump need?

30–50 gal (114–189 L) for a fixed industrial power unit at the 3× to 5× rule; 10–20 gal on mobile equipment at 1–2×. The tank also has to cover cylinder differential volume and keep oil below 140°F at end of shift — those checks override the multiplier when they bite.

Does pump displacement change with pressure?

No. Displacement is geometry — set by the rotating group, not the load. A fixed-displacement pump moves the same in³/rev at 500 psi and 3,000 psi. Pressure changes the input power and torque, which is exactly what the calculator above scales; slip reduces effective output slightly in worn pumps, but nominal displacement stays put.

Review & sources

Reviewed: CalcPanel Fluid Power Team

Updated: 2026-11-10