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Water Pump Size Calculator

Find the pump horsepower and watts needed to fill a water tank in the time you want, from the height the water is lifted and the pump's efficiency.

Rules last checked 10 November 2026Every step shown

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How much water to pump in one go.

minutes

Include the depth of the water below the pump, for a well or underground tank.

Long or narrow pipes, bends and valves add resistance. A few metres (or about 10 ft) covers a typical home run.

From the pump's data sheet if you have it. Small pumps are less efficient than large ones.

Pump size

0.33 hp

Pump power needed
245 watts
Flow needed
50 L/min
Total head
15 m

To fill 1,000 litres in 20 minutes against 15 m of head, you need about 0.33 hp (245 W). Pick the next standard size up.

How it's worked out

  1. Flow1,000 L ÷ (20 min × 60)0.833 L/s
  2. Total head12 + 3 m15 m
  3. Power1,000 kg/m³ × 9.81 × 0.00083 m³/s × 15 m ÷ 0.5245 W
  4. Horsepower245 W ÷ 745.70.33 hp
  • Check the pump's curve: it must deliver the flow at your total head, not just at zero head.
  • The watts are the power going into the pump. The motor draws more from the mains, by its own efficiency, unless the efficiency you entered is for the whole pump set (wire to water).
  • Suction pumps can only lift water a limited height from below them; for deep wells use a submersible pump.

How it is worked out

Flow = tank volume ÷ fill time. Power = water density × gravity × flow × head ÷ efficiency, which gives watts; dividing by 745.7 gives horsepower. Choose the next standard pump size above the result.

Questions people ask

What pump do I need to fill a 1,000 litre tank 12 m up in 20 minutes?

About 0.33 hp (245 W) at 50% efficiency, allowing 3 m extra for pipe friction. A 0.5 hp pump would do it comfortably.

What is head?

The height the pump has to lift the water, from the water level to the outlet, plus an allowance for friction in the pipes.

What efficiency should I use?

The figure on the pump's data sheet, if you have it. Small pumps are less efficient than large ones, so 50% is a cautious starting point.

Why check the pump curve?

A pump gives less flow the higher it lifts. Make sure it delivers your flow at your total head, not just at its best.

Formula and rules last checked 10 November 2026Written and tested by the CalculatorDex teamReport a problem