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CalcTheJob

Well Pump Size Calculator: Planning Horsepower

Turn a planning flow and total dynamic head into water horsepower and shaft horsepower. This is a hydraulic planning check, not a well design and not a shopping guide.

GPM

Peak household or irrigation flow you are planning for. The 8 GPM default is an example, not a design load.

ft

Depth from the discharge to the pumping water level, not the well’s total drilled depth.

psi

Converted to feet at 2.31 ft per psi.

ft

Optional extra head in feet. Default 10 ft is a CalcTheJob planning starting point, not a pipe-friction calculation.

%

Default 50% is a CalcTheJob planning starting point, not a pump-curve efficiency.

Your estimate

Planning shaft horsepower

1.22HP

Water HP 0.61 ÷ 50% efficiency

Total dynamic head

302.4 ft

200 ft level + 40 psi × 2.31 + 10 ft friction

Water horsepower

0.61 HP

GPM × TDH ÷ 3,960

Water horsepower = GPM × TDH ÷ 3,960, the standard hydraulic identity used in extension pumping-plant guides (for example NC State Extension). Shaft horsepower divides by the efficiency you enter. This is a planning check, not a well design, a pump curve, or a shopping guide.

Estimates only — confirm with your supplier and local code. First published · Last reviewed by CalcTheJob.

How to use the well pump size calculator

  1. Enter the planning GPM and the pumping water level in feet (not the total drilled depth).
  2. Enter the discharge pressure in psi (converted at 2.31 ft per psi) and an optional friction allowance in feet. The default 10 ft of friction is a CalcTheJob starting point, not a pipe calculation.
  3. Enter overall efficiency. The default 50% is a planning starting point, not a pump-curve efficiency.

This page does not pick a brand or model.

Water horsepower and shaft horsepower

Total dynamic head (ft) = pumping water level + pressure (psi) × 2.31 + friction allowance. Water horsepower = GPM × TDH ÷ 3,960, the standard identity used in extension pumping-plant guides such as NC State Extension. Shaft horsepower = water HP ÷ efficiency.

A worked example: 8 GPM at 200 ft and 40 psi

With 10 ft of friction, TDH is 200 + 40 × 2.31 + 10 = 302.4 ft. Water HP is 8 × 302.4 ÷ 3,960 = 0.61 HP. At 50% overall efficiency, shaft HP is 1.22 HP. That is a planning figure, not a recommendation to buy a particular pump.

Sources & assumptions

How this calculator gets its numbers. Full method: our methodology.

  • Total dynamic head (ft) = pumping water level + pressure (psi) × 2.31 + friction allowance. Water horsepower = GPM × TDH ÷ 3,960. Shaft horsepower = water HP ÷ efficiency.
  • Default 50% overall efficiency and default 10 ft friction allowance are CalcTheJob planning starting points, not a pump curve or a pipe-friction calculation. The default 8 GPM is an example planning flow.
  • This is a hydraulic planning check, not a well design and not a shopping guide.

Sources

  1. NC State Extension, Pumping Plant Performance. States water horsepower = GPM × total dynamic head (feet) ÷ 3,960, the identity used here.

Last reviewed . A pump curve, a well log and a design professional win.

Frequently asked questions

How much horsepower for an 8 GPM well at 200 ft and 40 psi?

With a 10 ft friction allowance, TDH is 302.4 ft. Water horsepower is 0.61 HP. At the default 50% efficiency, shaft horsepower is 1.22 HP. This is a planning check, not a pump selection.

Is 50% efficiency a measured pump efficiency?

No. It is a CalcTheJob planning starting point. Enter the efficiency from a pump curve when you have one.

Do I enter the drilled depth of the well?

No. Enter the pumping water level, plus pressure head and any friction allowance.

Is this a shopping guide?

No. It does not recommend a brand, model or motor size class to buy.

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