Well Pump Size Calculator

What the calculator actually sizes

A well pump is checked at a duty point: the required flow at the required total dynamic head (TDH). A label such as “10 GPM” is incomplete unless the pump can still deliver 10 GPM against the system head. Use both results on the manufacturer’s performance curve.

The calculator uses:

TDH = pumping water level + elevation rise + pressure head + friction loss

For US units, pressure head (ft) = pressure (psi) × 2.3067. For metric units, pressure head (m) = pressure (bar) × 10.1972. Franklin Water describes TDH as the sum of pumping water level, above-ground pressure converted to head and column friction loss.

Pumping water level matters. It is the depth to water while the well is producing at the design flow. Static water level is measured when the well is at rest, and total well depth may extend far below the water surface. Substituting either can materially misstate the lift. A pump or step-drawdown test is the appropriate source for pumping level and sustainable yield; Penn State Extension explains that sustained pumping capacity requires a pump test.

Michigan 7-minute planning method

The optional bathroom estimate reproduces the residential table in the State of Michigan Water Well Manual. It includes kitchen-sink, washing-machine and dishwasher use, but not simultaneous irrigation, farm or fire demand.

This is a Michigan planning method, not a universal code rule. Select custom demand when local requirements, measured fixture flow or a professional design gives a different value.

Interpreting a yield shortfall

The sustainable-yield margin is well yield − peak flow. If it is negative, the displayed short-peak deficit is:

theoretical deficit = max(0, peak volume − sustainable yield × peak duration)

Table mode uses the Michigan method’s seven-minute duration; custom mode uses the duration you enter.

For example, a 98-gallon table demand and an 8 GPM sustainable yield produce a theoretical 42-gallon gap during seven minutes. That number is a screening estimate, not a pressure-tank drawdown or recommended storage-tank capacity. It assumes the stated well yield continues throughout the peak and ignores usable borehole water, pump cycling and reserve. Penn State’s low-yield well guidance explains why a pressure tank provides limited usable storage and why an intermediate storage system may need a separate design.

Before choosing equipment

• Confirm pumping level and sustainable yield from current test data, especially after drought or changes in well performance.
• Estimate friction at the intended flow using the actual pipe length, diameter, material, valves and fittings.
• Check that the selected pump curve passes through the flow/TDH duty point with an appropriate operating range.
• Have a licensed or otherwise locally qualified well professional verify pump setting, electrical supply, cycling protection, pressure-tank drawdown and applicable code.