Size your well pump, pressure tank and septic tank
Three calculators for homes on a private well and septic system. The pump flow comes from a manufacturer's peak-demand table, the horsepower from that maker's published selection chart, the pressure tank from Boyle's law and the motor maker's one-minute run-time rule, and the septic tank from your own state's rule. Every figure links to where it came from.
1. Well pump: flow and horsepower
2. Pressure tank size
3. Septic tank minimum by state
49 states with a statewide figure read from the state's own rule on 2026-10-20. Not yet listed: Wisconsin.
How the numbers are worked out
Pump flow comes from Goulds Water Technology's peak-demand table for private homes: a seven-minute morning or evening peak of 45 gallons in a one-bathroom house rising to 122 gallons with three or four bathrooms, which sets a minimum pump of 7 to 17 gallons per minute. Penn State Extension's table by bedrooms and bathrooms is shown alongside for comparison.
Total dynamic head is the lift from the pumping water level, plus the rise to the tank, plus the switch's cut-out pressure converted to feet (H = 2.31 x psi), plus friction in the pipe from Goulds' Schedule 40 plastic pipe table. The pump is the smallest Goulds 10GS model whose published rating at your depth meets the flow at the cut-in pressure and is still rated at the cut-out pressure. The chart is read at the next listed depth and pressure at or above yours, so the answer errs on the safe side.
Pressure tank size follows from two rules: the motor should run at least 1 minute per start (Franklin Electric: "Run motors a minimum of one minute to dissipate heat build up from starting current."), so the tank must hold a minute of pump flow between cut-out and cut-in; and the precharge is set 2 psi below cut-in (AMTROL). Boyle's law gives the share of the tank that is usable water between the two pressures; it reproduces Goulds' published pressure-factor table. The methodology shows every formula and source.