PressureTankCalc Tip

Well pressure tank size calculator

A well pressure tank stores water under pressure so the pump does not start every time a tap opens. Its usable water, the drawdown, is only a fraction of the tank size, and that fraction depends on the pressure switch setting. PressureTankCalc works out the drawdown factor from the cut-in, cut-out and air precharge, then finds the total tank size that gives the pump at least a minimum run time per cycle.

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    A 10 GPM pump on a 40/60 switch needs about 39 gallons of total volume for a 1 minute run, so a 44 gallon tank.

    How the pressure tank calculator works

    PressureTankCalc treats the air in the tank as an ideal gas at constant temperature (Boyle’s law). With pressures converted to absolute by adding 14.7 psi, the share of the tank that is usable water between cut-out and cut-in is precharge × (1 ÷ cut-in − 1 ÷ cut-out). The precharge defaults to 2 psi below cut-in, the setting tank makers recommend. The drawdown you need is the pump flow × the minimum run time, 1 minute by default; dividing it by the drawdown factor gives the total tank volume, rounded up to a common size. Pump starts per hour peak when household use is half the pump flow: 60 × GPM ÷ (4 × drawdown).

    Drawdown by tank size and switch setting (gallons)

    Tank20/4030/5040/6050/70
    20 gal6.95.95.24.6
    32 gal119.48.37.3
    44 gal15.21311.310.1
    62 gal21.418.31614.2
    86 gal29.625.422.219.7
    119 gal4135.130.727.2

    Drawdown factor by switch setting

    SwitchPrechargeDrawdown factorTank per GPM (1 min run)
    20/40 psi18 psi0.3452.9 gal
    30/50 psi28 psi0.2953.4 gal
    40/60 psi38 psi0.2583.9 gal
    50/70 psi48 psi0.2294.4 gal

    Makers’ tables differ slightly. Follow the pump and tank instructions, and have a well contractor check a pump that short cycles. Results are estimates.

    Frequently asked questions

    What size pressure tank do I need?

    Multiply the pump GPM by the minimum run time (usually 1 minute) to get the drawdown, then divide by the drawdown factor for your switch setting: about 0.30 for 30/50 and 0.26 for 40/60.

    Is a bigger pressure tank better?

    Up to a point: a bigger tank means fewer pump starts and longer pump life, at a higher price and more space.

    How long should a well pump run each cycle?

    At least 1 minute so the motor cools after the start; many makers ask for 2 minutes on pumps above 1.5 HP.

    Is PressureTankCalc free?

    Yes, with no sign-up or ads.

    Are my numbers saved?

    Yes, in this browser only.

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