SumpPumpSize Tip

Sump pump GPM calculator

The best way to size a sump pump is to measure the water coming in during a wet spell. Unplug the pump, mark the water level, and time how long it takes to rise a few inches. SumpPumpSize converts that into gallons: a round pit holds π × radius² × height cubic inches, and 231 cubic inches make a gallon. An 18 inch pit holds 1.1 gallons per inch, so 6 inches in 30 seconds is 13.2 GPM.

Sump pump sizing

1. Inflow test: let the pit fill with the pump off and time how fast the water rises.

2. Discharge line: from the pump to where the water leaves the pipe.

    Saved you some time? Leave a tip, entirely optional.

    Do the test during the heaviest rain or snowmelt you can, and plug the pump back in before the pit is full.

    How the sump pump size calculator works

    Inflow comes from a fill test: with the pump off, the water rises a measured height in a measured time. A round pit holds π × (diameter ÷ 2)² ÷ 231 gallons per inch, so the inflow in gallons per minute is gallons per inch × inches ÷ minutes. SumpPumpSize adds your safety margin to get the design flow. The head is the vertical lift from the water level in the pit to the highest point of the discharge line, plus friction: the pipe length plus the equivalent length of each 90° elbow and the check valve, times the Hazen-Williams friction loss for schedule 40 PVC (C = 150) at the design flow. Water horsepower is GPM × head ÷ 3,960, and the motor estimate assumes about 30% wire-to-water efficiency for a small submersible pump.

    Gallons per inch of sump pit

    Pit diameterGallons per inchGallons in 10 in
    14 in0.676.7
    16 in0.878.7
    18 in1.111
    20 in1.3613.6
    22 in1.6516.5
    24 in1.9619.6
    30 in3.0630.6
    36 in4.4144.1

    Head at 10 ft of lift, 20 ft of 1½ in pipe, 3 elbows and a check valve

    InflowDesign flow (+50%)Total headSuggested motor
    5 GPM450 GPH10.2 ft1/4 HP
    10 GPM900 GPH10.8 ft1/4 HP
    15 GPM1,350 GPH11.6 ft1/4 HP
    20 GPM1,800 GPH12.8 ft1/3 HP
    30 GPM2,700 GPH15.9 ft3/4 HP
    40 GPM3,600 GPH20.1 ft1½ HP

    Pump curves differ between brands and models, so always check the flow at your head on the maker's curve. Discharge must go where local rules allow, away from the foundation and not into a sanitary sewer. Results are estimates for planning.

    Frequently asked questions

    How do I calculate sump pump GPM?

    Gallons per inch of pit × inches of rise ÷ minutes. An 18 in pit holds 1.1 gal per inch; a 24 in pit 1.96 gal per inch.

    How many GPM does a typical sump pit get?

    From under 5 GPM in dry areas to 20–40 GPM with a high water table during storms. That is why a fill test beats guessing.

    What safety margin should I add?

    About 50% for a test done in ordinary rain, less if you measured during a severe storm.

    Is SumpPumpSize free?

    Yes, with no sign-up or ads.

    Are my numbers saved?

    Yes, in this browser only.

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