SumpPumpSize Tip

What size sump pump do I need?

Sump pumps are rated in gallons per hour at a given head, for example 2,400 GPH at 10 feet. To choose one, you need two numbers: how much water comes in at the worst time, and how high and far the pump pushes it. SumpPumpSize gives both, with a safety margin, and lists the GPH you need at your head so you can compare pump curves from any brand.

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.

    If you cannot do a fill test, a common starting point is 1/3 HP for an average basement and 1/2 HP for a high water table or a deep basement.

    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 many GPH should my sump pump be?

    Inflow GPM × 60 × 1.5 for a margin. A 13 GPM inflow needs about 1,190 GPH at your total head.

    What size discharge pipe for a sump pump?

    Usually 1½ in for residential pumps, 1¼ in for small ones and 2 in for high-flow pumps. Keep velocity between about 2 and 8 ft/s.

    Should I get a backup sump pump?

    If your basement is finished or floods often, a battery or water-powered backup is worth it, because power cuts often happen in storms.

    Is SumpPumpSize free?

    Yes, with no sign-up or ads.

    Are my numbers saved?

    Yes, in this browser only.

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