SumpPumpSize Tip

Sump pit volume calculator

Knowing the gallons per inch of your sump basin lets you turn any rise into a flow and tells you how much water each pump cycle removes. SumpPumpSize uses the cylinder volume, π × radius² × depth, divided by 231 cubic inches per gallon. A standard 18 by 22 inch basin holds about 1.1 gallons per inch, or 24 gallons full; a 24 inch basin holds about 2 gallons per inch.

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.

    A bigger pit means fewer, longer pump cycles, which is easier on the switch and motor.

    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 gallons is an 18 inch sump pit?

    About 1.1 gallons per inch, so an 18 × 22 in basin holds about 24 gallons, and 10 inches of switch travel removes about 11 gallons per cycle.

    How many gallons is a 24 inch sump pit?

    About 1.96 gallons per inch, or 59 gallons for a 30 in deep basin.

    How often should a sump pump cycle?

    Ideally not more than about once a minute at peak inflow. Longer runs are easier on the motor; raise the float travel or use a larger pit if it cycles faster.

    Is SumpPumpSize free?

    Yes, with no sign-up or ads.

    Are my numbers saved?

    Yes, in this browser only.

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