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Sump Pit Size Calculator
Quick answer: Enter your sump basin's inside diameter, depth and the distance the water drops between pump-on and pump-off. The calculator returns gallons per inch, total gallons and gallons removed per cycle. Those numbers convert measured water rise into inflow and show whether a pump will short-cycle in your pit.
The formula
Volume of a cylinder is π × r² × height. Dividing cubic inches by 231 gives US gallons. An 18-inch pit holds about 1.10 gallons per inch; a 24-inch pit about 1.96. Results ignore the space the pump occupies and any taper, so real volume is slightly lower.
Why drawdown matters
Gallons per cycle divided by the pump's net flow tells you how long each run lasts. Very short runs — a few seconds — mean frequent starts and faster switch wear. A wider switch range or a larger basin lengthens cycles.
Worked example
A 24 × 30-inch basin with an 8-inch drawdown: about 1.96 gal/in, 59 gallons full and about 15.7 gallons per cycle. A pump moving 30 GPM net empties that in roughly 31 seconds.
Illustrative note (2026-10-29): Dimensions in this example were chosen on 2026-10-29 for illustration.
Limits
Measure the inside diameter at the water line. Planning estimate only; a licensed plumber can advise on basin replacement.