Sump Pump Capacity Calculator
Size a sump pump from how fast water rises in the pit — turns the inflow into GPM and GPH with a safety margin.
How it works
Inflow (GPM) = π ÷ 4 · pit² (in²) · rise (in/min) ÷ 231 · Recommended = inflow × safety factor
The honest way to size a sump pump is to measure the incoming water, not to guess. During a heavy rain, watch how many inches the water climbs per minute in the pit; the pit’s cross-section area turns that rise into a gallons-per-minute inflow. Rated pump capacity drops as discharge height rises, so a safety factor covers both peak storms and the head loss up your discharge pipe.
Worked example
An 18 in pit has an area of about 254 in². If water rises 1 inch per minute, that is 254 × 1 ÷ 231 ≈ 1.1 GPM, or about 66 GPH of inflow. With a 1.5× safety factor, choose a pump rated near 100 GPH at your actual discharge height.
Frequently asked questions
How do I measure the rise rate?
During heavy rain, unplug the pump briefly and time how far the water climbs — say 2 inches in 2 minutes is 1 in/min. Plug the pump back in promptly so the pit does not overflow.
Why does discharge height matter?
Pump capacity ratings fall as the water must be lifted higher (called head). A pump rated 3,000 GPH at 0 ft might deliver only 1,500 GPH at 10 ft of lift, so always check the pump’s curve at your real discharge height.
What safety factor should I use?
A 1.5× margin is common for typical basements; go higher (2×) if you have a high water table, a deep basement, or history of pump struggle. The margin also covers battery-backup pumps that pump more slowly.
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This calculator is for educational purposes. Double-check important results before acting on them.