Sump Pump Runtime in a Storm Calculator
Estimate how much water your sump pump moves in a storm, its total run time and duty cycle from rainfall and pump flow.
How it works
inflow = area × rainfall × 0.623 × fraction reaching sump · run time = inflow ÷ pump flow · duty cycle = (inflow ÷ duration) ÷ pump flow
Not all the rain that falls reaches your sump — much runs off or is absorbed — but the fraction that seeps to the footing drains is what the pump must move. Multiply the drainage area by the rainfall and the 0.623 gal per inch-per-sq-ft factor, then by the fraction that infiltrates, to get the storm’s total water. Dividing by the pump’s flow rate gives total run time, and comparing the average inflow rate to the pump’s capacity gives the duty cycle — how hard the pump is working. A duty cycle nearing 100% means the pit is barely keeping up.
Worked example
A 2-inch storm over 1,500 sq ft with 30% reaching the sump is 1,500 × 2 × 0.623 × 0.30 ≈ 561 gallons. A 2,000 gph pump clears that in 561 ÷ 2,000 ≈ 0.28 hr ≈ 17 minutes total. Over a 6-hour storm the average inflow is 561 ÷ 6 ≈ 94 gph, a duty cycle of just under 5% — plenty of margin.
Frequently asked questions
What duty cycle is too high?
Below about 30% is comfortable. Sustained duty above 70–80% means the pump runs almost continuously with little recovery, risking overheating and total failure if inflow spikes — that’s the signal to upsize the pump or add a second one.
Why do I need a battery backup if my pump has headroom?
Big storms and power outages travel together. A primary pump with plenty of capacity is useless with no electricity, so a battery-backup or water-powered secondary pump is what actually saves a basement during a storm-driven blackout.
The fraction reaching the sump seems like a guess — is it?
It varies with soil, grading, water table and how much of your yard drains toward the house, so 20–40% is a working range. If your pit fills faster than the estimate, raise the fraction; the run-time and duty figures scale with it directly.
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This calculator is for educational purposes. Double-check important results before acting on them.