What Size Sump Pump Do I Need?
A sump pump that's too small will run constantly and can fail during a heavy storm; one that's too big short-cycles and burns out its switch. This free sump pump size calculator estimates your basement's worst-storm water inflow in gallons per hour (GPH) and recommends the right pump horsepower (HP) — instantly, right in your browser.
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Head height is fixed at a standard 10 ft (typical basement lift). See “How it works” below.
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How to use this calculator
Enter your basement's floor area in square feet — the footprint of the space the sump protects. Then enter the worst storm rainfall in inches you want your pump to handle: a 6-inch storm is a strong design value for most US homes, while 3–4 inches covers typical heavy storms.
The calculator instantly estimates how many gallons of water that storm could send into your pit and the gallons-per-hour (GPH) rate your pump needs to clear it. The result tells you the recommended pump size in horsepower, plus whether the next size down would be marginal, and a basin diameter hint.
How it works
Water inflow is estimated as basement area (sq ft) × rainfall (ft) × 7.48052 (gallons per cubic foot). A 1,000 sq ft basement with 6 inches of rain gives 1,000 × 0.5 × 7.48052 ≈ 3,740 gallons. This is a design storm: it assumes the entire storm's water must be pumped out in about one hour, so required GPH equals the inflow in gallons.
Typical pump capacities are rated at a standard 10 ft head (vertical lift from pit to discharge): 1/4 HP ≈ 1,800 GPH, 1/3 HP ≈ 2,400–2,700 GPH, 1/2 HP ≈ 3,000–3,600 GPH, 3/4 HP ≈ 5,000+ GPH. The recommendation picks the smallest pump whose working band covers your required GPH. All results are estimates — real capacity varies by model, discharge pipe size, and check valve placement, and this is not a substitute for a plumber's advice on your specific drainage setup.
Frequently asked questions
What size sump pump do I need for my basement?
For most single-family homes, a 1/3 HP pump (about 2,400–2,700 GPH at 10 ft of head) is enough. Basements over about 1,000 square feet, or areas with heavy storms, usually need a 1/2 HP pump (about 3,000–3,600 GPH). Enter your basement area and worst storm rainfall above for a tailored estimate.
How many GPH should my sump pump handle?
Multiply your basement floor area in square feet by your worst storm's rainfall in feet (inches ÷ 12), then multiply by 7.48. That gives the gallons of water the pump should move in one hour — your target GPH. For example, a 1,000 sq ft basement with 6 inches of rain needs about 3,740 GPH.
Is a 1/3 HP sump pump enough?
It is enough for needs up to about 2,700 GPH — roughly a 700 sq ft basement in a 6-inch storm. If your estimate is above that, step up to a 1/2 HP pump. Running a pump near its limit means it runs constantly during storms, which shortens its life.
What is head height, and why does this calculator assume 10 feet?
Head height is the vertical distance the pump must lift water from the pit to the discharge outlet — usually 7 to 10 feet in a basement. Pumps move less water as head height rises, so ratings are quoted at a standard 10 ft head. If your lift is taller than that, size up one step.
Should I just buy the biggest sump pump to be safe?
No. An oversized pump cycles on and off rapidly (short cycling), which wears out the float switch and motor. The right size empties the pit in steady runs. If your estimate is borderline, go one size up — not two.