Sump Pump Sizing Calculator
Determine the minimum sump pump capacity (GPM) needed to protect your basement given its area, local peak rainfall, soil drainage, and discharge lift height. Use it before purchasing a pump or when upgrading an undersized unit.
Last updated: September 2026
Formula below · 2 sources (epa.gov, Wikipedia) · Updated Sep 2026
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About this calculator
A sump pump must remove water as fast as it enters the pit during a peak storm. This calculator estimates that inflow from the basement footprint, the peak rainfall rate and a soil factor, then adds a small allowance for lift. One inch of rain on one square foot is 0.623 gallons, so basementArea × rainfallRate × 0.623 is the rainfall volume in gallons per hour falling on an area the size of the basement. The soil factor scales how much of that reaches the drain tile: 0.5 for well-drained sand or gravel, where water percolates away, up to 1.5 for poorly drained clay, which holds water against the foundation. Dividing by 60 converts gallons per hour to gallons per minute, and subtracting 0.5 per foot of lift from that 60 adds roughly 1% per foot as a margin for higher discharge heads: Required GPM = (basementArea × rainfallRate × soilType × 0.623) / max(1, 60 − liftHeight × 0.5). Choose a pump whose rated flow at your actual lift height (from the manufacturer's pump curve) meets or exceeds the result, ideally with a 1.5× safety margin. This is a planning estimate; actual inflow depends heavily on grading, gutters and groundwater.
How to use
Assume a 1,000 sq ft basement, a peak rainfall rate of 2 in/hr, average soil drainage (1.0), and a lift height of 8 ft. Step 1: numerator: 1,000 × 2 × 1.0 × 0.623 = 1,246 gallons per hour. Step 2: denominator: max(1, 60 − 8 × 0.5) = 56. Step 3: divide: 1,246 / 56 ≈ 22.25, rounded to 22 GPM. Select a pump whose rated output at 8 ft of head meets or exceeds 22 GPM (about 1,335 GPH), preferably with a 1.5× safety margin, about 33 GPM.
Frequently asked questions
How does soil drainage type affect sump pump sizing calculations?
Soil drainage is a multiplier in the formula. Poorly drained clay keeps water pressed against the foundation and around the drain tile, so it gets the highest factor (1.5); average soil is 1.0; and well-drained sand or gravel, where water percolates down and away, gets 0.5. Using too low a factor can leave the pump undersized during a heavy storm. If you are unsure of your soil type, choose the higher factor, or use a local soil survey or percolation test.
What vertical lift height should I enter for sump pump sizing?
Vertical lift height is the straight vertical distance from the water level in the sump pit to the point where the discharge pipe exits the house or empties into a drain — not the total pipe length. Most residential installations range from 6 to 15 ft. Every additional foot of lift reduces a centrifugal pump's flow rate, which is why the formula's denominator increases with lift height. If your discharge line also has significant horizontal run or multiple elbows, add equivalent feet of head (typically 1 ft per 10 ft of horizontal pipe) to your lift height before entering the value.
When should I upgrade my existing sump pump based on calculated capacity?
If your calculated required GPH exceeds 80% of your current pump's rated capacity at your actual lift height, you are operating with insufficient safety margin and should consider upgrading. Pump capacity also degrades over time — impellers wear and seals weaken — so a pump that was correctly sized five years ago may now deliver 15–20% less flow. Other upgrade triggers include any basement flooding event during a storm your pump should have handled, or a switch to a finished basement where water damage costs are much higher. Running this calculator after any significant landscaping or addition changes that alter effective drainage area is also good practice.