Use the free Sump Pump Size Calculator below: your data never leaves your browser — all work happens on your device, and nothing is uploaded to any server.
A basement that floods does so because water arrives faster than the pump can remove it. Sizing a sump pump is therefore a race against inflow: the pump must beat the fastest water the basement is likely to see, with room to spare. The Sump Pump Size Calculator estimates that race. You enter the water inflow in gallons per minute and the vertical lift, the height the pump must raise the water, and it gives the required pumping rate in gallons per hour and a recommended standard pump size.
Using the tool is quick, but estimating the inflow is the hard part, and honesty matters. If you have an existing pump, time how long it runs and rests during heavy rain, or measure how fast the sump pit fills with the pump off. If you have no pump yet, use the worst inflow you have seen, not the average, because pumps are bought for storms, not for dry weeks. Enter that inflow, then the vertical lift from the pit to the discharge point, measured straight up, because every foot of lift makes the pump work harder.
The maths adds a safety margin and a lift penalty. The tool multiplies the inflow by 60 to get gallons per hour, multiplies by 1.5 for a 50 percent safety margin, and multiplies again by a lift factor of 2 percent per foot of lift. Ten gallons per minute of inflow with an 8-foot lift therefore needs about 1,044 gallons per hour, and the tool rounds up to a standard 1,200-gallon-per-hour pump. The safety margin is not waste: it covers storms heavier than the one you measured, partial blockages, and the pump’s own ageing.
Lift deserves special attention, because it is the most underestimated number in sump sizing. A pump rated at 3,000 gallons per hour on the box usually achieves that at zero lift, with no height to climb. At 10 or 15 feet of lift, the same pump may deliver far less, because it spends its energy raising water instead of moving it. That is why the tool penalises lift explicitly. If your discharge point is high, or the pipe run is long with many bends, size up rather than down, and check the pump’s performance curve at your actual lift, not just the headline rating.
This tool is an educational estimator only, not professional plumbing advice. It estimates pumping rate from inflow and lift alone, and it cannot capture pipe friction, check valve losses, power reliability, or the drainage design of your basement. Results are planning estimates. Check your local plumbing codes, and consult a licensed plumber before buying or installing a sump pump. Never work in a flooded basement with live electrics, and never rely on a single pump without a backup plan if flooding would damage your home.
Backup planning is part of sizing, not an extra. A battery backup pump, a water-powered backup where codes allow, or at least an alarm that warns you when the water rises, turns a pump failure from a flood into a phone call. Test the pump regularly by pouring water into the pit, keep the pit and the discharge pipe clear of debris, and make sure the discharge sends water well away from the house, not in a loop back to the foundation. A pump that discharges next to the wall is just recycling the same water.
The Sump Pump Size Calculator is free, it runs in your browser, and nothing you enter leaves your device. Use it before you buy a pump, replace an old one, or check whether your current pump is big enough for the storms you actually get. Enter the worst inflow and the real lift, and take the recommended size to a professional for confirmation. A basement stays dry not because the pump is big, but because the pump beats the water, and this tool tells you how big that win needs to be.
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