What size sump pump do you need?
Do the one-minute pit test, enter your sump pit diameter and how far the water rises in a minute, and get the pumping capacity — gallons or litres per hour — your pump must deliver at your discharge height. Sized to real inflow, not a horsepower guess.
Choose a pump that delivers at least this at your 3 mof lift — read the flow from the column of the pump's performance chart that matches your total head, not the headline “max” figure (which is measured at zero lift).
capacity = (rise per minute × π/4 × diameter²) × 60 × safety factor. Exact geometry; the safety factor is a rule of thumb, not a code value. The lift decides which pump meets that flow, not the flow itself.
Want the full method, the pit-capacity table and worked examples? Read: what size sump pump do I need →
Common questions
- What size sump pump do I need?
- Size it to the water coming in, not to a horsepower guess. During heavy rain, let the pit fill, then measure how many inches the water rises in one minute. Multiply that rise by the pit's gallons-per-inch (about 1.1 gallons per inch for a standard 18-inch pit) to get the inflow in gallons per minute, multiply by 60 for gallons per hour, then add a safety margin — commonly 1.5×. That gallons-per-hour figure, measured at your discharge height, is the capacity your pump must beat. Enter your own pit test above for the exact number.
- How do I do the sump pump pit test?
- During or right after a heavy rain, unplug the pump and let the water rise to the level where the float switches it on. Plug it back in only to keep the pit from overflowing, then, with the pump off, time how far the water climbs in exactly one minute and measure that rise in inches (or mm). That rise per minute is the number the calculator turns into an inflow rate. Doing it in the worst weather you get is the point — you are sizing for the storm, not a dry day.
- Is a 1/3 HP or 1/2 HP sump pump better?
- Neither is automatically better — horsepower is not a capacity. What matters is gallons per hour delivered at your actual lift. A 1/3 HP pump handles most ordinary homes; step up to 1/2 HP (or more) when your inflow is high, the water table is deep, the vertical lift is tall, or the discharge run is long, all of which cut a pump's real output. Match the pump's published flow at your head to the required capacity this calculator gives, and the horsepower will follow.
- Why does the pump's horsepower rating overstate what I get?
- Manufacturers headline the maximum flow, which is measured at zero lift — with the water going nowhere but straight out. Every foot the pump has to push water up (plus friction in the pipe and each elbow) cuts that flow, so a pump rated at, say, 3,000 gallons per hour at 0 feet may deliver far less at 10 feet. Always read the flow from the row of the performance chart that matches your total head. The pump head calculator helps you work that head out.
- How many gallons per hour should a sump pump handle?
- There is no single right number — it depends entirely on how fast water enters your pit, which is what the pit test measures. A modest inflow might need only a few hundred gallons per hour; a high water table in a downpour can demand a few thousand. Size to your measured inflow times a safety factor, then pick the next pump up whose chart meets that flow at your lift.
Reference & education only. Not professional, engineering, or code-compliance advice. Estimates are based on published model codes; local amendments and your Authority Having Jurisdiction (AHJ) govern. Always verify against the current adopted code and a licensed professional before doing work.
Last reviewed 2026-07.