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Size to the pit, not the box.
Most sump pumps are bought by horsepower and the GPH printed on the box — a number measured at zero lift that no basement ever sees. The plumber's method is two real numbers: your pit's measured inflow rate (the 60-second rise test) times a 1.5× safety margin, checked against your total dynamic head — vertical lift plus pipe friction, elbows, and the check valve. This calculator runs both and matches you to the pump class whose capacity at your head actually covers the inflow.
Homeowners replacing a dead pump use this to stop guessing at horsepower in the aisle — the 60-second rise test turns their actual pit into a GPH number, and the TDH math explains why the box rating is fiction. Waterproofing contractors use it to show customers the sizing logic, because "1/3 HP is fine" lands better with the inflow math next to it.
A 1/3 HP pump delivering roughly 2,400–3,200 GPH at a 10-foot lift covers the average basement. Step to 1/2 HP (3,500–4,500 GPH at 10 ft) for a high water table, a finished basement, or a deep pit with long discharge. The deciding number is never horsepower alone — it is the pump's capacity at your total dynamic head, read from its curve.
On a rainy day, run the pump down to shutoff, unplug it, and measure how many inches the water rises in exactly 60 seconds. An 18-inch pit holds 1.10 gallons per inch, a 24-inch pit 1.96 — so 1 inch per minute in an 18-inch pit is 66 GPH of inflow. Multiply by 1.5 for the sizing target, and never let the water rise past the inlet pipe during the test.
Oversized pumps short-cycle: they empty the pit in seconds, then restart over and over, which is what actually kills sump motors and float switches. A pump that clears peak inflow with a 1.5× margin and runs a reasonable cycle outlasts a giant that cycles every ten seconds. Capacity and cycle time are separate checks — this calculator handles the first, pit size handles the second.