Size a sump pump from your actual inflow — measure it with the built-in rise-rate guide or estimate it from drainage area — then get the head, HP, and an estimated cycle rate with a short-cycling warning so you don’t oversize.
US + metric
Formula shown
Shareable results
Sump Pump Sizing
in
in
in/min
ft²
in/h
C
×
ft
ft
How to measure inflow
During heavy inflow, unplug the pump and time how many inches the water rises in one minute — enter that above. This measured rate beats any rule of thumb.
Pit volume is 1 US gallon per 231 in³. Head is a planning estimate (vertical lift + discharge friction); verify against the pump curve. The cycle estimate assumes the pump moves the required flow — a much bigger pump would short-cycle. Size to the inflow, not to the shelf.
For preliminary sizing and educational use. Verify against the pump performance curve and local codes; for critical basements consider a battery backup or second pump.
The first rule of sump pumps is to size for your actual inflow — and almost nobody knows theirs. Most sizing advice jumps straight to horsepower, but the number that matters is how fast water enters the pit. This calculator lets you measure that directly, adds a safety margin, works out the head, and then does the step other tools skip: it estimates how often the pump will cycle, because an oversized sump pump cycles itself to death.
How to measure your inflow (the two-minute test)
During or just after heavy rain, unplug the pump and let the water rise. Time how many inches it climbs in one minute, then enter that rise rate and your pit diameter. The tool converts inches-per-minute into gallons-per-minute from the pit’s area — for an 18-inch pit, each inch of rise is about 1.1 gallons. This measured number beats any rule of thumb, because inflow depends on your soil, water table, and foundation drains, not on a chart. If you cannot measure it, the drainage-area option estimates inflow from roof/lot area, rainfall intensity, and a soil runoff factor.
Worked example
An 18-inch pit rising 5 inches per minute is about 5.5 gpm of inflow. With a 1.5× safety factor that is roughly 8 gpm; lifting 10 ft through 1½-inch discharge pipe is about 10 ft of head — a job for a 1/3 hp pump. With an 8-inch float differential the pit holds about 8.8 gallons between on and off, giving a comfortable cycle time.
Do not oversize — short-cycling kills sump pumps
It is tempting to buy the biggest pump on the shelf “to be safe,” but a pump far larger than the inflow empties the pit in seconds, shuts off, and refills — over and over. Each start heats the motor, and rapid on/off cycling is one of the most common reasons sump pumps fail early. This calculator estimates the cycles per hour and the run time per cycle and warns when the run time gets too short. If it does, the fix is usually a wider float differential (a deeper on/off gap) or a bigger pit — not a bigger motor. Match the pump to the inflow, give it room to run for a reasonable stretch each cycle, and it will last.
Variables
Symbol
Meaning
US unit
SI unit
Q_in
Measured or estimated inflow rate
gpm
L/min
SF
Safety factor on inflow
-
-
TDH
Vertical lift + discharge friction
ft
m
diff
Float on/off level differential
in
mm
Cu00b7iu00b7A
Rational method: runoff u00d7 rainfall intensity u00d7 drainage area
