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Sump Pump Capacity Calculator

Calculate the required sump pump GPM capacity based on water inflow rate, sump pit volume, and target pump runtime percentage.

Maximum groundwater/drainage inflow rate during peak storm events.
Volume of the sump pit. A typical 18" dia � 24" deep pit � 22 gallons.
Percentage of time the pump runs. 50% = runs half the time, 50% off.
Vertical height water must be pumped to the discharge point.

Results

Required Pump Capacity30.0 GPM
Estimated Pump HP0.076 HP
Cycles per Hour45.0 cycles/hr
Pit Fill Time80 seconds

📖What is it?

A sump pump must evacuate water faster than it enters during peak storm events. This calculator determines the required pump capacity (GPM) based on the inflow rate, pit volume (which acts as a buffer), and your target duty cycle (runtime percentage). Running a pump more than 80% duty cycle continuously accelerates wear.

🎯How to use

Estimate peak inflow by timing how fast the pit fills during a heavy rain, then convert to GPM. Enter the pit volume, your desired runtime percentage (50% is a common target for longevity), and the vertical discharge head. The result tells you the minimum pump GPM rating you need at that head.

💡Example scenario

A basement pit fills at 15 GPM during a heavy storm. The pit holds 20 gallons. At 50% duty cycle, the pump needs 30 GPM capacity. Cycles/hr = 15�60/20 = 45 � well within the 5�10 cycles/hr maximum for pump longevity. Select a 1/2 HP pump rated = 30 GPM at 10 ft head.

🏆Pro tip

Limit pump cycles to 5�10 per hour to prevent motor overheating and premature seal failure. If calculated cycles/hr exceed 10, increase pit volume (deeper or wider pit). For critical applications (finished basements), install a battery backup pump and a water alarm. A 1/3 HP pump handles most residential needs; upgrade to 1/2 or 3/4 HP in high-water-table areas.