Sump Pump Sizing Calculator

About this calculator

A sump pump that’s too small can’t keep up during heavy rain and lets a basement flood; one that’s oversized cycles constantly and wears out prematurely. Correct sizing starts with measuring your actual water inflow rate — how fast the sump pit fills during a rain event — then adding a safety margin, typically 1.5 to 2 times the measured rate, to handle worse-than-observed storms.

This calculator converts your measured inflow and chosen safety factor into the pump capacity you need in gallons per hour (GPH), the unit sump pumps are rated in. It’s used by homeowners shopping for a replacement pump after a basement flooding scare and by plumbers and waterproofing contractors specifying the right unit for a property’s drainage conditions.