How to Calculate Sump Size

Measure the width of your sump pit with a measuring tape.

Determine the number of inches of water that enter the sump pit during a heavy rain storm over a period of 60 seconds. For an 18-inch basin, one inch of water is equal to 1 gallon. For a 24-inch basin, 1 inch of water is about 2 gallons. This calculation will tell you the flow in gallons per minute (GPM).

Measure from the bottom of the sump pit to the maximum height of the discharge pipe. This is called the "static head."

Choose a pump that can handle the water flow GPM at the specific static head measurement for your house. Refer to the manufacturer's charts that show the pump's performance rate.