Sump Pump Size Calculator

Sump Pump Size formula

Inflow (gpm) = 3.1416 x (Pit diameter / 2)² x Rise per minute (in) / 231 x (1 + safety factor)
Total dynamic head (ft) = Vertical lift + Horizontal run x friction (0.1 for 1.5 in pipe) + 2 for the check valve and fittings
Pump = smallest pump whose curve delivers the inflow at that head (1/3 HP about 35 gpm at 10 ft, 1/2 HP about 50, 3/4 HP about 65, 1 HP about 80)
Pit capacity (gal) = 3.1416 x (Pit diameter / 2)² x 6 in of float travel / 231

Worked example

A 24 in pit in a high water table rises 8 in per minute in a storm, with a 12 ft lift and a 40 ft run of 1.5 in pipe. Inflow = 3.1416 x 12² x 8 / 231 = 15.7 gpm measured, times 1.5 for the safety factor = 23.5 gpm. Head = 12 + 40 x 0.1 + 2 = 18 ft.

At 18 ft a 1/3 HP pump delivers about 35 - 8 x 2 = 19 gpm, which is short, so step up to a 1/2 HP pump at about 50 - 8 x 2 = 34 gpm. The pit holds 3.1416 x 12² x 6 / 231 = 11.8 gal over 6 in of float travel, so the pump starts about every 0.5 minutes in that storm, which argues for a larger pit or a second pump.

Quick reference

Measuring inflow and head

The only inflow number worth having is one you measured in the pit during a hard rain. Unplug the pump, mark the water level, wait one minute, measure the rise, and plug the pump back in. The pit diameter turns inches of rise into gallons: an 18 in basin holds 1.1 gal per inch. Multiply by a safety factor because the next storm will be worse than the one you measured.

Head is what the pump works against. Measure the vertical distance from the bottom of the pit to the highest point in the discharge line, then add an allowance for friction in the horizontal run and for the check valve and elbows. A long run, undersized pipe or a discharge that climbs to a high window all add head and cut the flow the pump can deliver.

Choosing the pump and a backup

Read the flow at your head off the manufacturer's curve, not the headline gpm on the box, which is measured at zero head. A 1/3 HP pump handles most homes with a normal water table and a 10 ft lift; step up to 1/2 HP for a high water table, a lift above about 12 ft or a long discharge run, and to 3/4 HP or 1 HP only when the measured inflow demands it. Oversizing is not free: a pump that empties the pit in seconds short-cycles, which wears out the switch and motor.

Heavy rain and power outages arrive together, so a battery backup or a water-powered backup pump sized for the same inflow is cheap insurance against a flooded basement. Add a high-water alarm, keep the check valve in good order and clear the pit of gravel and debris once a year. This is a planning estimate; a licensed plumber and local code decide where the discharge may go and whether a pit or pump upgrade is needed.

Key facts

• A gallon is 231 cubic inches, so an 18 in pit holds about 1.1 gal per inch of depth and a 24 in pit about 2 gal per inch.
• A typical 1/3 HP sump pump moves about 35 gpm at 10 ft of head and a 1/2 HP about 50 gpm; 1/3 HP covers most homes, and 1/2 HP is for high water tables or lifts over 10 to 15 ft.
• Every foot of vertical lift is a foot of head, and a check valve plus elbows adds the equivalent of a couple more feet; pumps lose flow steadily as head rises.
• Sump pumps use 1.5 in discharge pipe; reducing to 1.25 in roughly doubles the friction loss and cuts the flow.
• Discharge water should exit at least 10 ft from the foundation, and most codes prohibit connecting a sump pump to the sanitary sewer.

Frequently asked questions

Sources and references

• •Model 53 Sump Pump product page (3/10 HP, 43 gpm max, 19.25 ft max head) (Zoeller Pump Company)
• •Technical Data Sheet FM2778, Mighty-Mate Models 53, 55, 57 and 59, with pump performance curve (Zoeller Pump Company)
• •CDU980E 3/4 HP and CDU1000 1 HP stainless steel sump pump specification sheet (gpm at 5, 10, 15 and 20 ft of head) (WAYNE Pumps)
• •Chapter 33 Storm Drainage, Section P3303 Sumps and Pumping Systems, 2021 IRC (ICC)