With a 50% safety margin that needs about 1,190 gallons per hour at 11.3 feet of total head: a 1/4 HP pump is enough on paper, though many homes choose 1/3 HP for heavier storms.
How the sump pump size calculator works
Inflow comes from a fill test: with the pump off, the water rises a measured height in a measured time. A round pit holds π × (diameter ÷ 2)² ÷ 231 gallons per inch, so the inflow in gallons per minute is gallons per inch × inches ÷ minutes. SumpPumpSize adds your safety margin to get the design flow. The head is the vertical lift from the water level in the pit to the highest point of the discharge line, plus friction: the pipe length plus the equivalent length of each 90° elbow and the check valve, times the Hazen-Williams friction loss for schedule 40 PVC (C = 150) at the design flow. Water horsepower is GPM × head ÷ 3,960, and the motor estimate assumes about 30% wire-to-water efficiency for a small submersible pump.
Gallons per inch of sump pit
| Pit diameter | Gallons per inch | Gallons in 10 in |
|---|---|---|
| 14 in | 0.67 | 6.7 |
| 16 in | 0.87 | 8.7 |
| 18 in | 1.1 | 11 |
| 20 in | 1.36 | 13.6 |
| 22 in | 1.65 | 16.5 |
| 24 in | 1.96 | 19.6 |
| 30 in | 3.06 | 30.6 |
| 36 in | 4.41 | 44.1 |
Head at 10 ft of lift, 20 ft of 1½ in pipe, 3 elbows and a check valve
| Inflow | Design flow (+50%) | Total head | Suggested motor |
|---|---|---|---|
| 5 GPM | 450 GPH | 10.2 ft | 1/4 HP |
| 10 GPM | 900 GPH | 10.8 ft | 1/4 HP |
| 15 GPM | 1,350 GPH | 11.6 ft | 1/4 HP |
| 20 GPM | 1,800 GPH | 12.8 ft | 1/3 HP |
| 30 GPM | 2,700 GPH | 15.9 ft | 3/4 HP |
| 40 GPM | 3,600 GPH | 20.1 ft | 1½ HP |
Pump curves differ between brands and models, so always check the flow at your head on the maker's curve. Discharge must go where local rules allow, away from the foundation and not into a sanitary sewer. Results are estimates for planning.
Frequently asked questions
What size sump pump do I need?
One that pumps your peak inflow plus a margin at your total dynamic head. Most homes need 1/3 HP (about 2,000–3,000 GPH at 10 ft); high water tables or lifts over 15 ft need 1/2 HP or more.
Is a bigger sump pump better?
Not always. An oversized pump empties the pit in seconds and short-cycles, which wears the switch and motor. Size to inflow with a reasonable margin.
What is total dynamic head?
The vertical lift plus the friction loss in the pipe, fittings and check valve, in feet of water. Pump curves show flow at each head.
Is SumpPumpSize free?
Yes, with no sign-up or ads.
Are my numbers saved?
Yes, in this browser only.