The result is the smallest standard size that should work. Confirm it on the pump curve, which gives the real gallons per hour at your head.
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
Is a 1/3 HP sump pump enough?
For most basements, yes: a 1/3 HP pump moves roughly 2,000–3,000 GPH at 10 ft of head, more than a typical inflow.
When do I need a 1/2 HP sump pump?
With high inflow (above about 35 GPM), a lift over 15 ft or a long discharge line, check whether a 1/3 HP pump still delivers enough at your head.
What is water horsepower?
The power the pump transfers to the water: flow in GPM × head in feet ÷ 3,960. The motor must be larger because of losses.
Is SumpPumpSize free?
Yes, with no sign-up or ads.
Are my numbers saved?
Yes, in this browser only.