Sump inflow = gallons per inch of pit × inches of water rise per minute. An 18 inch pit holds 1.1 gallons per inch, so water rising 2 inches in a minute is 2.2 gallons per minute (132 gallons an hour), and a 1/3 horsepower pump with 8 ft of lift handles it with room to spare.
How the sump pump size is calculated
Two facts size a sump pump: how fast water arrives, and how high the pump has to lift it. ePumps' guide describes the one-minute test for the first: unplug the pump during heavy rain, and time how far the water climbs in the pit in one minute.
gallons per inch = π × (pit diameter ÷ 2)² ÷ 231 cubic inches per gallon
inflow (GPM) = gallons per inch × inches of rise per minute; per hour = GPM × 60
The pump you buy must move more than that inflow at your lift, and lift is the height from the pump up to where the discharge pipe leaves the house.
Worked example: 18 inch pit, 2 inches a minute
| Step | Calculation | Result |
|---|---|---|
| Gallons per inch | π × 9² ÷ 231 | 1.10 gal/in |
| Inflow | 1.10 × 2 in per min | 2.2 GPM |
| Per hour | 2.2 × 60 | 132 GPH |
| Lift | 8 ft, to the discharge | needs a pump rated over 7.5 ft |
| Pump | first motor that lifts 8 ft | 1/3 hp |
Gallons per inch by pit size
| Pit diameter | Gallons per inch of depth |
|---|---|
| 18 in | 1.10 |
| 20 in | 1.36 |
| 24 in | 1.96 |
An 18 to 24 inch basin suits most homes, per ePumps: a wider pit gives longer pump cycles and less motor wear.
Sump pump size by horsepower
ePumps' horsepower table:
| Motor | Lift up to | Flow at 10 ft |
|---|---|---|
| 1/4 hp | about 7.5 ft | 20 to 30 GPM |
| 1/3 hp | about 10 ft | 30 to 40 GPM |
| 1/2 hp | about 15 ft | 40 to 60 GPM |
| 3/4 to 1 hp | 20 ft and up | 60 to 90 GPM |
The calculator picks the smallest motor whose lift covers yours and whose flow at 10 ft is above your inflow. Go up a size if you have a high water table, a discharge run longer than 10 ft or a finished basement, as ePumps advises. A pump's flow falls as lift rises, so a 1/3 hp pump moving 30 to 40 GPM at 10 ft moves less at 15 ft.
The next questions
- The other pump in your house: the well pump size calculator.
- Discharge line falling away from the house? The pipe slope calculator gives the drop.
- Patching the floor after the pit? Use the concrete calculator, and the cubic feet calculator for the gravel bed.
- See every heating, cooling and plumbing calculator.