Sump Size Calculator
Time how fast water rises in your sump pit, then enter the pit diameter, the rise in a minute, the vertical lift and the discharge pipe length. The calculator gives the inflow, the pump to buy and the pit depth.
- Pump size
- Inflow
- Pit depth
Do the fill test in the wettest weather you can.
Inflow = pit area × rise ÷ 231. In an 18 in pit, water rising 6 in a minute is about 6.6 gpm. With a 1.5 × margin you need about 10 gpm at a 12 ft head, so a 1/3 hp pump is plenty.
Sump pump size chart
About 10 ft head. Search for a pump.
| Pump | Flow at 10 ft (gpm) | Flow at 15 ft (gpm) | Best for |
|---|---|---|---|
| 1/3 hp | 35 | 29 | Most homes |
| 1/2 hp | 50 | 41 | Wet basements |
| 3/4 hp | 65 | 54 | High water tables |
| 1 hp | 80 | 66 | Very wet, high lift |
| 1 1/2 hp | 120 | 99 | Large inflows |
gpm.
Rise and pump
18 in pit, 10 ft head. Slide the rise.
The water and pump grow with the inflow.
How to size a sump pump
Fill test to pump. Step through it.
Test
Time the rise.
Fit a check valve so water does not run back into the pit.
Pedestal, submersible or battery backup
Sump pumps. Compare them.
In the water.
Motor above.
Second pump.
Submersible pumps are quieter and suit most basements.
Before you buy
Sump checks. Tick each item.
Tick each item as you go.
Test the pump each spring by filling the pit with a bucket.
What you get
Sump sizing. Each tile explains one result.
Horsepower.
gpm.
Feet.
Depth.
Battery.
A pit that is too small makes the pump switch on and off too often.
Frequently Asked Questions
No question matches that search.
What size sump pump do I need?
One that pumps one and a half times your inflow at your total head.
How do I measure sump inflow?
Time how many inches the water rises in a minute in the pit.
Is a third horsepower sump pump enough?
For most homes, yes, up to about ten feet of lift.
How big should a sump pit be?
About eighteen to twenty four inches across and two to three feet deep.
What is total head?
The vertical lift plus about one foot for every ten feet of pipe.
Do I need a battery backup sump pump?
If a power cut could flood a finished basement, yes.
Can a sump pump be too big?
Yes. It short cycles and wears out faster in a small pit.
A dry basement
Size the pump to the water coming in and it keeps up even in the heaviest rain.
More size calculators
Each one has its own calculator, worked examples and answers to common questions.