Sump Pump Sizing Calculator
Timing the Pit, Not Guessing the Pump
A sump pump has to keep pace with how fast groundwater actually enters the pit, and that inflow rate varies a lot by property, soil, and season — a spec sheet can't know it in advance. The most reliable way to size a pump is a basin fill-rate test: plug the pump's outlet path, let water rise in the pit, and time it. This calculator turns that field measurement directly into an inflow rate and a recommended pump horsepower tier.
The Formula
Volume per Rise = Pit Area × Rise Height
Inflow Rate = Volume per Rise / Rise Time
Once the inflow rate in GPM is known, this calculator maps it to a recommended pump tier: 1/3 HP for up to 30 GPM, 1/2 HP for 30–50 GPM, 3/4 HP for 50–80 GPM, and 1 HP or larger above 80 GPM.
Where This Calculation Matters
- Replacing a failed pump — confirming the replacement matches actual inflow rather than just duplicating a possibly undersized original.
- Basement waterproofing projects — sizing a pump appropriately for a property's specific groundwater conditions rather than a generic recommendation.
- High water table properties — homes with persistently active sumps benefit from sizing based on measured worst-case inflow, not average conditions.
- Backup pump selection — a battery backup pump should be rated to handle the same measured inflow as the primary during a power outage.
Worked Example
An 18 in diameter pit where water rises 6 in in 1 minute after the pump is disabled:
| Quantity | Value |
|---|---|
| Pit area | 1.77 ft² |
| Volume per rise | 6.61 gal |
| Inflow rate | 6.61 GPM |
| Recommended pump | 1/3 HP |
Run the test during the wettest conditions you reasonably expect (heavy rain, spring thaw) — sizing off a dry-season measurement risks an undersized pump when it matters most.
How to Use This Calculator
- Enter the sump pit diameter in inches.
- Let water rise after disabling or removing the pump, then enter the observed rise height in inches.
- Enter the time that rise took, in minutes.
- Select Calculate to get the inflow rate and recommended pump size.
Related Calculations
To check the pressure the pump needs to overcome to lift water to a discharge point, see the Water Pressure Calculator. For sizing the discharge pipe itself, use the Pipe Size Calculator.