Sump Pump Sizing Calculator
Pit gallons and empty time from pit size and pump rate.
What it does
Sump pumps are rated in gallons per hour, but the pit they sit in holds only tens of gallons. Sizing means checking that the pump clears the pit faster than groundwater fills it, with margin left for storms and power flickers.
The tool above takes pit diameter, pit depth, and pump rate. It returns pit volume with the empty time at rated flow, so an undersized pump or an oversized pit shows up before the basement floor tells you.
The defaults describe a common 18 in diameter, 24 in deep pit with a 2,000 GPH pump: 26.4 gal, emptied in about 0.79 min at rated flow. The metric mirror (45.7 cm, 61 cm, 7,571 L per h) gives 100.1 L with the same 0.79 min.
Volume math follows NIST conversion factors with one US gallon equal to 231 cubic inches. Flow versus head sizing follows Hydraulic Institute practice for rotodynamic pumps, where rated flow always belongs to a stated lift.
How to use
- Pick imperial (gallons) or metric (liters).
- Enter pit diameter and depth.
- Enter pump rate from its curve at your lift.
- Read pit volume first.
- Read empty time at rated flow.
- Upsize the pump or add backup if inflow nears outflow.
Formula
- d
- Pit diameter in inches (or cm when metric).
- h
- Pit water depth in inches (or cm when metric).
- GPH
- Rated pump flow at the stated lift.
- 231
- Cubic inches per US gallon.
Pump ratings belong to a head value, and flow falls as vertical lift plus pipe friction rises. This page uses rated flow directly; total dynamic head, check valves, and inflow rate sit outside the line and belong in the limits below.
Worked example
Example: 18 in pit, 24 in deep, 2,000 GPH
Defaults: imperial, 18 in diameter, 24 in depth, 2,000 gal per h.
- Radius: 18 / 2 = 9 in.
- Area: pi x 81 = 254.5 sq in.
- Volume: 254.5 x 24 = 6,107.3 cu in.
- Gallons: 6,107.3 / 231 = 26.44 gal.
- Time: 26.44 / 2,000 x 60 = 0.79 min.
- Tool shows 26.4 gal, 0.79 min, 2,000 gal per h. Metric mirror gives 100.1 L, 0.79 min.
Reference table
| Pit | Volume | Note |
|---|---|---|
| 12 in x 24 in | 11.7 gal | Small |
| 18 in x 24 in | 26.4 gal | Common |
| 24 in x 24 in | 47 gal | Large |
| 18 in x 36 in | 39.7 gal | Deep |
Limits
- Rated flow belongs to a stated head; real lift cuts it.
- Inflow during storms can beat a fairly sized pump.
- Check valve and discharge freezing sit outside this math.
- No battery backup is counted here.
Common mistakes
- Sizing from the headline GPH at zero head.
- Ignoring vertical lift to the discharge.
- Skipping the check valve and re pumping the same water.
- Using a tiny pit that short cycles the motor.
- Never testing the pump before storm season.
Try it yourself
Quick checks. Answers sit tucked under each question.
24 in by 24 in pit holds what?
47 gal at full depth.
GPH to clear 26.44 gal in 5 min?
26.44 / (5/60) = 317 GPH at rated flow.
Why does real flow sit below rating?
Lift height plus pipe friction cut flow below the zero head rating.
Questions
How many gallons does my pit hold?
Enter diameter and depth above; an 18 by 24 in pit holds about 26.4 gal.
What GPH sump pump do I need?
One that clears measured inflow with margin at your real lift; this page checks the pit side of that.
Why does head matter?
Pumps push less water the higher they lift; curves show flow at each head.
Does a bigger pit help?
It adds buffer minutes but needs a pump that still cycles normally.
How often should it run?
Short rapid cycling wears motors; steady occasional runs are healthier.
Battery backup needed?
Storms kill power when pits fill fastest, so backup matters where flooding hurts.