How to Size a Sump Pump: GPM, Head & Backup
Quick answer: time the water rise in your basin during the heaviest rain, convert to gallons per minute, add 50% headroom, and pick a pump whose capacity at your total head beats that number. Most homes settle on 1/3 HP at 1,500–2,000 GPH.
Measure the inflow
Everything starts with how fast water actually enters your basin. During a heavy storm (or pour a bucket in to test):
- Mark the water level and time 60 seconds.
- Measure the rise in inches.
- Basin area (sq in) = diameter² × 0.785. Inflow (GPM) = area × rise × 0.00433.
An 18-in basin rising 1 in/min pulls about 1.1 GPM (66 GPH). Rising 3 in/min pulls 3.3 GPM — three times the pump demand. That's why the rise number, not the basin size, decides the pump.
Total head & the curve
Pumps are rated on capacity curves: GPM falls as head rises. Total head = vertical lift (floor to discharge) + 1 ft per 10 ft of horizontal pipe + ~10% for fittings. A 1/3 HP pump doing 35 GPM at 5 ft of head does only ~25 GPM at 10 ft. Match the curve at your head — a pump that's plenty on paper can underdeliver on a long horizontal run.
HP by situation
| Situation | Pump | Typical GPM @ head |
|---|---|---|
| Small basin, low inflow | 1/4 HP | ~25 @ 5 ft |
| Most homes (the standard) | 1/3 HP | ~35 @ 5 ft |
| High water table, 30+ ft runs | 1/2 HP | ~50 @ 5 ft |
| Commercial-grade inflow | 1 HP | ~75 @ 5 ft |
Step up from 1/3 HP only when your measured inflow needs it — a bigger pump in a small basin short-cycles and burns out faster.
The backup decision
Storms that flood basements are exactly when power fails. If a wet basement is unacceptable:
- Battery backup: 5–7 hours of normal inflow; needs yearly battery replacement.
- Water-powered backup: runs on city water pressure, runs indefinitely, but uses water and needs flow.
- Generator: covers everything, but only if the generator starts — pair a portable with the pump circuit.
Even a basic backup beats a replaced carpet. If you own a finished basement, a backup pump isn't optional — it's insurance with a motor.
Sizing mistakes
- Buying by HP instead of the curve — capacity at your head is the real spec.
- Guessing the rise — measure it during the worst rain you get.
- No check valve — water backflows every cycle and the pump runs twice as often.
- Oversizing the basin pump — short-cycling burns motors.
- No backup, finished basement — the savings are a one-time event you'll regret.
- Never testing it — pour a bucket in monthly; find the failure in June, not January.
Size it once, dry forever
Measure your basin and rise, run the sump pump size calculator, match the curve at your head — and add the backup before the storm, not after the flood.