Sump Pump Size Calculator
| Pump class | GPH at 5 ft lift | GPH at 10 ft | GPH at 15 ft |
|---|---|---|---|
| 1/4 HP | 1,700 | 1,150 | 700 |
| 1/3 HP | 2,900 | 2,500 | 1,900 |
| 1/2 HP | 3,800 | 3,300 | 2,600 |
| 3/4 HP | 5,000 | 4,400 | 3,500 |
| 1 HP | 6,000 | 5,300 | 4,500 |
Unplug the pump, time how long the pit takes to fill between the float marks, and enter the lift to your discharge line. The calculator turns that one measurement into the gallons per hour a pump must move at your height, then names the horsepower class.
The trap this page exists for is buying on box horsepower: a 1/2 hp pump rated 3,800 gph at 5 feet delivers barely 2,600 by 15 feet, and a pit collecting 20 gpm floods with either if the lift margin is wrong. Flow against actual lift is the only rating that matters.
How to use
- With the pump unplugged, time the water climbing from the off level to the float-switch level; that seconds figure is your inflow clock. A standard 18-inch pit moves about 8 gallons over a typical draw.
- Measure vertical lift from the pit bottom to where the discharge pipe exits, then add a foot per elbow as friction.
- Enter battery amp-hours if you run backup - runtime assumes about 8 running amps at 80 percent usable depth. The verdict updates live.
Frequently asked questions
How do I know what GPM my sump pit collects?
Time the fill between float marks with the pump unplugged: gallons between the marks, divided by seconds, times 60. Eight gallons refilling in sixty seconds is 8 gpm; the same pit filling in twenty seconds after a storm is 24 gpm and needs the next class up.
Is a 1/3 hp sump pump enough?
For most houses, yes - NDSU Extension calls a 1/3 hp pump satisfactory for typical homes. It moves roughly 2,500 gph at a 10-foot lift, which covers a pit seeping under 25 gpm with margin. Size up only when the fill test or the lift says so.
What does lift mean on a sump pump?
Vertical feet from water line to discharge point. Pump charts show flow falling as lift rises, and elbows plus long horizontal runs act like extra feet: a pump sold as 3,800 gph can be under 2,600 by the time it lifts 15. Read the flow-versus-lift chart, not the headline number.
How long will a battery backup run my sump pump?
Amp-hours times 0.8 usable depth, divided by about 8 running amps: a 75 Ah marine battery carries a typical 1/3 to 1/2 hp pump roughly seven hours. Cycling matters more than the headline - the pump only draws while the float is up.
How often should a sump pump cycle?
A right-sized pit cycles a handful of times per hour in wet weather and rarely in dry. Rapid on-off clicking means the pump is oversized for the pit or the float range is short - either way the switch wears out long before the motor does.