What size sump pump do I need?

One that moves more water than enters your pit, at your lift. Measure inflow with this tool, then compare it with the gallons-per-hour figure on the pump curve at your lift height.

Is 1/3 HP or 1/2 HP enough?

Horsepower does not tell you flow at your lift. Two pumps with the same horsepower can move different amounts at 10 ft. Use the published curve.

What if I cannot measure during rain?

Measure in the wettest weather you can and add margin. A dry-week reading will undersize the pump.

Do I need a backup pump?

A backup covers power cuts and pump failure. Whether it is worth it depends on what is stored below grade and how often the pit runs.

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Sump pump size calculator

Measure how fast your pit fills. That, plus your lift, is what sizes a pump. Horsepower alone does not.

A sump pump size calculator should start with how much water actually enters your pit. Time the water rising a marked distance, enter the pit diameter, and this tool converts it to gallons per minute and per hour.

Then choose a pump whose published performance curve moves more than that at your total lift. Horsepower is not a flow rate, so it cannot size a pump on its own.

Sump pump size calculator

Measure how fast your pit fills. That, plus your lift, is what sizes a pump. Horsepower alone does not.

— Inflow

— Volume in that rise

— What to look for

— When to measure

What this assumed —

Measured inflow, not a recommendation of a model.

How to measure your inflow

Wait for steady inflow. The best reading comes during or right after heavy rain, when the pit refills on its own.

Mark, unplug, time. Put a mark on the pit wall, unplug the pump, and time how long the water takes to rise to a second mark a few inches up. Plug the pump back in straight away. Never leave it off in a storm.

Convert to gallons. A cylinder holds π × radius² × height cubic inches, and a US gallon is 231 cubic inches. An 18 inch pit rising 3 inches holds about 3.3 gallons.

Read the pump curve. Manufacturers publish gallons per hour at each foot of lift. Pick one that beats your inflow at your lift with room to spare. SDSU Extension suggests testing by filling the sump slowly at about the rate water would normally enter; see sump pump maintenance.

Questions this calculator answers

What size sump pump do I need?: One that moves more water than enters your pit, at your lift. Measure inflow with this tool, then compare it with the gallons-per-hour figure on the pump curve at your lift height.
Is 1/3 HP or 1/2 HP enough?: Horsepower does not tell you flow at your lift. Two pumps with the same horsepower can move different amounts at 10 ft. Use the published curve.
What if I cannot measure during rain?: Measure in the wettest weather you can and add margin. A dry-week reading will undersize the pump.
Do I need a backup pump?: A backup covers power cuts and pump failure. Whether it is worth it depends on what is stored below grade and how often the pit runs.

Sources and methodology

Figures dated 26 October 2026. Last reviewed August 16, 2026.

Flooding and Sump Pumps: testing a pump by filling the sump slowly (South Dakota State University Extension, retrieved 2026-10-26)

Sump pump maintenance The yearly checklist.
How long does a sump pump last Signs it is wearing out.
Basement waterproofing Where the pit fits in.
Water intrusion diagnostic Where the water comes from.

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Sump pump size calculator

How to measure your inflow

Questions this calculator answers

Sources and methodology

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