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Sump pump sizing: how much water flows in, and what pump do you need?

Measure how fast water rises in the sump pit to find the inflow rate, then the pump capacity and head you need and how often it will cycle.

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Results

Water flowing into the pit

6.6

Recommended pump capacity (1.5 × inflow)

9.9

Read this flow on the pump's curve at the total head below
Approximate total head
12.5
Vertical lift plus about 1 ft per 10 ft of horizontal pipe; add more for many elbows
Water pumped per cycle
8.8
Pump cycles per hour at this inflow
15.0
At peak inflow; far fewer in dry weather

Keeping a basement dry

A sump pump sits in a pit, the sump, at the lowest point of a basement or crawl space. Drain tile around the foundation carries groundwater into the pit, and when the water reaches a set level, a float switch starts the pump, which sends the water outside. Choosing the right pump means knowing how much water comes in and how high it must be lifted.

Measuring the inflow

The most reliable method is to measure it during heavy rain or snowmelt:

  1. Let the pump empty the pit, then unplug it or wait until it stops.
  2. Time how long the water takes to rise a few inches.
  3. The inflow is the pit area × the rise ÷ the time.

Inflow = π × (diameter ÷ 2)² × rise ÷ time

Sizing the pump

  • Capacity: at least the peak inflow, with a margin of about 50%.
  • Total head: the vertical lift from the pump to the highest point of the discharge pipe, plus friction losses, about 1 foot for every 10 feet of horizontal pipe, and more for elbows and the check valve.
  • Read the pump’s performance curve at your total head: a pump rated 3,000 gallons per hour at zero head may deliver half that at 10 feet.

Example

An 18-inch pit where the water rises 6 inches in 1 minute, with a 10-foot lift and 25 feet of horizontal pipe:

  • Inflow: about 6.6 gallons per minute, or 397 gallons per hour
  • Recommended capacity: about 9.9 gallons per minute at the total head
  • Total head: about 12.5 feet
  • With an 8-inch pump-down range, each cycle removes about 8.8 gallons, and the pump starts about 15 times an hour at this inflow

Good practice

  • Install a check valve on the discharge line so pumped water does not flow back into the pit.
  • Discharge at least 10 to 20 feet from the foundation, away from neighbors, and check local rules: many cities forbid connecting sump pumps to the sanitary sewer.
  • Consider a battery backup or water-powered backup pump, since storms often cause power outages.
  • Test the pump every few months by pouring water into the pit, and clean the inlet screen.
  • Cast iron pumps last longer and handle heat better than plastic ones in continuous use.

Frequently asked questions

How do I measure the inflow into my sump pit?

During heavy rain, turn the pump off or wait until it stops, then time how long the water takes to rise a few inches. The calculator converts the rise and the pit diameter into gallons per minute.

What size of sump pump do I need?

Choose a pump that delivers at least the peak inflow, with a margin of about 50%, at your total head. Most homes are well served by a 1/3 or 1/2 horsepower pump; check the flow on the pump's performance curve at your head, not the maximum flow.

What is total head?

The height the pump must lift water, from the pump to the highest point of the discharge pipe, plus friction losses in the pipe, roughly 1 foot for every 10 feet of horizontal pipe, and more for elbows and check valves.

How often should a sump pump run?

It depends on the water table. Frequent short cycles, many times an hour, wear the motor and switch. A larger pit, a wider pump-down range or a check valve that works properly reduces cycling.

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Results are estimates provided for information purposes. They do not replace professional advice.

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