Sump Pump Size Calculator

Two measurements tell you what a sump pump really has to do: how fast water flows into the pit during a heavy rain, and how hard the pump has to push to get it out of the house. This calculator turns your pit size, a timed fill test and your discharge pipe layout into a target flow in gallons per minute at a total dynamic head in feet.

Your result appears here after you answer the questions and press the button.

This tool gives planning ranges only. It is an estimate, not a quote or inspection. A local provider who reviews the property sets the actual scope and price. Nothing you enter here is saved or sent; only the request form sends information.

Want a provider to look at it? Call or send a short request, and a local provider may contact you.

Call (484) 275-9544 Send a request

Why horsepower is the wrong question

Pumps are often sold by horsepower, but a pump's real capacity depends on the head it works against. A pump that moves a lot of water at zero feet of lift may move far less once it has to push water up eight feet and through forty feet of pipe with a check valve. Pump makers publish a performance curve or chart for each model showing flow at different heads. The goal is to find the flow you need, find the head your system creates, and pick a pump whose chart meets both.

Step 1: measure inflow

Do this during or right after a heavy rain, when the pit is filling fastest. Unplug the pump briefly, mark the water level, and time how many seconds it takes to rise a few inches. Plug the pump back in before the water nears the top of the pit. An 18-inch round pit holds about 1.10 gallons per inch of depth. If water rises four inches in 30 seconds, that is about 4.4 gallons in half a minute, or roughly 8.8 gallons per minute.

Step 2: measure head

Static head is the vertical lift: the distance from the low water level in the pit to the point where the discharge pipe turns horizontal. Friction head is the extra pressure lost as water rubs through pipe and fittings. Every elbow and check valve adds the equivalent of several feet of pipe. The calculator adds those equivalents to your horizontal run, applies the friction loss for your pipe size and flow, and adds the static head. The result is total dynamic head, rounded up.

Reading the result

You will get a flow, in both GPM and gallons per hour, at a head in feet. Look up that head on the chart for any pump you are considering and confirm it delivers at least that flow. Size a battery backup the same way.

Chester County context

Many basements here sit on stone or block foundations with interior drains feeding a sump, and some take groundwater for days after a big storm. A pump that keeps up in a normal rain can fall behind when the ground is saturated. If your fill test comes in near the top of the table, or above 35 gallons per minute, have a plumber or waterproofing contractor size the system.

If your pump has already failed and water is on the floor, call (484) 275-9544 or send the request form. A qualified local provider may contact you about pump-out and drying.

FAQ

What if my pit is not round?

Choose rectangular and enter length and width in inches.

Can I do the fill test on a dry day?

You can test the pump with a bucket, but inflow needs a real rain to measure.

Should I round up?

Yes. The calculator rounds flow up to the next table row and head up to the next foot.

Does a bigger pump always help?

An oversized pump can short-cycle. Match the pump to your measured flow and head.

Sources

Zoeller at Home, "5 Ways to Properly Size Your Sump Pump" (Sept. 2023): pit volume, fitting equivalent lengths, friction loss table and total dynamic head method. https://zoellerathome.com/2023/09/15/properly-sizing-your-sump-pump/

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