Sump Pump Sizing Calculator: Flow and Total Head

Horsepower is a headline, not a specification. What decides whether a pump clears your pit is total head: the vertical lift to the discharge plus the friction losses in the pipe and fittings. This calculator works out your real inflow from the pit size and how fast it refills, then tells you the output you need at your own head.

When this tool helps most

Use it before buying a replacement pump, and again before accepting a quote. A pump advertised at a large gallons-per-hour figure is usually quoting that figure at almost no lift.

How to use this tool

1. Measure the pit diameter across the inside of the basin.
2. During heavy rain, mark the water level, wait one minute, and measure how far it rose.
3. Measure the vertical lift from the pit water line to the highest point of the discharge run.
4. Measure the horizontal pipe run and count the elbows plus the check valve.
5. Set a safety margin for a worse storm than the one you measured.

Worked example

An 18 inch pit rising 1 inch per minute, with 9 feet of lift, a 20 foot run and 4 fittings, returns a required output of 99 GPH at 10.6 feet of head, from a measured inflow of 66 GPH. The pit holds 1.10 gallons per inch of depth. Note how modest the flow figure is and how much the head matters: read the pump curve at 10.6 feet, not at zero.

Understanding your results

The required output is your measured inflow plus your margin. The total head is the number to take to the flow curve. A pump that moves 3,000 GPH at zero lift may move far less at ten feet, and that lower figure is the one that matters.

Common mistakes to avoid

• Measuring the refill rate in dry weather, which undersizes the pump.
• Comparing pumps on horsepower instead of flow at your own head.
• Forgetting the check valve and elbows, which add real friction.
• Ignoring pit size, so the pump short-cycles and wears out early.

How to get the most accurate result

• Measure your own basement or crawl space with a tape measure instead of using typical sizes.
• Use the numbers printed on your product labels, manuals and data sheets.
• Change one input at a time to see which assumption matters most.
• Print or save the result and bring it to your installer or contractor.

Next steps

Then size the backup with the Sump Pump Backup Runtime Calculator, because the storm that floods you often cuts the power too.

Frequently asked questions

What is total head?

The vertical distance the pump must lift water, plus the equivalent resistance of the pipe, elbows and check valve. Pumps deliver less flow as head increases, which is why the flow curve matters more than the headline figure.

How do I measure inflow?

During genuinely heavy rain, mark the water level in the pit, wait exactly one minute, and measure the rise. The calculator converts that to gallons per hour using your pit diameter.

Why add a margin?

Because the storm you measured is probably not the worst storm you will get. A margin also covers the pump losing a little output as it ages.

This tool is for planning and education only. It does not store your entries. Structural judgement belongs to a licensed structural engineer, radon mitigation design to a certified mitigator, and mold over roughly 10 square feet to a remediation professional. New circuits are a job for a licensed electrician. Read our disclaimer.