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Sump Pump Sizing Calculator

Enter your measured inflow and safety factor to size your sump pump.

How it is calculated

Two steps convert the measured inflow into a pump rating: inflow_GPH = inflow_GPM × 60, then required_GPH = inflow_GPH × safety. inflow is how many US gallons per minute enter the pit at the worst moment you have measured (volume gained divided by the minutes it took), and safety is a multiplier of 1 or more that you choose to cover storms worse than the one you observed; 1 means no margin. Both inputs must be above zero. Results are rounded to whole gallons per hour. The tool does not take the pump's lift height or discharge pipe length into account, but pumps deliver less than their headline rating as the water must be lifted higher. Compare the result with the pump's flow rating at your actual lift, taken from its performance chart.

Worked example

During heavy rain the pit fills at 12 gallons per minute and you choose a safety factor of 1.5. Step 1: convert to hours, 12 × 60 = 720 GPH of inflow. Step 2: apply the margin, 720 × 1.5 = 1,080 GPH of required pump capacity. The tool echoes the factor as 1.50×. A pump rated at 1,080 GPH at your lift or more would cope with the measured rate plus a 50 percent allowance; one rated only at 720 GPH would be just level with the inflow and have no spare capacity.

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Common mistakes

  • Entering the inflow in gallons per hour. The field is gallons per minute; the tool does the hour conversion itself, so hours input gives an answer 60 times too high.
  • Measuring inflow on a dry day. The number must come from the heaviest inflow you have seen, otherwise the pump will be undersized for a storm.
  • Setting the safety factor as a percentage such as 50 instead of 1.5. It is a multiplier, so 50 would multiply the capacity fifty times.
  • Comparing the result with a pump's maximum (zero-lift) rating. Capacity falls as the vertical lift increases, so check the pump curve at your actual lift.
  • Sizing for volume only. A larger pump cycles in short bursts and can wear sooner, so don't over-correct with a very high factor.

About this calculator

A sump pump that’s too small can’t keep up during heavy rain and lets a basement flood; one that’s oversized cycles constantly and wears out prematurely. Correct sizing starts with measuring your actual water inflow rate — how fast the sump pit fills during a rain event — then adding a safety margin, typically 1.5 to 2 times the measured rate, to handle worse-than-observed storms.

This calculator converts your measured inflow and chosen safety factor into the pump capacity you need in gallons per hour (GPH), the unit sump pumps are rated in. It’s used by homeowners shopping for a replacement pump after a basement flooding scare and by plumbers and waterproofing contractors specifying the right unit for a property’s drainage conditions.

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  • Editorial team

    The rating you need is read at your actual lift, since pumps move less water the higher they lift it. A pit gaining 15 gallons a minute is 900 GPH, and with a 1.5 margin you look for 1,350 GPH at that height on the pump's own chart. Time a worst-case rise in the pit to get the inflow. Tell us what inflow you measured during your heaviest storm.

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