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Sump Pump Sizing Calculator

Calculate the pump capacity (GPM) needed for a sump pump based on your basement's water intake rate.

Result

Minimum Required Capacity
4 GPM
Recommended Pump Capacity (with margin)
6 GPM

Adds a 50% safety margin over the bare minimum measured inflow rate, since pump capacity also decreases with vertical lift height and pipe friction - always check a specific pump's GPM rating at your actual discharge height (listed on its performance curve), not just its rating at 0ft of lift.

About the Sump Pump Sizing

The Sump Pump Sizing Calculator estimates the pump capacity, in gallons per minute, needed to keep a basement sump basin from overflowing during heavy inflow. It's built around a simple field test: timing how long your existing basin takes to fill from empty to the level where a pump normally activates.

How It Works

You enter the sump basin's diameter, the minutes it takes to fill from empty to the pump's trigger level, and the vertical lift from the pump to the discharge point. The calculator treats the basin as a cylinder, computes the water volume in the section that fills during your timed interval, divides that volume by the fill time to get the required inflow rate, and then adds a margin to produce a recommended pump rating.

Volume (gal) = pi x (diameter / 2 / 12)^2 x 1.5 ft x 7.48 gal per cu ft. Required GPM = volume / fill time (min). Recommended GPM = required GPM x 1.5.

Formula & Methodology

The calculation assumes the water level rises a fixed 1.5 feet between the empty mark and the float trigger point, regardless of basin size, and converts that cylindrical volume to gallons using the standard 7.48 gallons-per-cubic-foot conversion. Dividing by your measured fill time in minutes gives the actual inflow rate in gallons per minute, and multiplying by 1.5 builds in a margin so a pump isn't running at its bare rated minimum during a heavy storm.

Examples

Standard 18-inch basin

An 18 inch diameter basin that fills in 5 minutes computes to roughly 4 GPM required, with a recommended pump rating near 6 GPM after the 50% margin.

Fast-filling 24-inch basin

A wider 24 inch basin filling in just 3 minutes during a storm computes to about 12 GPM required, with a recommended capacity around 18 GPM.

Advantages

  • Uses a field-measurable input, basin fill time, rather than asking you to estimate groundwater inflow directly, which is hard to know otherwise.
  • Builds a safety margin into the recommendation automatically instead of requiring a separate manual markup.
  • Flags that lift height matters for real-world pump selection even though it isn't part of the GPM formula itself.

Common Mistakes

  • Comparing the calculator's GPM figure to a pump's maximum rating at zero lift instead of its rated output at the actual discharge height.
  • Timing the fill test on a dry day instead of during or right after rain, which understates true inflow and undersizes the pump.
  • Assuming a larger basin alone fixes an inadequate pump, when basin size mainly changes how often the pump cycles, not how fast it can move water.

Edge Cases to Watch For

  • Fill time is floored at 0.1 minutes even if zero or a negative value is entered, which prevents a divide-by-zero error but should prompt a double check of the input.
  • The 1.5-foot fill height is a fixed assumption; a basin whose float switch travels a noticeably shorter or longer distance will produce a proportionally inaccurate GPM figure.
  • The vertical lift field is captured but does not feed into the GPM math itself; it exists as a reminder to check a candidate pump's rated output at that specific lift height, since output drops as lift increases.

Common Use Cases

  • Homeowners replacing a failed or undersized sump pump after a basement flooding incident.
  • Contractors sizing a battery backup or secondary pump to match a primary pump's demonstrated capacity.
  • Home inspectors and buyers evaluating whether an existing sump system looks adequately sized for the property.
Written & fact-checked by the Calculateus TeamLast updated August 5, 2026How we verify our formulas

Frequently asked questions

Why does a pump's GPM rating change with lift height?

A sump pump's flow rate is highest with no vertical lift and decreases as it has to push water higher against gravity, which is why manufacturers publish a performance curve showing GPM at several lift heights rather than a single number - always check the rated GPM at your actual discharge height, not the pump's maximum rating at 0 feet of lift, which overstates real performance for most installations.

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Conclusion

By anchoring the calculation to a timed fill test rather than a guess, the Sump Pump Sizing Calculator gives a GPM target grounded in how the specific basin behaves during wet conditions. Pair the recommended GPM with a pump's published performance curve at your real lift height before buying.