Sump Pump Sizing Calculator
Time how fast your basin fills, enter its dimensions — get the recommended pump horsepower.
| Basin volume (this depth) | |
|---|---|
| Required inflow rate |
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Don't stop at primary pump sizing — plan the backup too
A correctly sized primary pump doesn't help during the exact scenario sump pumps most need to handle: a power outage during heavy rain. Battery backup systems are sized by runtime at your basin's actual inflow rate, not by horsepower — a battery backup rated for, say, 2,000 gallons of pumping capacity at a given battery size will drain faster in a basin with high inflow than a slow-filling one. Use this calculator's required-GPM output as the input to sizing a backup system's runtime, not just the primary pump's horsepower. A water-powered backup (no battery, runs off household water pressure) is a lower-maintenance alternative worth asking a plumber about if your water pressure and local code support it.
How to use this calculator
This works from a real, on-site measurement rather than an assumption — time how long it takes water to rise a measured depth in your actual basin during active groundwater inflow (wet weather is the best time to check). Enter the basin diameter, that rise depth, and the fill time. The calculator converts that into your basin's actual inflow rate and recommends a pump sized to keep up with it.
Why a real fill-time measurement beats a rule of thumb
Groundwater inflow varies enormously by property — soil type, water table depth, drainage setup, and weather all affect it in ways a generic "basement square footage" rule of thumb can't capture. A timed fill test measures your specific situation directly, which is far more reliable than a generic estimate.
Worked example
An 18-inch diameter basin, water rising 24 inches in 2 minutes: basin volume for that rise is about 26.4 gallons, so required inflow is 26.4 ÷ 2 = 13.2 GPM. That's comfortably within a 1/3 HP pump's typical capacity (roughly 38 GPM at 10 ft of head).
Frequently asked questions
What if I can't get a wet-weather fill-time measurement?
Run a hose into the basin at a known flow rate and time how long it takes to rise the measured depth, as a rough substitute — it won't perfectly match real groundwater inflow, but it's more grounded than a guess.
Does head (vertical lift to discharge) matter for sizing?
Yes — a pump's rated GPM capacity drops as head increases, so the standard capacity figures used here (rated at roughly 10 ft of head) assume a typical residential discharge height. A much higher discharge point reduces real-world capacity below the nameplate rating.
Should I always round up to the next pump size?
Yes, and consider a backup (battery or water-powered) pump regardless of primary pump size — a sump pump failing during a power outage is one of the most common and costly basement flooding scenarios, unrelated to whether the pump itself was sized correctly.
Based on a real basin measurement, but published pump GPM ratings vary by brand and model. Reference only — verify against your specific pump's performance curve, and consult a licensed plumber for installation.