Sizing calculator
Work out the sump pump you need from the water, not the price tag
A sump pump is bought on one number — gallons per hour, printed large on the box — and installed against a completely different one. The advertised figure is measured at some particular lift, often zero feet or ten feet, in a laboratory with a short straight pipe. Your basin sits at the bottom of a stairwell with nine feet of vertical rise, twenty-five feet of buried line, three elbows and a check valve. The pump that moved 3,000 gallons an hour on the box will move considerably less than that here, and the gap is not a defect in the pump. It is the difference between the head it was rated at and the head it actually has to work against.
This tool starts from the opposite end. It asks what you can see in your own basement — how wide the crock is, how far the water climbed while you watched it, how high the pipe goes and how far it runs — and it derives the two numbers you need in order to shop: the flow the pump has to deliver, and the head it has to deliver that flow against. Those two together are a specification. Either one alone is marketing.
Size the pump from your own basin
Take this to any pump curve
Deliver at least 6.6 gallons per minute (397 gallons per hour) at 9.2 feet of total head.
Both halves matter. A pump that hits the flow at a lower head than yours does not hit it here.
Inflow, low to high 3.7 to 5.1 gpm
Biggest swing factor How representative your timed test was of a real storm. A measurement taken in light rain understates the peak you are actually sizing for; repeat it during heavy rain and take the worst reading.
- Discharge velocity is only 1.0 ft/s over a 25 ft run. Slow-moving water in a long horizontal line drops silt and stays in the pipe between cycles, which is what freezes first in winter.
What this specification deliberately leaves out
- Any performance claim about a specific pump. We compute what you need; the manufacturer curve says what a pump gives.
- Groundwater arriving through the slab or the walls rather than the footing drain, which no pump specification can fix.
- A failed or collapsed footing drain, which changes the answer completely and is a drainage repair, not a pump purchase.
- Sewer backup, which is a different failure with a different device — a sump pump does not protect against it.
The number that actually kills sump pumps
Motors rarely wear out first. Switches do, and switch life is counted in starts rather than in hours. With your figures the pump runs about 240 seconds, rests about 120 seconds, and therefore starts roughly 10 times an hour while water is arriving at that rate.
This is why buying a much larger pump than you need can make a system less reliable rather than more. A bigger pump empties the same small basin faster, which shortens the run and leaves the start count unchanged or worse. The three things that genuinely reduce starts are a wider basin, a longer float travel, and less water arriving in the first place. Only the last of those is free, and it is usually a downspout.
Your pit holds 1.10 gallons per inch of depth, so the 8-inch drawdown you entered moves 8.8 gallons per cycle. Adding two inches of float travel would add 2.2 gallons to every cycle and cut the start count proportionally, which costs nothing but an adjustment.
The two assumptions on this page, stated plainly
If you switch the inflow method to the estimate, two further assumptions come into play — the capture fraction (35%) and the placeholder rainfall intensity (1.0 in/hr). Both are described on the methodology page, and the capture fraction is frankly the weakest number on this site. Do the pit inflow test during real rain instead. A measured rise rate replaces this assumption entirely and is the number the sizing tool prefers whenever you have it.
Where the numbers underneath this come from
Exactly one figure on this page came from outside your own basement, and it is the conversion that turns a roof and a driveway into gallons. The United States Geological Survey publishes it outright, under the water-equivalents heading on its rain and precipitation page:
One inch of rain falling on 1 acre of ground is equal to about 27,154 gallons and weighs about 113 tons.
What that sentence does for your decision is set a ceiling, not give an answer. USGS says about and means it — the figure is rounded — but the rounding is a fraction of a percent, which is why our own cross-check against the roof example on the same page lands two gallons apart in seventeen hundred. It pins down how much water reaches the ground, which means every remaining doubt about the estimate path is doubt about what share of it reaches your footing drain — and that share is our 35% capture assumption, not the USGS figure. The federal half of the chain is arithmetic; the half we supply is a guess that can be wrong by a factor of five. That asymmetry is the entire argument for spending ten minutes on the inflow test instead of trusting the estimate, and it is why the tool above prefers a measured rise whenever you have one.
| Constant | Value | Label | Source | Retrieved |
|---|---|---|---|---|
| Rain per acre-inch | 27,154 gal | sourced | U.S. Geological Survey, Water Science School | 2026-08-06 |
| Pit gallons per inch of depth | 0.00340 x diameter squared | derived | Cylinder geometry and the 231-cubic-inch gallon | 2026-08-06 |
| Sizing margin | 1.5x | assumed | Our own adjustment, disclosed above | 2026-08-06 |
| Capture fraction | 35% | assumed | Our own adjustment, disclosed above | 2026-08-06 |
Every figure on this site carries a label of exactly one of three kinds — sourced, derived, or assumed — and the methodology page lists all of them with their quotes and retrieval dates. Nothing on the site is a number we liked the look of.
Where to go from here
- Run the pit inflow test properly — the ten-minute procedure that replaces the weakest assumption on this page with a measurement.
- Break down the head — see how much of your 9.2 feet is lift you cannot change and how much is pipe you can.
- Decide pedestal or submersible — a fit question your basin usually answers for you.
- Check a quote against the scope — which line items this job genuinely requires.