Size your sump pump
Do this on a wet day when water is actively entering the pit. Run the pump until it shuts off on its own, then switch it off, wait, and measure how far the water rises. How the test works
Most pits are 18″. Measure straight across the top of the liner.
Required pump capacity
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How the number breaks down
| Step | Value |
|---|---|
| Pit volume per inch | — |
| Water in the test | — |
| Inflow before margin | — |
| Safety margin | — |
| Required capacity | — |
| Recommended size | — |
How sump pump sizing works
The pit test, step by step
- Wait for a wet day. The test only means something when water is actively entering the pit — ideally during or right after steady rain.
- Run the pump to shutoff. Let your existing pump run until it stops on its own, then switch it off (or unplug it). Mark the water level with tape or a marker.
- Wait and measure. Wait 1 minute, then measure how far the water rose. That's your inflow.
- Convert. An 18″ pit holds about 1.1 gallons per inch of depth (a 24″ pit holds about 2.0). Inches × gallons-per-inch × 60 = your inflow in gallons per hour.
- Add the margin. Multiply by 1.5 — sizing guides recommend a pump about 50% stronger than your measured need.
The math — a worked example
The pit is a cylinder, so gallons per inch = π × (diameter ÷ 2)² ÷ 231 (a US gallon is exactly 231 cubic inches). For an 18″ pit: π × 9² ÷ 231 = 1.10 gal/in; a 24″ pit holds 1.96 gal/in — confirming the published rule of thumb (“an inch of water in an 18″ basin is about a gallon”).
Say the water rises 20 inches in 1 minute in an 18″ pit: 20 × 1.10 = 22 gallons × 60 = 1,320 GPH of measured inflow × 1.5 = ~1,980 GPH required → a 1/3 HP pump. The simplified rule of thumb (1 gallon per inch) gives 1,800 GPH for the same test — this calculator uses the exact 1.10 figure, which sizes you about 10% higher. That direction is the safe one.
Why the 50% safety margin
- Your test wasn't the worst day. It measured one rainstorm, not the spring-thaw-every-gutter-fails storm.
- Water gets worse, not better. Drain tile silts up, gutters clog, landscaping settles — inflow trends upward over the years.
- Full-tilt running kills pumps. A pump running constantly at its limit short-cycles less but wears faster and has no headroom when the rain doubles.
- It matches the guides. Published sizing methods recommend increasing your rough GPH figure by 50%, and more if you see regular heavy rain or snowmelt.
HP vs. GPH: what actually matters
Pumps are sold by horsepower but rated by gallons per hour at a given head height — and two pumps with the same HP can differ a lot. Two real 1/3 HP pumps are rated 2,040 and 3,060 GPH at 10 feet of lift. So: use horsepower to pick the category, then confirm the model against your GPH number at your actual lift height.
| Category | Typical required GPH | Example published ratings @ 10 ft |
|---|---|---|
| 1/3 HP | up to ~2,000 | Zoeller M53: 2,040 · Wayne CDU790: 3,060 |
| 1/2 HP | ~2,000–3,500 | Wayne CDU800: 3,840 · Zoeller M98: 3,660 |
| 3/4 HP | 3,500+ | Wayne CDU980E: 4,600 · Zoeller 1099: 4,743 |
Published manufacturer ratings at 10 ft of discharge lift. Ratings vary by model; always check the spec curve.
What head height does to flow
Head height is the vertical distance from the pump's resting water level to where the discharge pipe leaves the house (add a little for elbows and pipe friction). Every foot of lift costs you flow — dramatically. A Zoeller M53 (1/3 HP) publishes 2,680 GPH at 5 ft, 2,040 at 10 ft, and just 1,140 at 15 ft: more than half the flow is gone. A Wayne CDU790 (1/3 HP) publishes 3,810 at 5 ft, 3,060 at 10 ft, and 2,220 at 15 ft. Never size from the headline “max GPH” — that number is measured at 0 feet of lift, which no installed pump ever sees.
No pit? The estimate path
Without a pit to test, published planning figures say a house sees roughly 8 gallons per minute per 1,000 sq ft of basement on clay soil and 14 GPM per 1,000 sq ft on sandy soil; the calculator's “moderate” setting interpolates to 11. Multiply by 60, add the 1.5× margin, and the result is shown as a ±25% planning range. Treat it as a rough envelope: sandy soil surprises are always upward, so if you're choosing between sizes, choose up.
Don't skip these two
- Check valve. A check valve in the discharge pipe keeps pumped water from draining back into the pit when the pump shuts off. Without one, the pump re-pumps the same water every cycle and burns out early. Also drill a 3/16″ weep hole in the discharge pipe just above the pump (below the check valve) to prevent airlock.
- Battery backup. The storms that flood basements are the storms that knock out power. A backup system takes over automatically when the power fails or the primary pump falls behind — size it against the backup unit's own, lower GPH rating.
Frequently asked questions
How do I do the sump pump pit test?
Wait for a wet, rainy day when water is actively entering the pit. Let your pump run until it shuts off on its own, then switch it off (or unplug it), mark the water level, and wait 1 minute. Measure how far the water rose in that minute. An 18-inch pit holds about 1.1 gallons per inch of depth, so the inches of rise, times the gallons-per-inch for your pit, times 60, is your inflow in gallons per hour — then add the 50% safety margin to get the pump size you need.
What does GPH mean on a sump pump, and how much do I need?
GPH (gallons per hour) is how much water the pump can move in an hour at a given lift height. Measure your basement's inflow with the pit test, add a 50% safety margin, and buy a pump whose published rating at your actual head height covers that number — typically a 1/3 HP pump for up to about 2,000 GPH, 1/2 HP for 2,000–3,500 GPH, and 3/4 HP above that.
Why does the calculator add a 50% safety margin?
Sizing guides recommend buying a pump 50% more powerful than your measured need. The pit test measures one rainy day, not the worst one; storms vary, groundwater rises in spring, and a margin keeps the pump from running constantly at its limit, which shortens its life. The margin is cheap insurance — an oversized pump costs a little more, an undersized one costs a flooded basement.
Does a higher horsepower sump pump always pump more water?
Not exactly. Horsepower describes the motor; what moves water is the GPH rating at a given head height, and that varies between models of the same horsepower. For example, two 1/3 HP pumps can deliver 2,040 and 3,060 GPH at 10 feet of lift. Shop by the GPH on the pump's spec curve at your actual lift height, and use horsepower as a rough category.
How does head height (lift) affect a sump pump's capacity?
Head height is the vertical distance the pump must lift water, and every foot of lift reduces flow. A typical 1/3 HP pump moves about 2,680 GPH at 5 feet of lift, 2,040 GPH at 10 feet, and only 1,140 GPH at 15 feet — more than half the flow is gone. Measure from the pump's resting water level to the point where the discharge pipe exits, add a little for pipe friction and elbows, and size from the rating at that height.
Do I need a battery backup for my sump pump?
Strongly consider one. The storms that flood basements are the same storms that knock out power, and a primary pump without electricity is a sculpture. A battery backup system watches the pit and takes over automatically when the power fails or the primary pump can't keep up. Size it to cover your measured inflow at the backup unit's own (lower) GPH rating, not the primary's.
Should I install a check valve on my sump pump discharge?
Yes. A check valve in the discharge pipe keeps the water in the pipe from draining back into the pit when the pump shuts off — without one, the pump re-pumps the same water every cycle and wears out faster. While you're at it, drill a small 3/16-inch weep hole in the discharge pipe just above the pump (below the check valve) to prevent airlock.