Sump Pump Size Calculator
A sump pump size calculator turns a pit fill test into GPH at your total head. An 18 in pit rising 8 in in 20 s needs 2,379 GPH at 12 ft, a 1/3 HP class.
Sump pump size calculator: from a bucket-style test to GPH
A sump pump size calculator finds how fast your pit fills, adds a safety margin, and compares the result with the total head the pump must push against. An 18 in pit that rises 8 in in 20 seconds takes in 26.4 GPM, or 1,586 GPH. With a 1.5 safety factor you need 2,379 GPH at 12 ft of head, which usually points to a 1/3 HP class pump.
The pit fill test
Picture a round bucket 18 in across. Every inch of water in it holds about 1.1 gallons. You are timing how fast that bucket fills.
- Let water into the pit with the pump off, usually during or right after a rain.
- Note the water level and start a timer.
- Stop when the water has risen a few inches. Record the rise and the seconds.
- Measure the pit diameter across the inside.
The formulas
- Gallons in the rise = π × (diameter ÷ 2)² × rise in inches ÷ 231.
- Inflow in GPM = gallons ÷ seconds × 60.
- Inflow in GPH = GPM × 60.
- Flow needed = inflow × safety factor (1.5 by default).
- Total head = vertical lift + (horizontal run ÷ 10) + 1.5 ft per pair of 90 degree elbows.
Worked examples
These match the calculator exactly.
Example 1: 18 in pit, 8 in rise in 20 s, 9 ft lift, 15 ft run, 1 pair of elbows.
- Gallons: π × 9² × 8 ÷ 231 = 8.81 gal
- Inflow: 8.81 ÷ 20 × 60 = 26.4 GPM, or 1,586 GPH
- Flow needed: 1,586 × 1.5 = 2,379 GPH (39.7 GPM)
- Head: 9 + 1.5 + 1.5 = 12 ft
- HP class: 1/3 HP
Example 2: 24 in pit, 5 in rise in 15 s, 10 ft lift, 20 ft run, 2 pairs of elbows.
- Gallons: π × 12² × 5 ÷ 231 = 9.79 gal
- Inflow: 9.79 ÷ 15 × 60 = 39.2 GPM, or 2,350 GPH
- Flow needed: 2,350 × 1.5 = 3,525 GPH (58.8 GPM)
- Head: 10 + 2 + 3 = 15 ft
- HP class: 3/4 HP or larger, or two pumps
Flow and horsepower classes
| Flow needed | Common class | Basis |
|---|---|---|
| Up to 2,400 GPH | 1/3 HP | Supplier guides: 30 to 40 GPM at 10 ft |
| 2,401 to 3,000 GPH | 1/2 HP | Supplier guides: 2,500 to 3,000 GPH at 10 ft |
| Over 3,000 GPH | 3/4 HP or larger, or two pumps | Above common residential output |
These classes are a screening step. Horsepower rates the motor and does not guarantee flow. Read the pump curve at your total head before you buy.
How to measure the head
- Measure vertical lift from the floor of the pit to the highest point the water must reach, which is often the outlet outside.
- Add the length of any horizontal pipe.
- Count the 90 degree elbows and enter them as pairs.
- A check valve and a long, small-diameter pipe add more friction than this estimate shows.
Common mistakes
- Sizing from the box GPH. That number is for near-zero lift.
- Testing on a dry day. Inflow is much lower than in a storm, so test during rain or repeat with a hose to simulate it.
- Measuring the pit at the top instead of the inside. A tapered pit changes the gallons per inch.
- Leaving out horizontal runs and elbows.
- Using a pit smaller than the pump needs. Zoeller guidance calls for at least 24 in of water depth in the pit to avoid short cycling.
Sources
- SupplyHouse, How to Size a Sump Pump: inflow from pit rise, gallons formula, 1 ft of head per 10 ft of horizontal pipe, 1 to 2 ft per pair of 90 degree elbows, and a 1.5 safety factor (blog.supplyhouse.com/how-to-size-a-sump-pump/).
- Zoeller Pump Company, sizing and installation guidance: pit water depth for run time (zoellerpumps.com).
- Manufacturer data sheets for Zoeller and Wayne pumps list output falling as head rises. Published GPH at 10 ft varies by model, which is why the HP classes above are a screen only.
The HP bands are rules of thumb from supplier guides and not from a single manufacturer table. Confirm against a pump curve.
Related calculators
For a longer pipe run, work out the loss with the pipe friction loss calculator. Convert between flow units with the GPM calculator. The pipe volume calculator shows how much water the discharge line holds.
Frequently asked questions
How do I test how fast my sump pit fills?
Unplug the pump or lift the float so it cannot run, and let water enter the pit. Mark the water level, start a timer, and note how many inches the water rises. Measure the pit diameter. The calculator turns the rise into gallons and the time into a flow rate.
What is total head?
Total head is the vertical lift from the bottom of the pit to the highest point of the discharge pipe, plus an allowance for friction in the pipe. This calculator adds 1 ft per 10 ft of horizontal pipe and 1.5 ft per pair of 90 degree elbows. Long runs and small pipe lose more, so check the pump curve.
Why is there a safety factor?
Rain events get heavier than your test, and a pump does not run at its best point all the time. A factor of 1.5 gives the pump a 50% margin over the inflow you measured. You can change it in the calculator.
Is 1/3 HP or 1/2 HP better?
Pick by the flow you need at your head, not by horsepower. Supplier guides place 1/3 HP pumps near 1,800 to 2,400 GPH and 1/2 HP pumps near 2,500 to 3,000 GPH at 10 ft of head. A pump's output falls as the lift rises, so compare the pump curve at your total head.
Why does the pump box say a higher GPH than the calculator?
Box ratings are often measured at or near zero lift. Real output at 10 or 15 ft of head can be much lower, so use the curve in the pump's data sheet.
Estimates only. Verify quantities with your supplier before purchasing.