Sump pump sizing calculator
Time how fast the water rises in your pit with the pump off, or enter the inflow if you know it. Add the lift and the discharge pipe. You get the flow and total dynamic head to look up on a pump curve, and how often the pump will cycle.
Inputs
Result
Gallons per inch of sump basin
How much water each inch of depth holds in a round basin: diameter squared x π ÷ 4 ÷ 231. Your basin is marked.
| Basin diameter | Gallons per inch | Gallons per foot |
|---|---|---|
| 14 in | 0.67 | 8.0 |
| 15 in | 0.76 | 9.2 |
| 18 in | 1.10 | 13.2 |
| 20 in | 1.36 | 16.3 |
| 22 in | 1.65 | 19.7 |
| 24 in | 1.96 | 23.5 |
| 30 in | 3.06 | 36.7 |
| 36 in | 4.41 | 52.9 |
How a sump pump is sized
A sump pump is chosen by the flow it has to move, in gallons per minute, at the total dynamic head it pumps against, in feet. That pair is the duty point; look for it on the pump’s performance curve. Horsepower alone says little: two 1/3 hp pumps can deliver very different flows at 10 ft of head.
Step 1: measure the inflow
During heavy rain, turn the pump off and time how far the water rises. A round 18 in basin holds 1.10 gallons per inch of depth, and a 24 in basin 1.96. If the water rises 8 in in a minute in an 18 in pit, the inflow is 8 x 1.10 = 8.8 gpm. The pump has to move more than that, with room for a bigger storm.
Step 2: the total dynamic head
The vertical lift runs from the bottom of the pit to the highest point of the discharge line. Friction depends on the pipe size, its length and the flow; each elbow and the check valve adds the friction of a length of straight pipe.
Worked example
8.8 gpm of inflow plus 50% is 13.2 gpm. The discharge lifts 10 ft and runs 25 ft more in 1-1/2 in PVC with three 90° elbows and a check valve, which add 25.5 ft of equivalent pipe. Friction over 60.5 ft at 1.14 ft per 100 ft is 0.7 ft, so the pump needs 13.2 gpm at about 11 ft of head.
Friction loss in PVC discharge pipe
Feet of head per 100 ft of Schedule 40 PVC (Hazen-Williams, C = 150). These match Zoeller’s published table for plastic pipe.
| gpm | 1-1/4 in | 1-1/2 in | 2 in |
|---|---|---|---|
| 10 | 1.4 | 0.7 | 0.2 |
| 15 | 3.1 | 1.4 | 0.4 |
| 20 | 5.2 | 2.5 | 0.7 |
| 25 | 7.9 | 3.7 | 1.1 |
| 30 | 11.1 | 5.2 | 1.5 |
| 40 | 18.8 | 8.9 | 2.6 |
| 50 | 28.5 | 13.5 | 4.0 |
| 60 | 39.9 | 18.9 | 5.6 |
Elbows and check valves
Equivalent length of straight pipe for each fitting: 30 pipe diameters for a 90° elbow, 16 for a 45° elbow and 100 for a swing check valve (Crane). At 2 in they come to the 5.2, 2.8 and 17.2 ft in Zoeller’s sizing worksheet.
| Pipe | 90° elbow | 45° elbow | Check valve |
|---|---|---|---|
| 1-1/4 in | 3.4 ft | 1.8 ft | 11.5 ft |
| 1-1/2 in | 4.0 ft | 2.1 ft | 13.4 ft |
| 2 in | 5.2 ft | 2.8 ft | 17.2 ft |
How often the pump will run
Each cycle the pump empties the water between its start and stop levels: the switch range times the basin’s gallons per inch. With water coming in at a steady rate, the pump runs most often when the inflow is half of what it pumps; then the most starts an hour is 15 x pump gpm ÷ gallons per cycle. A short switch range in a small pit makes the pump start often, which wears the switch and motor; a larger basin or a wider range spreads the starts out.
Questions
What size sump pump do I need?
One that delivers your inflow plus a margin at your total dynamic head. Time how fast the pit fills with the pump off, add the lift and pipe friction, and find the pump whose curve meets that point: for example 13 gpm at 11 ft.
What size sump pump for a 2000 sq ft house?
Floor area doesn't size a sump pump: the water coming into the pit does, and that depends on the soil, the water table and the drainage around the house. Time the inflow in a heavy rain and size from that.
Is a 1/3 hp or 1/2 hp sump pump better?
Compare the flow each one delivers at your total dynamic head on the makers' curves, not the horsepower. At low lift a 1/3 hp pump is often enough; a higher lift or a long discharge run is where the larger pump earns its place.
How many gallons per inch is an 18 inch sump pit?
1.10 gallons per inch of depth (18 x 18 x π ÷ 4 ÷ 231). A 24 in pit holds 1.96 gallons per inch.
How do I measure sump pump inflow?
In a heavy rain, switch the pump off and time how far the water rises: inches of rise times the basin's gallons per inch, divided by the minutes, is the inflow in gpm. Switch it back on before the water gets near the top.
Does a check valve reduce sump pump flow?
A little: a swing check valve has the friction of about 100 pipe diameters of straight pipe, 13.4 ft in 1-1/2 in PVC. On a short discharge that is usually less than a foot of head.
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