Sump pump sizing calculator
"What size sump pump do I need" has a two-number answer: the gallons per minute the pit takes on in a storm, and the total head the pump has to push that water against. Measure the first in the pit, add up the second from the lift and the pipe, and read both against a manufacturer's curve. Horsepower never enters the arithmetic.
Pre-filled with a storm reading from an 18 in. pit. Put your own pit, lift and run in.
19.8
gpm the pump must move
14.0
ft of total dynamic head
Find a pump whose published curve sits on or above the point 19.8 gpm at 14.0 ft. Horsepower does not enter into it.
- Pit holds
- 1.10gal per inch
- Measured inflow
- 16.5gpm
- Inflow plus 20% margin
- 19.8gpm
- Static lift
- 12.0ft
- Pipe length, lift plus run
- 52.0ft
- Fittings and check valve
- 23.5equiv. ft
- Friction rate at the duty flow
- 2.60ft per 100 ft
- Friction head
- 2.0ft
Water speed in the discharge at the duty flow: 3.2 ft/s. Pit volume between switch points: 4.7 gal.
| Reference pump | gpm at 14.0 ft | Meets duty? | Starts per hour |
|---|---|---|---|
| Zoeller Model 53 3/10 hp | 22.1 | Yes | 53 |
| Zoeller Model 73 1/3 hp | 17.3 | No, keeps up without margin | 10 |
Starts per hour assume your measured inflow and the switch differential you entered. Curves read from Zoeller technical sheets FM2778 and FM2960; between published points the line is straight.
What the calculator is actually sizing
A pump curve is a line of flow against head. The job sets one point on that chart, the duty point: the flow the pit needs taken away, at the total dynamic head (TDH) the discharge piping puts on the pump. TDH here is the static lift, measured vertically from the pump to the high point of the discharge, plus the friction head of every foot of pipe and every fitting along the way. Zoeller's own sizing material describes TDH the same way and adds an operating-head term for pressurised outlets, which a sump dumping to grade does not have.
Any pump whose published curve passes on or above the duty point will keep up. A pump whose curve passes below it will fall behind in exactly the storm you bought it for. The friction rate is estimated with the Hazen-Williams formula Spears publishes for PVC (C = 150) against schedule 40 average inside diameters; if you have a manufacturer friction table for your pipe, switch the calculator to enter the rate yourself. The diameters are the ones on the pipe size chart, and the head arithmetic is the same one the water pressure calculator runs: one psi is 2.31 ft of head.
How to measure inflow in the pit
The University of Illinois Extension method needs a tape measure, a phone timer and a real storm. Take the reading during heavy rain, because that is the flow the pump has to keep up with.
- 1
Pump the pit down
Let the pump run to its shut-off level during heavy rain, then unplug it.
- 2
Time the rise
Mark the water line and time it. One minute is the Extension's window; a slower pit can run longer, which is why the calculator takes seconds.
- 3
Measure and restore
Measure the rise in inches and plug the pump back in before the water reaches the drain-tile inlet. Water should never stand above that pipe.
- 4
Convert to gallons
An 18 in. pit holds about 1.10 gal per inch, a 24 in. pit about 1.96. Inches times gallons-per-inch over minutes is your inflow in gpm.
No pit yet?
Reading a pump curve, and why horsepower is the wrong question
Here are two real curves from Zoeller's own sheets. The Model 53 is rated 3/10 hp. The newer Model 73 is rated 1/3 hp, a bigger motor on paper. At every published head the smaller motor moves more water.
| TDH, ft | Model 53, 3/10 hp | Model 73, 1/3 hp |
|---|---|---|
| 5 | 43 | 38 |
| 10 | 34 | 30 |
| 15 | 19 | 14 |
| Shut-off | 19.25 ft | 18 ft |
Horsepower describes the motor. The impeller and volute decide where that power goes, and two pumps with the same label can have very different curves. Read the curve at your head, not the box. The shut-off head is the height at which flow stops altogether. A pump working near shut-off is barely moving water, so keep the duty point well inside the curve.
Worked example: the pre-filled numbers
An 18 in. pit rises 15 in. in one minute during a storm: 16.5 gpm of inflow, 19.8 gpm with a 20 percent margin. The pump lifts 12 ft to the rim joist and the line runs 40 ft to daylight through three 90° elbows and a check valve. Change only the pipe size and the answer changes with it:
| Sch 40 PVC | ft per 100 ft | Friction, ft | TDH, ft | Model 53, gpm | Model 73, gpm |
|---|---|---|---|---|---|
| 1-1/4" | 5.56 | 4.2 | 16.2 | 13.8 | 8.6 |
| 1-1/2" | 2.60 | 2.0 | 14.0 | 22.1 | 17.3 |
| 2" | 0.76 | 0.6 | 12.6 | 26.2 | 21.6 |
On 1-1/4 in. pipe neither pump keeps up. On 1-1/2 in. the Model 53 clears the duty point and the 1/3 hp Model 73 does not. On 2 in. both pass. Before you reach for a bigger pump, try the next pipe size up.
Check valve, discharge size and the code
IPC 1113.1.4 In the 2021 International Plumbing Code the discharge from a sump pump needs a gate valve and a full-flow check valve, and the pipe may not be smaller than the pump's discharge tapping. One- and two-family dwellings need only the check valve. A pump with a 1-1/2 in. outlet, like the Model 53, therefore cannot be reduced to 1-1/4 in. downstream. IPC 1113.1.2 sets the pit at a minimum of 18 in. diameter and 24 in. deep unless otherwise approved, with a removable cover, and IPC 1113.1.1 asks only that capacity and head suit the anticipated use, which is the duty point above. The adopted edition and any local amendments are set by the authority having jurisdiction (AHJ); see IPC basics and plumbing permit cost before you pull one.
Bigger is not safer
The check valve counts as fittings in the friction sum, often the largest single one, so use the valve maker's equivalent length when you have it. On whether the pump sits in the water or above it, see pedestal vs submersible. To quote the install, carry the pump, basin, pipe and hours into the job price calculator at your own shop rate.