Sump pump size calculator: time the pit, convert the rise to GPH, add the head, then read a pump curve instead of a horsepower label
Gallons per inch of rise is pi x radius squared / 231. An 18 in pit rising 10 in a minute needs 661 GPH at 15 ft of head; a 3/10 HP Zoeller 53 gives 1,140.
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Short answer
A sump pump is sized by gallons per hour at a stated head, never by horsepower. Time how many inches the pit rises in one minute during heavy inflow, then convert with the cylinder formula: gallons per inch equals pi times the radius squared divided by 231, because NIST Handbook 44 defines the US gallon as 231 cubic inches. An 18 in basin holds 1.10 gallons per inch, so a 10 in rise in one minute is 11.0 GPM, or 661 GPH. Head is the vertical lift plus friction: Zoeller’s sizing guide puts 13 ft of lift with 100 ft of 1.5 in pipe, two elbows and a check valve at 14.4 ft, rounded up to 15 ft. Read a published curve at that head. Zoeller’s FM2778 data sheet gives the 3/10 HP Model 53 19 GPM (1,140 GPH) at 15 ft; Wayne’s manual gives the 1/3 HP CDU790 2,220 GPH at 15 ft. Both cover 661 GPH.
The picks
An inflow under about 2,000 GPH with 15 ft or less of total head
WAYNE CDU790 1/3 HP Submersible Sump Pump
Wayne's manual prints 4,600 GPH at 0 ft, 3,810 at 5 ft, 3,060 at 10 ft, 2,220 at 15 ft and 1,200 at 20 ft of head, cast iron volute, vertical float, 120 V
The same line one step up, for a longer discharge run or a head near 20 ft
WAYNE CDU800 1/2 HP Submersible Sump Pump
Wayne's manual prints 5,100 GPH at 0 ft, 4,500 at 5 ft, 3,840 at 10 ft, 3,060 at 15 ft and 2,040 at 20 ft of head, cast iron motor housing and volute
If you are replacing a pump that always kept up, buy the same curve or better and skip the timing: this page is for a pit nobody has measured.
Inflow in GPH, total dynamic head, and whether two published curves cover it
Formula: GPH = (pi x (diameter / 2)^2 x rise / 231) / minutes x 60. Head = lift + (pipe length + fitting equivalent length) x friction loss per 100 ft / 100. Fitting lengths and friction loss are Zoeller's published tables; the curves are Zoeller FM2778 and Wayne 600002W-001 J.
What is the formula?
Inflow is how fast water enters the pit, in gallons per hour. Zoeller’s sizing guide measures it by the amount of water the basin collects during a high-flow period; the pump must remove water faster than that.
Gallons per inch follows from NIST Handbook 44, Appendix C, which defines the gallon as 231 cubic inches: a cylinder holds pi x (diameter / 2)^2 / 231 gallons per inch of rise.
| Basin diameter | Gallons per inch of rise |
|---|---|
| 14 in | 0.67 |
| 18 in | 1.10 |
| 24 in | 1.96 |
| 30 in | 3.06 |
Total dynamic head is the pressure the pump works against, in feet of water. Zoeller defines it as static head (the vertical distance from the pump to where the discharge pipe turns horizontal) plus friction head: horizontal pipe length plus fitting equivalent length, times the friction loss per 100 ft at the system’s flow, divided by 100.
| Fitting, per Zoeller | 1.25 in pipe | 1.5 in pipe | 2 in pipe |
|---|---|---|---|
| 90 degree elbow | 3.5 ft | 4 ft | 5.2 ft |
| 45 degree elbow | 1.8 ft | 2.2 ft | 2.8 ft |
| Check valve | 11.5 ft | 13.4 ft | 17.2 ft |
Friction loss is Zoeller’s table: 0.33 ft per 100 ft of 1.5 in pipe at 6 GPM, 1.16 at 12 GPM, 2.96 at 20 GPM. The calculator reads the row at or above your inflow. The pump runs further up its curve than the inflow, so real friction is slightly higher; Zoeller’s example rounds 13.4 ft up to 14 ft, and the calculator rounds up too.
What does a worked example look like?
An 18 in basin, timed during a storm, rises 10 in in one minute.
- Gallons per inch: pi x 9^2 / 231 = 1.10.
- Inflow: 1.10 x 10 = 11.0 GPM, x 60 = 661 GPH.
- Equivalent length on 1.5 in pipe: two 90 degree elbows (8 ft) plus one check valve (13.4 ft) = 21.4 ft. Add 100 ft of horizontal pipe: 121.4 ft.
- Friction head: 121.4 x 1.16 (the 12 GPM row) / 100 = 1.41 ft.
- Total dynamic head: 13 ft of lift + 1.41 = 14.4 ft, use 15 ft.
- Read the curves at 15 ft.
| Pump, maker’s document | GPH at 5 ft | GPH at 10 ft | GPH at 15 ft | GPH at 20 ft |
|---|---|---|---|---|
| Zoeller Model 53, 3/10 HP, FM2778 | 2,580 (43 GPM) | 2,040 (34 GPM) | 1,140 (19 GPM) | 0 (shut-off 19.25 ft) |
| Wayne CDU790, 1/3 HP, 600002W-001 J | 3,810 | 3,060 | 2,220 | 1,200 |
| Wayne CDU800, 1/2 HP, 600002W-001 J | 4,500 | 3,840 | 3,060 | 2,040 |
Both smaller pumps cover 661 GPH at 15 ft; the margin is for the storm you did not time.
Why is horsepower the wrong number to shop by?
Because two pumps with the same motor deliver different water at the same head, and one pump delivers wildly different water at different heads. Zoeller’s Model 53 moves 43 GPM at 5 ft and nothing at 19.25 ft.
Wayne’s manual makes the point within one line. Its 1/2 HP CDU800 delivers 2,040 GPH at 20 ft; its other 1/2 HP pump, the SPF50, delivers 1,260 GPH there. The 1/3 HP CDU790 delivers 1,200 GPH at 20 ft, within 5 percent of the SPF50 with a smaller motor.
Zoeller’s guide calls 1/3 HP standard for most homes, then spends five steps on head. Wayne’s troubleshooting chart lists “pump not properly sized for application” under both “pump will not shut off” and “delivers little or no water”.
Where do the numbers come from?
| Number | Document |
|---|---|
| 231 cubic inches per gallon | NIST Handbook 44, Appendix C |
| Timed-fill method, total dynamic head, fitting equivalent lengths, friction loss per 100 ft | Zoeller at Home, 5 Ways to Properly Size Your Sump Pump |
| Model 53 curve: 43, 34, 19 GPM at 5, 10, 15 ft; shut-off 19.25 ft; tested to UL 778 | Zoeller Technical Data Sheet FM2778 |
| CDU790, CDU800, SPF50 GPH at 0 to 20 ft; check valve and rigid PVC required; 11 in and 14 in minimum basin | Wayne manual 600002W-001 J |
The calculator does not decide the installation. Wayne’s manual requires a check valve and rigid PVC discharge pipe, no flex hose, sets minimum basin diameters of 11 in (vertical float) and 14 in (tether float), 22 in high, and says the installation must follow the National Electrical Code and local ordinances on a GFCI receptacle. Where the discharge may go is a local code question.
Time the pit before the cart
The calculator needs a diameter, a rise and a lift, and no box knows them. One wet night with a tape measure decides whether 3/10 HP is generous or 1/2 HP is short.
Once the head is real, the Wayne CDU790 is the published-curve pick for a pit under about 2,000 GPH with 15 ft of head or less, and the CDU800 for a run that lands near 20 ft. Skip both if your pit fills faster than the curve at your head: that is the sump pump and battery backup review, where the second pump matters more than the first.
A pump sized right still fails silently when the float sticks or the power goes out; a pit that has overflowed once earns a water sensor beside it, see the leak detector review.
Frequently asked questions
How many gallons is one inch of water in a sump pit?
Volume of a cylinder divided by the gallon. NIST Handbook 44 defines the US gallon as 231 cubic inches, so gallons per inch of rise is pi x radius squared / 231. An 18 in basin holds 1.10 gallons per inch and a 24 in basin holds 1.96. Measure the diameter rather than trusting a rule of thumb.
Is a 1/3 HP or 1/2 HP sump pump better?
Neither, until you know the head. Wayne's own manual lists two 1/2 HP pumps, the CDU800 at 2,040 GPH and the SPF50 at 1,260 GPH at 20 ft of head, while its 1/3 HP CDU790 delivers 1,200 GPH there. Compare pumps at your calculated head, not by the horsepower on the box.
What is total dynamic head on a sump pump?
Zoeller defines it as static head plus friction head. Static head is the vertical distance from the pump to where the discharge pipe turns horizontal. Friction head is the loss in the pipe and fittings: total pipe length plus fitting equivalent length, times the friction loss per 100 ft from the table, divided by 100.
How much head does a check valve or elbow add?
Zoeller's sizing guide gives equivalent lengths by pipe size. On 1.5 in pipe a 90 degree elbow counts as 4 ft of pipe, a 45 degree elbow 2.2 ft and a check valve 13.4 ft. On 2 in pipe those become 5.2, 2.8 and 17.2 ft. Add them to the actual pipe length before applying the friction loss.
Does the discharge pipe have to be rigid PVC?
Wayne's manual says rigid PVC pipe is required and flex hose is not to be used in a permanent application, and that a check valve is required to prevent back-flow after each cycle. Both also affect sizing: a check valve adds 13.4 ft of equivalent length on 1.5 in pipe. Local codes govern the discharge point.
Fact sheet and how to cite this page
- Answer
- Time the rise in the pit, convert inches to gallons with pi x radius squared / 231 (NIST Handbook 44 defines the gallon as 231 cubic inches), multiply by 60 for GPH, add vertical lift to Zoeller’s friction head, then read a published curve at that head. An 18 in pit rising 10 in per minute is 661 GPH; at 15 ft a Zoeller 53 delivers 1,140 GPH and a Wayne CDU790 2,220 GPH.
- Formula
GPH = (pi x (diameter / 2)^2 x inches of rise / 231) / minutes x 60; total dynamic head = vertical lift + ((actual pipe length + fitting equivalent length) x friction loss per 100 ft / 100); pick a pump whose curve delivers at least the GPH at that head- Inputs
- Basin diameter: 18 in
- Water rise: 10 in in 1 minute
- Vertical lift: 13 ft
- Discharge pipe: 1.5 in, 100 ft horizontal
- Fittings: two 90 degree elbows (4 ft each), one check valve (13.4 ft)
- Output
- 1.10 gallons per inch, 11.0 GPM, 661 GPH inflow; 121.4 ft total pipe length x 1.16 ft per 100 ft (the 12 GPM row) = 1.41 ft friction head; 14.4 ft total dynamic head, rounded up to 15 ft. Zoeller 53: 19 GPM (1,140 GPH) at 15 ft. Wayne CDU790: 2,220 GPH at 15 ft. Either covers 661 GPH.
- Assumptions
- The basin is a vertical cylinder and the timed rise is taken during a high-flow period, which is when Zoeller’s sizing guide says to measure what the basin collects
- Friction loss is read from the next row of Zoeller’s table at or above the required GPM; the pump will actually run further up its curve than the inflow, so real friction head is slightly higher and the rounding up covers it
- The two curves are Zoeller’s FM2778 data sheet (Models 53/55/57/59) and Wayne’s manual 600002W-001 J; any other pump is read from its own maker’s curve
- Method
- The calculator converts the timed rise to GPH with the 231 cubic inch gallon, builds equivalent length from Zoeller’s fitting table (1.25, 1.5 and 2 in pipe), reads friction loss per 100 ft from Zoeller’s table at the required flow, adds vertical lift to get total dynamic head, then interpolates both published curves at that head and reports whether each pump delivers at least the required GPH.
- Primary source
- Zoeller Pump Company, Technical Data Sheet FM2778, Mighty-Mate Series Models 53/55/57/59 (PDF) (manufacturer or vendor document)
- Facts verified
- Last updated
- Cite as
- OwnerSpec, "Sump pump size calculator: time the pit, convert the rise to GPH, add the head, then read a pump curve instead of a horsepower label", https://ownerspec.com/plumbing/calculators/sump-pump-size-gph-calculator/, facts verified 24 September 2026.
Every figure above also appears, with its source, in the article. All sources ยท How OwnerSpec verifies numbers
Sources
- Zoeller Pump Company, Technical Data Sheet FM2778, Mighty-Mate Series Models 53/55/57/59 (PDF) (manufacturer or vendor document)
- Zoeller at Home, 5 Ways to Properly Size Your Sump Pump (manufacturer or vendor document)
- WAYNE, Submersible Sump Pumps Operating Instructions and Parts Manual, 600002W-001 J (PDF) (manufacturer or vendor document)
- NIST Handbook 44 (2023), Appendix C, General Tables of Units of Measurement (PDF) (government or regulatory source)