Sump pump horsepower, explained
Horsepower is a proxy for flow, and flow only means anything at a stated height. Here is how to read the numbers on the box.
Horsepower measures the motor, not the water. Two pumps with the same motor rating can move different amounts of water depending on impeller design, volute shape and discharge size. It is a useful shorthand within a single manufacturer's range and a poor one across brands.
The number that actually matters
Flow at your lift. A pump's output falls as the height it has to push water increases, and the shape of that fall-off — the flow curve — is the real specification. Most packaging prints the zero-lift figure because it is the biggest one.
| Lift | What typically happens |
|---|---|
| 0 ft | The headline number. Describes a pump lying on a bench |
| 5 ft | Perhaps 10–20% below the headline figure |
| 10 ft | Commonly 25–40% below. This is your basement |
| 15 ft | Small pumps approach zero; larger ones keep working |
| Shut-off head | The height at which the pump moves nothing at all |
Reading a flow curve
- Find your total lift on the horizontal axis — including the horizontal-run allowance.
- Read the flow at that point. That is what the pump will actually deliver in your house.
- Check the shut-off head is comfortably above your lift. A pump operating near shut-off moves almost nothing while drawing full current.
- Compare pumps at the same lift, never one at 0 ft against another at 10 ft.
Zoeller 1073 (1/3 HP cast iron submersible)
A cast-iron 1/3 HP with published flow at several lifts, which is what makes it useful as a reference.
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| Type | Submersible, cast iron |
|---|---|
| Horsepower | 1/3 HP |
| Line | Zoeller at Home consumer range |
We point at this pump throughout the site because the manufacturer publishes enough data to argue with. A brand that only quotes a zero-lift figure is not giving you anything to check.
What it gets right
- Published curve
- Cast iron
- Typical of the category
Where it falls short
- Needs pit width
- Marginal above 12 ft
Specifications from Zoeller at Home sump pump range.
Wayne CDU980E
3/4 HP, and a useful illustration of how the curve flattens out as motors get bigger.
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| Type | Submersible, cast iron and stainless steel |
|---|---|
| Horsepower | 3/4 HP |
| Electrical | 120 V, 10 A |
| Discharge | 1-1/2" NPT |
| Switch | Vertical float, on at 9", off at 4" |
| Intake | Top suction, to keep debris out of the impeller |
| Seals | Carbon/ceramic shaft seal, ball bearings in a sealed oil bath |
| Flow, by lift | 5,490 GPH at 0 ft · 4,600 GPH at 10 ft |
| Warranty | Five years, limited |
The honest way to describe additional horsepower is that it buys you flow at height. At zero lift the difference between pumps is much smaller than the specification sheet implies.
What it gets right
- Holds flow at height better than smaller pumps
- Cast iron and stainless
- Top suction
Where it falls short
- Short-cycles in a small pit
- More current
Specifications from Wayne CDU980E specification listing, Wayne 58321 CDU980E distributor listing.
Why we store flow with lift attached
Every flow figure on this site is stored as a pair: gallons per hour, and the lift it was measured at. The content model makes it impossible to record one without the other. That is a deliberate constraint, because quoting a zero-foot number as real-world performance is the most common misleading convention in this category and several large publishers still do it.
Where a manufacturer publishes only the zero-lift figure, that is all you will see from us, and the page will say so rather than interpolating a number we do not have.
Questions people ask
Does more horsepower mean more water?
Generally yes within a manufacturer's range, but the gain is largest at high lift and smallest at low lift. Across brands, impeller and volute design matter as much as the motor rating.
What is total dynamic head?
The total resistance the pump works against: vertical lift plus friction losses from pipe length and fittings. It is the number to size against, and it is always higher than the vertical height alone.
Why do manufacturers quote flow at zero lift?
Because it is the biggest number. It describes a pump discharging freely at the same level as the water, which is not an arrangement that exists in any basement.
What is shut-off head?
The height at which the pump can no longer move water at all. Your working lift should be well below it — a pump near shut-off draws full current and delivers almost nothing.
Sources
Read next
What size sump pump do I need?
Two measurements and ten minutes give you a real answer. Almost everyone skips both and buys on horsepower instead.
1/3 HP vs 1/2 HP sump pump
The most common sizing question in the category, and the one most often answered by buying more pump than the pit can use.
1/2 HP vs 3/4 HP sump pump
Past half a horsepower, the pump usually stops being the limiting factor. Here is how to tell whether you are in the small group that genuinely needs more.
Sump pump discharge pipe size
The pump is rated against a certain resistance. Every reduction, elbow and yard of corrugated hose adds to it — and the pipe is often the real bottleneck.
The best sump pumps, ranked by what they move at real lift
Five pumps that cover almost every residential pit, compared on the number that matters: how much water they move at the height your basement actually needs.