Sump Pump Flow Rate & Horsepower Sizing Tool
Work out the gallons per hour your pit actually has to move — from basement size, site conditions, pit depth and vertical lift — then match it to a 1/3, 1/2, 3/4 or 1 HP pump at your real discharge head.
Pump ratings are quoted at total dynamic head, not at zero lift — a pump rated 2,940 GPH moves far less once it pushes water up and out. This tool sizes on head. For planning purposes only.
| Pump | GPH at head | GPM | Vs. required |
|---|
Against a requirement of 1,121 GPH at 7.9 ft. Curves are typical cast-iron submersible ratings — check the curve on the model you actually buy.
How to use the sump pump calculator
Four quick steps turn your basement and your discharge line into a pump spec — the gallons per hour you actually need, the head the pump works against, and the horsepower class that covers it — with a downloadable PDF.
Enter the water load
Basement footprint, design rainfall, your site conditions, and any roof piped into the tile.
Measure the lift
Pit depth, discharge height, horizontal run, elbows, and pipe size set the total head.
Set reserve & pit size
Choose a safety margin, then the pit diameter and float drawdown for cycle checks.
Read results & save a PDF
Get the required GPH, total head, horsepower match, cycle rate, and a downloadable summary.
How the pump size is calculated
Sizing a sump pump is two calculations that meet on a pump curve: how much water arrives in the worst hour, and how hard the pump has to push to get rid of it. Here is the exact math.
Worked example — a 1,200 sq ft basement on an average lot
Base collection: 1,200 sq ft × 1 in/hr × 0.623 = 748 GPH
Peak inflow: 748 × 1.00 (average lot) = 748 GPH
Required: 748 × 1.5 safety = 1,121 GPH (18.7 GPM)
Static lift: 30 in pit ÷ 12 + 4 ft discharge = 6.5 ft
Head: 6.5 + 1.3 friction (43.8 ft equivalent) = 7.9 ft
≈ a 1/3 HP pump, which delivers about 2,270 GPH at 7.9 ft — roughly 100% reserve over the load.
Sump pump sizing charts
Handy lookups for the questions people ask most — pump curves at real head, capacity by basement size, what your site conditions do to the load, and what the discharge pipe costs you.
| Total head | 1/3 HP | 1/2 HP | 3/4 HP | 1 HP |
|---|---|---|---|---|
| 0 ft | 2,940 | 4,200 | 5,400 | 6,600 |
| 5 ft | 2,580 | 3,720 | 5,040 | 6,180 |
| 10 ft | 2,040 | 3,180 | 4,560 | 5,700 |
| 15 ft | 1,320 | 2,520 | 3,900 | 5,040 |
| 20 ft | — | 1,560 | 3,000 | 4,200 |
| 25 ft | — | — | 1,800 | 3,180 |
Typical cast-iron submersible ratings in gallons per hour. The 0 ft row is the number printed on the box, and it is the one figure you will never see in an installed pump. Always read the curve at your own head.
| Footprint | Average lot | Wet / high table | Typical pump |
|---|---|---|---|
| 800 sq ft | 748 GPH | 1,196 GPH | 1/3 HP |
| 1,000 sq ft | 935 GPH | 1,495 GPH | 1/3 HP |
| 1,200 sq ft | 1,121 GPH | 1,794 GPH | 1/3 HP |
| 1,500 sq ft | 1,402 GPH | 2,243 GPH | 1/3 HP |
| 2,000 sq ft | 1,869 GPH | 2,990 GPH | 1/3 – 1/2 HP |
| 2,500 sq ft | 2,336 GPH | 3,738 GPH | 1/2 HP |
At 1 in/hr design rainfall with a 1.5× safety factor. The pump column assumes an average lot at roughly 8 ft of head — a higher lift or a wet lot moves it up a class.
| Site condition | Factor | Peak inflow | Required at 1.5× |
|---|---|---|---|
| Dry / well drained | ×0.55 | 411 GPH | 617 GPH |
| Average lot | ×1.00 | 748 GPH | 1,121 GPH |
| Wet / high water table | ×1.60 | 1,196 GPH | 1,794 GPH |
| Severe / spring seepage | ×2.20 | 1,645 GPH | 2,467 GPH |
Figures shown for a 1,200 sq ft basement at 1 in/hr. Roof downspouts tied into the drain tile sit on top of all of this — 1,200 sq ft of roof adds another 673 GPH on its own.
| Pipe size | Per 90° elbow | Check valve | Friction / 100 ft | Velocity |
|---|---|---|---|---|
| 1-1/4 in PVC | 3.1 ft | 10.4 ft | 7.5 ft | 4.9 ft/s |
| 1-1/2 in PVC | 3.8 ft | 12.5 ft | 3.1 ft | 3.4 ft/s |
| 2 in PVC | 5.0 ft | 16.7 ft | 0.8 ft | 1.9 ft/s |
Elbow and check valve figures are equivalent pipe length, added to the real run before friction is calculated. Friction and velocity shown at 18.7 GPM. Keep velocity under 8 ft/s — above that you are buying horsepower to fight your own plumbing.
footprint & site
drawdown & cycles
lift & friction
Two numbers decide the pump: the gallons per hour arriving in the worst storm, and the feet of head the pump fights to get rid of them.
- Inflow — footprint, rainfall, and site conditions set the gallons per hour.
- Pit — diameter and float drawdown decide how often the motor starts.
- Discharge — lift plus pipe friction is the head the curve is read at.
Everything the calculator works out
One basement and one discharge line give you the whole picture — the pump to buy, the head it works against, and whether your pit can keep up.
Key figures behind pump sizing
Built for any sump pump decision
Replacing a pump that gave up, finishing a basement, or bracing for storm season — the same math sizes the pump and checks the discharge line.
The old pump died or could not keep up in the last storm, and they want to know whether it was ever the right size.
- Measure the real lift, not the pit depth
- Check the old pump against the curve
- Count the elbows in the run
About to put flooring, drywall and furniture into a space that has to stay dry, and wants headroom in the sizing.
- Size for the worst hour, not the average
- Add a safety factor above 1.5×
- Plan the discharge route before the walls
On a wet lot or a hillside with spring seepage, where the tile runs most of the year and outages come with the storms.
- Use a wet or severe site factor
- Take downspouts to daylight
- Budget for a backup pump
7 tips for sizing a sump pump
A few habits keep a pump matched to the basement, running in short bursts, and lasting far longer than the one it replaced.
Sump pump calculator FAQ
The sizing, head, horsepower, and cycling questions people ask most before buying a sump pump.
Size on the gallons per hour your pit has to move in the worst hour, then check that a pump actually delivers that much at your discharge height. A 1,200 sq ft basement on an average lot in a 1 in/hr storm collects roughly 750 GPH, and a 1.5× margin puts the requirement near 1,120 GPH.
At a typical 8 ft of head a 1/3 HP pump delivers about 2,250 GPH, so it covers that load with room to spare. Wet lots, spring seepage, and roof downspouts tied into the tile push the requirement up fast.
Total dynamic head is the real resistance the pump works against: the vertical lift from the pit up to the discharge point, plus the friction water loses moving through the pipe, elbows, and check valve.
Manufacturers headline the flow at zero lift, which no installed pump ever sees. A 30 in pit discharging 4 ft above the floor through 20 ft of pipe with three elbows works out to roughly 8 ft of head, and every published curve should be read at that number instead.
Multiply the basement footprint by the design rainfall in inches per hour and by 0.623, the gallons that one inch of water over one square foot represents. Adjust with a site factor for your water table, add anything the roof contributes, then apply a safety margin.
An average lot uses a factor of 1.0, a high water table around 1.6, and severe seepage around 2.2. A 1.5× safety margin is the common design allowance, so the pump is not running flat out during the peak hour.
Neither is better on its own. A 1/3 HP pump handles most average basements with lifts under about 10 ft, and it costs less to buy and to run.
Step up to 1/2 HP when the lift is high, the run is long, the water table is high, or the basement is large. Horsepower buys performance at head, not just raw flow — which is why the deciding number is GPH delivered at your actual head, not the motor rating.
More than most people expect, because friction rises steeply as the pipe narrows. At roughly 19 GPM, 1-1/4 in pipe loses about 7.5 ft of head per 100 ft, 1-1/2 in loses about 3, and 2 in loses under 1.
Keep discharge velocity under about 8 ft/s. Above that, friction eats into the pump curve and you end up buying horsepower to overcome your own plumbing rather than to move water.
Each run should last at least 30 seconds. Short, repeated starts are what wears out these motors, far more than total running time.
If the pump runs only a few seconds per cycle, the pit is too small or the float drawdown too short for the pump you have. A 24 in pit holds about 2 gal per inch of depth, so 10 in of drawdown is roughly 20 gallons per cycle.
An undersized pump runs continuously during heavy rain and still loses ground, so the pit level climbs until water backs up through the drain tile and onto the basement floor.
Running flat out also overheats the motor and shortens its life, which means the failure tends to arrive during the exact storm you needed it for. Sizing with a margin above peak inflow keeps the pump working in bursts instead.
A backup makes sense wherever storms and power cuts arrive together, since a primary pump on mains power is useless in an outage and that is exactly when inflow peaks.
Battery backups run off a deep-cycle battery for several hours of intermittent pumping; water-powered backups need no electricity but consume municipal water and are not an option on a well. Either one is also insurance against the primary simply failing.
General Estimating Notice: This calculator estimates sump pump flow requirements, total dynamic head, and horsepower class from your basement footprint, design rainfall, site conditions, pit geometry, and discharge line, using standard hydraulic methods and typical published pump curves. Actual performance depends on the specific model’s curve, the condition of the drain tile, local rainfall intensity, groundwater behaviour, and the installed plumbing. Discharge routing, electrical work, backup systems, and permits should be confirmed by a licensed plumber or electrician to local code. For planning purposes only.