Calculator
Plumbing Sump Pump Sizing Calculator
Measure the real inflow with a basin drawdown test, or estimate it from the roof and foundation area draining to the tile. Add static lift, discharge pipe and fittings to get total dynamic head, then see the GPM the pump must deliver at that head, the horsepower class that covers it, and whether the basin is big enough to stop short-cycling. Free, no signup.
Free
Discharge line & total dynamic head
Friction in the riser adds to the static lift. Every elbow and the check valve counts as extra pipe.
Drawdown test1/4 HPTDH 10.3 ft6.6 GPM in
Pump capacity needed
9.9 GPM
at 10.3 ft of total dynamic head — read that point on the pump curve, not the zero-head maximum on the box.
The pump at a glance
Required capacity
9.9 GPM6.6 GPM measured inflow × 1.5× safety factor
Total dynamic head
10.3 ft10.0 ft static lift + 0.34 ft friction
Recommended class
1/4 HP22.5 GPM at this head — 2.26× the requirement
Starts per hour
25.4 /hr42 sec running, 100 sec refilling
Basin drawdown
11 gal1.1 gal per inch × 10 in float band
Band for 30/hr
8.5 in9.3 gal of drawdown holds starts to two minutes apart
How the head was built
Static lift
10.0 ftBasin water level to the discharge point
Friction loss
0.34 ftHazen-Williams over 50.5 ft of equivalent length
Fittings as pipe
25.5 ft3 × 90°, 0 × 45°, swing check
Discharge velocity
1.56 ft/s1.61 in inside diameter at the design flow
Net pump-down rate
15.8 GPMDelivered flow minus the inflow that keeps arriving while it runs
Class curve reference: Liberty Pumps 240-Series. Horsepower alone does not tell you the flow — two pumps of the same rating can differ by 20% at the same head, so confirm the pump you buy on its own published curve.
Industry benchmarks
- Basin gallons per inch
- π × (ID/2)² ÷ 231
- 18" / 24" basin
- 1.10 / 1.96 gal per inch
- Drawdown inflow
- gal per inch × rise ÷ minutes
- Rainfall inflow
- 0.0104 × C × in/hr × ft²
- Safety factor
- 1.5× measured peak inflow
- Total dynamic head
- static lift + friction + pressure
- Starts per hour at design storm
- 30 target · 60 hard limit
- Continuous-duty starts per hour
- 6–12 (normal seepage)
- Residential minimum basin
- 18" diameter × 22–24" deep
What you'll get
Measure the inflow
Unplug the pump during real inflow and time how far the water climbs in the basin. Basin diameter converts the rise to gallons per minute. No basin to test yet? Switch to the footprint mode and estimate from drainage area, design rainfall intensity, and a soil runoff coefficient.
Build the total dynamic head
Enter the static lift from basin water level to the discharge point, then the discharge pipe material, size and length plus every 90, 45 and the check valve. Fittings convert to equivalent pipe length and friction comes from Hazen-Williams — head is never just the vertical rise.
Get capacity, HP class and cycle rate
See the GPM required at the calculated head with your safety factor applied, the smallest published horsepower class that delivers it, and the run time, starts per hour and float band you need to keep the motor from short-cycling.
How it works
- 1
Measure the inflow
Unplug the pump during real inflow and time how far the water climbs in the basin. Basin diameter converts the rise to gallons per minute. No basin to test yet? Switch to the footprint mode and estimate from drainage area, design rainfall intensity, and a soil runoff coefficient.
- 2
Build the total dynamic head
Enter the static lift from basin water level to the discharge point, then the discharge pipe material, size and length plus every 90, 45 and the check valve. Fittings convert to equivalent pipe length and friction comes from Hazen-Williams — head is never just the vertical rise.
- 3
Get capacity, HP class and cycle rate
See the GPM required at the calculated head with your safety factor applied, the smallest published horsepower class that delivers it, and the run time, starts per hour and float band you need to keep the motor from short-cycling.
Why sump pumps are sized in GPM at head, never in horsepower
The number on the box is flow at zero head, and no sump pump ever runs at zero head. Every foot of static lift plus the friction in the riser, the elbows and the check valve pulls the pump back along its curve, so a unit advertised at 50 GPM may deliver half that at a real 12 foot total dynamic head. Sizing starts with a measured inflow rate — the basin drawdown test, where you unplug the pump during active inflow and time the rise — multiplied by a safety factor of about 1.5, and ends by reading that flow off the manufacturer curve at your calculated head. Horsepower is a shorthand for a family of curves, not a capacity: a high-flow three-quarter horsepower vortex will out-pump a one horsepower high-head unit at low lift.
The second half of the job is the basin, and it is the half that kills motors. A pump that comfortably beats the inflow will empty the drawdown band in seconds and restart the moment it refills; each start pulls roughly three times running amperage through the windings, and at a start every thirty seconds the motor never cools. That is why this calculator reports gallons per inch of basin, the drawdown volume your float band actually gives you, the run time per cycle and the starts per hour — and tells you the float band width needed to keep starts two minutes apart. When the numbers go red the fix is usually a wider band or a bigger basin, not a bigger pump.
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View statesSump Pump Sizing Calculator FAQs
How do I measure sump pump inflow with a drawdown test?
On a wet day with water actively entering the pit, unplug the pump and time how many inches the water rises — sixty seconds is the usual window, and you stop well before the level reaches the inlet pipe. Basin gallons per inch is pi times the inside radius squared divided by 231, so an 18 inch basin holds about 1.10 gallons per inch and a 24 inch basin about 1.96. Multiply gallons per inch by the rise and scale to a minute to get GPM.
What is total dynamic head on a sump pump?
Total dynamic head is the static lift from the water level in the basin to the discharge point, plus friction loss in the discharge pipe, plus the equivalent length of every fitting and the check valve, plus any pressure at the discharge. Only calculating the vertical lift is the most common sizing error and it always undersizes the pump. A swing check valve alone is worth roughly 100 pipe diameters of extra length.
What size sump pump do I need — 1/3 or 1/2 HP?
It depends on flow at your head, not horsepower. One third horsepower is the standard residential choice and typically moves 34 to 37 GPM at 10 feet of head; half horsepower covers heavier inflow or longer lifts at roughly 55 to 61 GPM at the same head. Size for 1.5 times your measured peak inflow, then confirm that number on the published curve of the exact pump you intend to fit — models of the same rating can differ by 20 percent.
How often should a sump pump cycle before it is short-cycling?
Manufacturers publish 6 to 12 starts per hour as continuous-duty guidance for ordinary seepage. During a design storm faster cycling is unavoidable, and field practice is that a start every two minutes or longer is normal while under a minute is short-cycling. Runs shorter than about 10 seconds are the classic cause of premature motor failure. The fix is a wider float band or a deeper basin, since a bigger pump empties the band even faster.
