About the Sump Pump Size Calculator
This sump pump size calculator works out the two numbers you need to choose a pump: the flow rate in gallons per hour (GPH) and the total dynamic head (TDH) the pump must deliver it against. Instead of guessing from horsepower, it starts from a real measurement of how fast water enters your pit, then adds a safety margin and calculates the friction loss in the discharge pipe.
To measure inflow, wait for a heavy rain or snowmelt, unplug the pump (or note when it shuts off) and time how many seconds the water takes to rise a few inches. Enter the pit diameter, the rise and the time; the calculator converts that to gallons per minute and per hour. A safety factor of 1.5 to 2 covers heavier storms than the one you measured.
Head is the vertical lift from the pump to the highest point of the discharge plus friction in the pipe, valve and elbows, computed with the Hazen-Williams equation for PVC pipe. Pick a pump whose published performance curve delivers at least the required GPH at the calculated TDH — not just at zero feet, where the box rating is often quoted.
With the default inputs, the pump capacity needed is 595 GPH. Change any value above to recalculate instantly.
How to use the sump pump size calculator
- 1Measure the pit diameter in inches.
- 2During heavy inflow, time how long the water takes to rise a few inches.
- 3Choose a safety factor, 1.5 for most homes or 2 for a high water table.
- 4Measure the vertical lift and horizontal run of the discharge pipe and count the elbows.
- 5Choose a pump whose curve meets the required GPH at the total dynamic head.
Formula and method
A round pit of diameter D inches holds π(D/2)² cubic inches per inch of depth, and 231 cubic inches make a US gallon. The measured rise in a given time therefore gives the inflow in gallons per minute, which is multiplied by the safety factor and by 60 for gallons per hour.
Friction loss uses the Hazen-Williams equation in US units with C = 150 for smooth PVC, at the required flow Q (GPM) and the Schedule 40 inside diameter d (in). The equivalent length adds vertical and horizontal pipe plus about 30 pipe diameters per standard 90° elbow and 100 diameters for a swing check valve, typical published resistance values. Total dynamic head is the static lift plus that friction; velocity is 0.4085 × Q ÷ d².
- D
- Pit diameter (in)
- SF
- Safety factor
- Q
- Required flow (GPM)
- C
- Hazen-Williams roughness coefficient (150 for PVC)
- L_eq
- Pipe plus fitting equivalent length (ft)
Worked examples
18 in pit rising 6 in per minute, 9 ft lift
An 18 in pit holds 1.10 gallons per inch, so a 6 in rise in a minute is 6.6 GPM or 397 GPH. With a 1.5 safety factor the pump must move about 595 GPH. In 1-1/2 in PVC the friction is only about 0.37 ft, so the total dynamic head is about 9.4 ft.
High-inflow 24 in pit, long 1-1/4 in discharge
A 24 in pit rising 4 in every 30 seconds takes in about 940 GPH, so the pump needs about 1,410 GPH. At 23.5 GPM through 1-1/4 in pipe, friction adds about 5.5 ft to the 12 ft lift, for a TDH near 17.5 ft.
Light inflow, 2 in pipe, safety factor 2
A 2 in rise per minute in an 18 in pit is about 132 GPH; doubling it gives 264 GPH. In 2 in pipe friction is negligible, so any pump whose curve shows at least 264 GPH at about 7 ft is enough.
Frequently asked questions
What size sump pump do I need?+
Measure how fast water enters the pit during heavy rain, convert it to gallons per hour and multiply by 1.5–2. Then choose a pump that delivers at least that flow at your total dynamic head, read from the manufacturer’s performance curve rather than the zero-head box rating.
How do I calculate total dynamic head for a sump pump?+
Add the vertical distance from the pump to the highest point of the discharge pipe to the friction loss in the pipe and fittings. Friction depends on flow, pipe diameter and length; elbows and check valves add the equivalent of several feet of pipe each.
Is 1/3 hp or 1/2 hp sump pump better?+
Horsepower alone does not tell you capacity. Two pumps with the same horsepower can have very different curves. A 1/2 hp model generally moves more water and handles higher lifts than a 1/3 hp model, but the right choice is whichever pump’s curve meets your required GPH at your head.
What size discharge pipe should a sump pump have?+
Most residential pumps have a 1-1/2 in outlet, and 1-1/2 in PVC is typical. For long runs or high flow, stepping up to 2 in reduces friction and lets the pump deliver more water at the same head.
How many gallons are in a sump pit?+
An 18 in diameter pit holds about 1.1 gallons per inch of depth, and a 24 in pit about 2 gallons per inch. A typical 18 × 24 in pit therefore holds around 26 gallons when full.