What flow and head does your sump pump need?

Most sump pumps are bought by horsepower, but what the basement needs is a flow at a head. This calculator finds both from things you can measure with a tape and a watch: the width of the basin, how fast the water rises, how high the pipe lifts the water and how long the pipe is.

What the number means

A sump pump lifts groundwater that collects in a basin below the floor, and its job is to keep the water from rising to the slab. There are two separate questions. How much water arrives (the inflow, in gallons per minute) and how hard the pump must push it (the head, in feet). The answer is a flow at a head: "8.8 gpm at 7.3 feet" is something you can read off a pump curve, while "1/3 horsepower" is not.

Method and variables

Inflow = rise per minute × gallons per inch of the basin. Design flow = inflow × the margin. Static head is the vertical distance from the lowest water level in the basin (where the float turns the pump off) to where the pipe discharges. Friction head uses the Hazen-Williams equation with C = 140, which reproduces SSPMA's chart value of 1.3 feet per 100 feet for 25 gpm in 2 inch pipe within 0.05 foot per 100 feet, on the pipe length plus the equivalent length of the fittings. The cycle uses the basin volume between the float's on and off points: run time = volume ÷ (pump flow − inflow), refill time = volume ÷ inflow.

Worked example: 18 inch basin, 7 foot lift

The water rises 4 inches a minute, so the inflow is 4.41 gpm. With a margin of 2 the pump must deliver 8.8 gpm. The discharge pipe is 30 feet of 1½ inch PVC (bore 1.61 inches) with two 90° elbows and a check valve, about 21.5 feet of equivalent length; friction is 0.32 foot and the head 7.3 feet. An 8 inch float travel holds 8.8 gallons: the pump runs 2.0 minutes, the basin refills in 2.0 minutes, and the pump starts 15.0 times an hour while that inflow lasts.

Limits of this method

The inflow is the one you measured on one day; after a storm or snowmelt it can be many times larger, and the calculator cannot predict it. The margin is yours to choose; there is no code number behind it. The calculator assumes the pump delivers the design flow at the computed head, which is true only if you select it from its curve. Power outages stop any pump, so a battery backup or a second pump is a decision of its own. Do not discharge into a sanitary sewer or a septic system unless local rules allow it, and check your local code for the discharge location and the check valve requirements.

How do I measure how much water comes into my sump?

Unplug the pump, or lift the float so it cannot start, and time how far the water rises in one minute. Multiply the inches by the basin's gallons per inch: π × diameter² ÷ 4 ÷ 231. An 18 inch basin holds 1.10 gallons per inch and a 24 inch basin 1.96. A rise of 4 inches a minute in an 18 inch basin is 4.41 gallons per minute. Measure in wet weather, since the number you need is the worst one, and keep the basin from overflowing while you test.

What size sump pump do I need?

The pump must move more than the inflow, at the head it works against. The calculator multiplies the measured inflow by a margin you choose (2 by default) and finds the head. No standard sets the margin, and the sizes manufacturers quote by horsepower are broad ranges at assumed heads. For the example here, 4.41 gpm at a margin of 2 means 8.8 gpm at 7.3 feet; a margin of 1.5 gives 6.6 gpm and 3 gives 13.2 gpm. Look at the pump curve, not the carton.

What is total dynamic head?

It is the work the pump does against gravity and the pipe, expressed in feet of water: static head plus friction head. SSPMA defines static head as the vertical distance from the lowest water level in the basin to the point of discharge, and friction head as the resistance of the pipe, fittings and valves, with each fitting converted to an equivalent length of straight pipe. In the example the lift is 7 feet and friction adds 0.32 foot, so the total dynamic head is 7.3 feet.

How much do the fittings matter?

SSPMA gives these equivalent lengths for 2 inch fittings: 5.2 feet for a 90° elbow, 2.8 for a 45° elbow and 17.2 for a swing check valve. The check valve is the biggest single item. For other pipe sizes this calculator scales those lengths with the bore, a standard approximation but still an assumption. On short residential runs the lift usually dominates: in the example the fittings and 30 feet of pipe add 0.32 foot to a 7 foot lift.

Can a pump be too big?

For the pump, yes, because of cycling. A bigger pump empties the same few inches of basin faster and then waits for it to refill. With the same inflow and switch travel and a pump delivering 8.8 gpm at its head, the example basin starts 15.0 times an hour. SSPMA says more horsepower or flow is not always better, especially in a small basin, and that a longer cycle is better for pump life. A wider basin helps: a 24 inch basin with the same inflow and travel stores 15.7 gallons per cycle instead of 8.8 and starts 8.4 times an hour.

How big must the basin be?

The International Residential Code requires a sump to be at least 18 inches across and 24 inches deep unless the building official approves otherwise, with a removable cover that supports the loads in the area, the floor solid and the sump placed so that all drainage flows to it by gravity (P3303.1.2). Local codes can be stricter, so ask your building department. A bigger basin is also the cheapest way to lengthen the pump's cycles.

Sources: Sump and Sewage Pump Manufacturers Association, “Sewage Pump Sizing” (2011, sspma.org), sections on total dynamic head, equivalent lengths of fittings and basin size; International Residential Code 2024, P3303.1.2 sump pit (up.codes, read October 27, 2026; the 18 by 24 inch size is the same in the 2015 and 2021 editions per ICC summaries); Hazen-Williams equation as in our friction loss calculator. Zoeller and Wayne technical pages were not used; horsepower ranges quoted by sellers are not a standard. Related: French drain calculator, pipe size calculator, pressure tank drawdown calculator. How we check our calculators.