Sump Pump Sizing Guide

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Basin Diameter Is Gallons Per Inch, Not “A Big Enough Crock”

A sump basin is a cylinder of water the pump is allowed to move between float-on and float-off. Diameter sets how many gallons live in each inch of that travel. Height of the liner sets how deep you can bury it and how much emergency storage you have above the off level. Cycle time cares about the inches the float actually uses, not the sticker height of the crock.

231 cubic inches is one U.S. gallon. An 18 in pit is about 1.10 gallons per inch. A 24 in pit is about 1.96 gallons per inch. Same 8 in float travel: 8.8 gallons versus 15.7 gallons. That is why a “correct” 1/3 HP pump short-cycles in a 15 in laundry-tub crock and looks civilized in a 24 in liner. The motor did not change. The volume per cycle did.

Square or oval pits still use the water-surface area at the float. Measure the diameter (or length × width) at the waterline, not the flare at the lid. A corrugated liner that necks in will lie to a tape at the rim.

The calculator uses that cylinder math and a selectable drawdown (default 8 in, typical tethered-float residential). It does not invent a basin for you.

How to Measure Inflow in the Field (Do Not Trust the Old Pump’s GPM)

The number on the failed pump is what that pump could move at some catalog head, on a good day, with a clean impeller. It is not how much water is entering the pit. Size to inflow. Confirm with a watch.

Timed-rise test (best default)

Unplug the pump. Mark the waterline. Let the pit rise a known number of inches — 2 in is enough if you can see it; 4 in is better. Time it. Gallons in = gallons per inch × inches risen. GPM = gallons in ÷ minutes. Do this during the weather you actually care about: a soaking rain, not a sunny Tuesday, unless sunny Tuesday is when the pit still weeps.

Catch-and-weigh / bucket test

If you can temporarily route the discharge into a 5-gallon bucket while the pump is running, you can time how long the pump takes to move 5 gallons, convert that to pump GPM at today’s head, then unplug and time the rise for inflow. Do not run a pump dead-headed against a closed valve to “see what it does.”

What a quiet-day test will lie about

Foundation drains see storms. A 3 GPM weep in July can be 20 GPM when the downspouts dump next to the footing and the clay is saturated. The calculator multiplies measured inflow by 1.5 as a transparent storm margin. If you already timed a peak rain, you can treat that measurement as the inflow and still have the 1.5 factor as extra conservatism — or you can mentally back it out. The results page states the factor so you can disagree with it.

Total Discharge Head Is Lift Plus Friction, Not “How Tall Is the Basement”

Total dynamic head (TDH) is the pressure the pump works against, expressed as feet of water. Two pieces: static lift, and equivalent friction from pipe, fittings, the check valve, and the outlet.

Static lift is vertical distance from the water surface in the pit (not the basement floor, not the lid) to the highest point in the discharge. If the pipe goes up 7 ft to a joist, across, then down 3 ft to a daylight window well, the lift is 7 ft, not 4. The downhill after the high point does not give the pump a refund on the way up.

Friction is the rest. A 1-1/2 in PVC run that is short and straight barely moves the number. A 1-1/4 in line with four elbows, a swing check, and 40 ft of pipe to the property line does. Order-of-magnitude adders plumbers actually use on residential sump discharge:

• About 3 ft of equivalent head per 50 ft of 1-1/2 in PVC at typical sump flows (more if you stay on 1-1/4 in).
• About 1.5 ft per 90° elbow (equivalent length converted to feet of head, not a law).
• About 2 ft for a swing check valve that actually opens.
• A crushed corrugated “flex” discharge can add more head than the entire vertical rise. Rigid PVC is the default for a reason.

The calculator wants one head number. Add lift and those equivalents yourself. If you only enter the 8 ft from pit to rim, and the real discharge is 8 ft up plus 40 ft out, you will pick a pump that looks fine on paper and dies on the curve.

Interior sanitary drains are not this pipe. Slope on gravity DWV is IPC Table 704.1. A sump discharge is a force main to a legal outlet, not a 1/4 in/ft house drain.

Horsepower Is a Carton. GPM at Head Is the Spec.

Residential sump cartons cluster on 1/3, 1/2, and 3/4 HP. Catalog curves for common thermoplastic and cast-iron pumps in those classes, in round numbers, look like this (not a named model):

Those are the bands the calculator interpolates. A 1/3 HP that still makes 26 GPM at 15 ft covers an 18 GPM requirement. The same 1/3 HP at 22 ft is essentially done. That job is 1/2 or 3/4 HP, or less head (shorter run, fewer fittings, larger pipe), not a pep talk to the 1/3 HP you already own.

Cycle time is the check on whether the basin and the float match the pump. Manufacturers want the motor to run long enough to dissipate heat and to avoid a check valve that slams 40 times an hour. If pump-on is under ~45 seconds, lengthen the tether, switch to a vertical float, or install a larger liner. More horsepower in a 15 in pit makes short-cycling worse: the net rate goes up, the on-time goes down.

Submersible vs pedestal does not change these numbers. It changes where the motor lives, noise, and how you service it. That choice: submersible vs pedestal.

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