Size at head, not at zero lift
A pump's headline GPH is measured at 0 ft of lift. Against real vertical lift plus pipe friction it delivers far less — always read the pump curve at your total head.
Sizing calculator
Sump Pump Sizing & Discharge Calculator
Size a sump pump to the water reaching the pit and the head it must push against — the required flow, total dynamic head, horsepower, and whether your discharge pipe can carry it.
Use the result wisely
What this sizes
- Required GPM from a measured pit rise or an estimate.
- Total dynamic head — vertical lift plus pipe friction.
- Recommended horsepower and a discharge-pipe check.
- Sizing only — it is not a price or a contractor quote.
Confirm the number on the pump's published performance curve, and have any electrical work near water reviewed by a qualified professional.
Sizing calculator
Size the pump to the water and the head
Enter how much water reaches the pit and how far the pump has to push it. The result is the flow and horsepower you need at the real head — not the advertised rating at zero lift.
in/min
During heavy rain, unplug the pump safely and time how fast the water rises — inches per minute. Even a 2–3 minute watch gives a good number.
Used for the inflow reading and the short-cycling check.
The tool checks whether this size can carry the flow.
ft
From the pit water level up to the highest point of the discharge pipe, in feet.
ft
How far the pipe runs horizontally to the outlet, in feet.
Watch
Watch: the calculator in 35 seconds
A quick walkthrough of sizing a pump the right way — time the pit fill rate in heavy rain, add the lift and run, and read the horsepower at your real head instead of the zero-lift number on the box.
Discharge pipe is part of the pump
Too small a discharge line throttles the flow and hammers the fittings. Most homes use 1½", but higher flow or long runs often need 2".
Water and electricity
Never reach into a pit near a powered pump or standing water. Have the outlet, GFCI, and any wiring checked by a qualified electrician.
Quick reference
Sump pump horsepower chart: HP by flow
A sump pump sizing chart is a rough guideline once you know the flow your pump must move. The right choice also depends on total head — deeper lifts and longer runs push you toward the higher end, which is why the calculator bumps the horsepower when the head is high.
| Required flow | Typical HP | Common use |
|---|---|---|
| Under 20 GPM | 1/4 HP | Low inflow, shallow lift |
| 20–40 GPM | 1/3 HP | Most basements |
| 40–60 GPM | 1/2 HP | Higher inflow or deeper lift |
| 60–80 GPM | 3/4 HP | Heavy water, tall lift |
| Over 80 GPM | 1 HP+ | Very high inflow or long runs |
How to measure
Get the two numbers that matter
- Inflow: during heavy rain, with the power off and hands clear of the pit, mark the water level and time how many inches it rises per minute.
- Vertical lift: measure from the water level in the pit to the highest point the discharge pipe reaches before it heads outside.
- Horizontal run: measure the length of pipe from that high point out to the discharge, and note whether it's 1½" or 2".
- Basin size: a bigger basin gives the pump a longer run between starts, which matters when inflow is heavy.
A bigger pump isn't always the answer
If the pit fills faster than any single pump can clear, the fix is usually more basin, a second pump, or reducing the water reaching the sump with drainage and downspout work — not just more horsepower. And every primary pump still needs a backup for the power outages that come with the same storms.
Questions homeowners ask
Sump pump sizing FAQs
What size sump pump do I need?
Size the pump to the water entering the pit at the head it must pump against. Measure the pit's inflow in gallons per minute, add the vertical lift plus pipe friction for total dynamic head, then choose a pump that still delivers that GPM at that head — roughly 1/3 to 1/2 HP for most basements, more for deep lifts or heavy inflow.
How do I measure my pit's inflow rate?
During heavy rain, with the power off and your hands well clear of the pit, mark the water level and time how many inches it rises per minute. Multiply that rise by the basin's area to get the inflow in gallons per minute. Even a two- to three-minute reading is enough to size from.
What is total dynamic head, and why does it matter?
Total dynamic head is everything the pump works against: the vertical lift from the pit to the pipe's high point, plus the friction of the pipe run and fittings. It matters because a pump's advertised gallons-per-hour is measured at zero lift — at 8 to 10 feet of real head it may move only half that.
How do you determine sump pump size step by step?
Four steps. First, measure the inflow: time how fast the pit rises during rain and convert it to gallons per minute. Second, add up the head: vertical lift from the pump to the pipe's high point, plus an allowance for pipe length and fittings. Third, read the pump's flow-versus-head curve and pick a pump that still delivers your GPM at your head. Fourth, check that the discharge pipe is large enough for that flow. The calculator above does all four from your measurements.
How do I calculate sump pump head?
Start with static head: the vertical distance from the pump in the pit to the highest point of the discharge line, usually 8 to 12 feet for a basement. Then add friction head for the pipe run, roughly 1 to 2 feet of head per 10 feet of 1½-inch pipe at typical flows, plus about 1 to 2 feet for each check valve or elbow. The total is the total dynamic head you size the pump at.
How many horsepower do I need — 1/4, 1/3, or 1/2?
Most basements land at 1/3 or 1/2 HP. Under about 20 GPM a 1/4 HP pump can be enough; 40 to 60 GPM points to 1/2 HP, and higher inflow or a tall lift moves you to 3/4 HP or more. The calculator raises the recommendation when your total head is high.
What size discharge pipe should a sump pump have?
Most residential pumps use a 1½-inch discharge, but higher flow or a long run often needs 2 inches. An undersized line makes the water move too fast, which raises friction and cuts the flow the pump can actually deliver — the calculator flags when your pipe is the bottleneck.
How big should the sump basin (sump pit) be?
The sump pit size calculation is simple: big enough that the pump isn't switching on and off every few seconds during heavy rain. An 18-inch by 22- to 24-inch basin is common. Too small a basin fills fast, short-cycles the pump, and wears the motor out early.
Does a battery backup pump need to be the same size?
A battery backup moves less water than a primary pump at the same head, so treat it as insurance for power outages, not a full-capacity replacement. Size the primary pump for the storm and add a backup sized to keep the basement safe until power returns.