What size sump pump do I need?

1. Why horsepower is the wrong starting point

Walk the pump aisle and everything is labelled by horsepower — 1/4, 1/3, 1/2, 3/4 HP — as if a bigger motor is simply better. It is not. Horsepower describes the motor, not the job. The job is to remove water at least as fast as it arrives, and how fast it arrives depends on your soil, your water table and the storm, none of which are printed on the box. Two houses with identical 1/3 HP pumps can be worlds apart: one stays dry through a hurricane, the other floods in an ordinary downpour, because their inflow rates differ. Sizing starts with measuring the water, and only then choosing a pump whose real, charted output beats it.

The second trap is the headline flow rating. Manufacturers advertise the maximum gallons per hour a pump can move, but that figure is measured at zero lift — water going straight out the top with nothing to climb. In a real basement the pump has to push water up several feet and along a discharge pipe, and every foot of that total head cuts the flow. A pump that claims 3,000 GPH at 0 feet may deliver closer to 2,000 GPH at 10 feet. That is why the number this method produces is a flow you must meet at your head, read from the pump's own performance chart, not from the sticker.

2. Step one — the pit test

The only honest way to know your inflow is to measure it, and the best time is during or right after a heavy rain, when the ground is saturated and the water table is high. Let the pit fill until the float would normally switch the pump on, then switch the pump off (or unplug it) and watch the water. With a tape measure or a marked stick, note how many inches — or millimetres — the water climbs in exactly one minute. That rise per minute is the raw measurement everything else builds on. Keep an eye on the pit so it does not overflow; a minute is usually plenty, and you can switch the pump back on the moment you have your number.

Test in the worst conditions you realistically get, not on a dry week, because you are sizing for the storm that would otherwise flood you. If you cannot catch a big rain, size conservatively and lean on the safety factor. Homes near the bottom of a slope, with a high water table, or on clay that sheds water toward the foundation, all tend to show fast rises and need more capacity.

3. Step two — turn the rise into an inflow rate

A rise of so many inches per minute only becomes a flow rate once you know how much water one inch of the pit holds. For a round pit that is pure geometry — the cross-sectional area times one inch of depth:

volume per inch = π ÷ 4 × diameter² (then converted to gallons or litres)

A standard 18-inch liner works out to about 1.10 gallons per inch; a big 24-inch pit holds nearly twice that. Multiply your measured rise per minute by this figure and you have the inflow rate — gallons per minute — then multiply by 60 for gallons per hour. This table gives the volume per inch (and per millimetre) for the common round pit sizes, all from the exact geometry, so you can check the calculator or work by hand:

4. Step three — add a safety factor

Sizing a pump to exactly match the inflow leaves no room for error — a slightly worse storm, a partly clogged intake, or an ageing motor and the pit overflows. The common rule of thumb is to multiply the inflow by a safety factor of about 1.5, giving the pump headroom to keep ahead of the water and cycle less frantically. This is a practical margin, not a code requirement, and you can adjust it: a critical finished basement in a flood-prone area might use a larger factor, while a rarely-wet crawlspace could use less. The calculator lets you set it.

required capacity = inflow per hour × safety factor

That is the number to shop with — but remember it is a flow the pump must hit at your total head, so the last step is to read the pump's performance chart at your lift, not its zero-lift maximum.

5. Worked examples

A standard basement, 18-inch pit. The water rises 8 inches in a minute in a heavy storm. An 18-inch pit is 1.10 gallons per inch, so the inflow is 8 × 1.10 = 8.81 gallons per minute, or about 529 gallons per hour. Apply a 1.5 safety factor and you need a pump delivering about 793 gallons per hour at your discharge height. A common 1/3 HP pump meets that comfortably at a 10-foot lift — but confirm it on the chart, don't assume.

A wide 24-inch pit, gentler rise. A 24-inch pit holds 1.96 gallons per inch. If the water rises 5 inches a minute, inflow is 5 × 1.96 = 9.79 gallons per minute (about 588 GPH); with a 1.5 factor the required capacity is about 881 gallons per hour. Note how a slower rise in a bigger pit still adds up — the wider pit collects more per inch.

A metric example. A 450-mm pit rising 200 mm in a minute holds 0.159 litres per mm, so inflow is 200 × 0.159 = 31.8 litres per minute — about 1,909 litres per hour. With a 1.5 safety factor that is roughly 2,863 litres per hour the pump must move at its lift. Enter any of these in the calculator to see the full breakdown.

6. From required flow to the right pump

With a required capacity in hand, choosing the pump is a matter of reading charts, not guessing horsepower. Every quality pump publishes a performance curve: gallons (or litres) per hour at 5 feet, 10 feet, 15 feet of lift and so on. Find the row nearest your total head and check the flow beats your required capacity; if it does, the horsepower on the label is simply whatever that pump happens to be. Where two pumps both meet the flow, the smaller motor is usually the more efficient and longer-lived choice.

Your total head is more than the vertical drop from pit to the outside — it also includes friction in the discharge pipe and every elbow and check valve. To work that out properly, use the pump head calculator for total dynamic head and the pipe friction loss calculator for the losses along the run. A wider, smoother discharge pipe and fewer sharp bends lower the head, which lets a smaller pump deliver the same flow.

Finally, keep this in perspective: the method sizes the pump to your measured inflow, but it does not replace good drainage, a working check valve, a properly set float, or — in a flood-prone home — a battery or water-powered backup for when the power fails mid-storm. Size the pump correctly first, then build the redundancy around it. The rest of the water tools on the plumbing hub follow the same measure-first approach.

Common questions

What size sump pump do I need?

Size it to your inflow, not to a horsepower number. Measure how many inches the water rises in the pit in one minute during heavy rain, multiply by the pit's gallons-per-inch (about 1.1 for an 18-inch pit), multiply by 60 for gallons per hour, then add a safety factor of about 1.5×. That is the capacity the pump must deliver at your discharge height.

How do I do the sump pump pit test?

In heavy rain, let the pit fill to the pump's turn-on level, switch the pump off, and time how far the water rises in exactly one minute. That rise per minute, times the pit's cross-section, is your inflow rate. Test in the worst weather you get — you are sizing for the storm.

Is a 1/3 HP or 1/2 HP sump pump better?

Horsepower is not a capacity. A 1/3 HP pump suits most ordinary homes; move to 1/2 HP or more when inflow is high, the water table is deep, the lift is tall, or the discharge run is long — all of which reduce real output. Match the pump's published flow at your head to the required capacity.

Why does the horsepower rating overstate what I get?

The headline flow is measured at zero lift. Every foot of vertical lift, plus friction in the pipe and elbows, cuts the flow, so a pump rated at 3,000 GPH at 0 feet delivers far less at 10 feet. Always read the chart row for your total head.

How many gallons per hour should a sump pump handle?

It depends entirely on your inflow. A modest pit might need a few hundred GPH; a high water table in a downpour can demand a few thousand. Size to your measured inflow times a safety factor, then choose the next pump up whose chart meets that flow at your lift.