Home › What Size Sump Pump

What Size Sump Pump

Quick answer: The right sump pump size is the one that moves your basement's peak inflow, plus 25–50% margin, at your actual total dynamic head. Measure how fast the pit fills in heavy rain, convert to gallons per hour, estimate lift plus friction, and choose a pump whose published curve meets that point. Horsepower alone does not answer it.

Why "what size" is really two questions

When people ask what size sump pump they need, they usually mean horsepower: 1/4, 1/3, 1/2 or 3/4 HP. Horsepower is a convenient label on the box, but it does not tell you how much water a pump will move in your basement. Two 1/3 HP pumps from different manufacturers can deliver very different flows at ten feet of head because of impeller design, motor speed and discharge size. The useful question is: how many gallons per hour must leave my pit, and against how many feet of head? Answer that and horsepower becomes a by-product.

The first half — inflow — depends on your soil, your lot grading, the depth of your footing drains, the water table, roof drainage and the weather. The second half — head — depends on your plumbing: how far the water climbs, how long the pipe is, how many elbows it turns and what kind of check valve it passes through. A sandy lot on a hill with a short discharge might need a small pump. A clay lot at the bottom of a slope with a discharge that climbs eleven feet and runs forty feet to daylight needs much more.

Step 1: measure your inflow on a wet day

Inflow is best measured during the conditions you are sizing for — a heavy rain, a spring melt or the day after a storm when the water table is high. A dry August reading tells you almost nothing.

If the pump is running continuously and never catches up, your inflow is greater than its output at your head. That is a strong sign you need more capacity, a second pump, or a plumber to look at the drainage system itself.

Step 2: estimate total dynamic head

Total dynamic head (TDH) is vertical lift plus friction. Vertical lift is measured from the water level in the pit (not the pit floor) to the highest point the discharge pipe reaches, usually where it passes through the rim joist. Friction comes from pipe length and fittings. A common planning shortcut adds one foot of head for every ten feet of horizontal pipe in 1½-inch PVC at typical residential flow, plus an allowance per elbow and for the check valve. The TDH calculator applies those rules of thumb and labels the result as an estimate.

Step 3: add a sensible margin

One storm is a sample. Next spring may be wetter, footing drains can silt up and change flow, and pumps lose some performance as they age. Many planners add 25% for a typical lot and up to 50% where flooding has happened before, the basement is finished, or the pit is small. The margin is a judgment call, not a law of physics; a licensed plumber who has seen your neighbourhood can refine it.

Step 4: read the pump curve, not the headline number

Every reputable sump pump publishes a performance table or curve showing GPH at 5, 10, 15 and 20 feet, and a "shut-off head" where flow drops to zero. Find your TDH on that table and check the flow. If the flow at your head is comfortably above your target, the pump fits. If it only meets the target at a lower head, keep looking. Avoid choosing a pump whose shut-off head is only a few feet above your TDH; near shut-off, flow collapses quickly.

Worked example

A homeowner in a two-storey house measures a 24-inch pit, which the calculator puts at about 2.0 gallons per inch. During a March thaw the water rises 3 inches in 60 seconds: about 6 GPM or 360 GPH. Because the basement is finished, they add 50%, targeting 540 GPH. The discharge rises 8 feet from the water line to the rim joist and then runs 30 feet underground to a pop-up emitter, through a check valve and four elbows. The TDH estimate is roughly 12 feet. A pump curve showing 1,800 GPH at 10 feet and 1,300 GPH at 15 feet clearly clears the target. In this case, the bigger question becomes pit size: at 2 gallons per inch, a pump moving more than 1,300 GPH empties the pit very quickly, so switch type and cycle length matter more than raw power.

Illustrative note (2026-10-29): This worked example was prepared on 2026-10-29 for illustration only. The pit, inflow, head and pump curve values are hypothetical and do not describe any specific product or property.

Common sizing mistakes

When the answer is not "a bigger pump"

If inflow seems enormous, a plumber or waterproofing contractor may find a broken downspout connection, poor grading, a blocked footing drain or groundwater from a neighbour's yard. Fixing the source can be cheaper and more reliable than adding horsepower. Likewise, if the pump keeps up but you worry about power outages, a battery backup protects you more than an upgrade.

Frequently asked questions

Is a 1/3 HP sump pump enough?

For many homes with moderate inflow and 10 feet or less of head, yes. Check the curve at your head; see HP explained.

Should I size for the worst storm ever?

Size for a realistic heavy event with margin, then add backup and an alarm for the rare extreme. No single pump guarantees a dry basement.

Who should confirm my sizing?

A licensed plumber, plus a licensed electrician if circuits or a generator connection are involved.

Related pages