What size sump pump do I need?

It is not a horsepower question, it is two numbers: how many gallons a minute arrive in the pit, and how many feet of head the pump has to push against. A pump is then chosen off its published curve, which shows what it still moves at that head. The advertised headline flow on the box is measured at almost no lift and is not the figure to buy on. A pump rated at 60 gallons a minute may only move 35 at ten feet.

How do I do the one-minute test?

Wait for a properly wet day, because the answer on a dry week is meaningless. Run the pump until the water drops to its shutoff level, switch it off, wait exactly one minute, then measure with a ruler how far the water rose. In an 18-inch pit each inch is one gallon, and in a 24-inch pit each inch is roughly two. Six inches of rise in an 18-inch pit is six gallons a minute.

What is total dynamic head, in plain words?

How hard the pump has to work to get the water out. It is the vertical lift from the pump to where the pipe turns horizontal, plus the friction of pushing water along the pipe and round every corner. The published formula is static head plus friction head, where friction head is the actual pipe length plus the equivalent length of the fittings, multiplied by the friction loss per hundred feet and divided by a hundred.

Why does the calculator ask me to look up the friction loss?

Because that one number depends on pipe diameter as well as flow rate, and no readable published table covers both without ambiguity. Inventing a constant to sit inside a formula is the same defect as inventing a price: it looks authoritative and nobody can check it. The field is pre-filled with the source's own worked example, 5.25 feet per 100 feet for 18 gallons a minute through inch-and-a-quarter pipe, and labelled as exactly that. Your pump maker publishes the real table.

Is a bigger pump always better?

No, and oversizing has its own failure mode. A pump too large for the pit empties it in seconds, shuts off, refills and starts again, and that short-cycling wears out the switch and the motor faster than steady running does. The published advice is that above 30 gallons a minute you want a wider pit rather than simply a stronger pump, precisely so the cycle has room to be a cycle.

Does the discharge pipe size matter?

A great deal, because friction rises steeply as the pipe narrows. The published method notes that going up a pipe size can lower the friction head enough to let you use a smaller, cheaper pump. If your total dynamic head is coming out awkwardly high, widening the discharge line is often a better answer than buying more horsepower.

What size sump pump do you actually need?

Last updated: August 26, 2026By Below-Grade Records Desk

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Horsepower is the wrong question. What selects a sump pump is how much water arrives in the pit each minute, and how far the pump has to lift it against how much pipe friction. Get those two numbers and a pump curve answers the rest. The first one you measure with a ruler and a watch, on a wet day, in about ninety seconds.

1. How much water arrives (system capacity)

Is there a pit in the floor already?

Yes, and I can do the one-minute testOn a properly wet day, run the pump until it shuts off, wait exactly one minute with it off, then measure how far the water rose.No, nothing is built yetThen it is estimated from the footprint of the house and the soil it sits in.

How far did the water rise in 60 seconds?

inches

How wide is the pit?

18 inch diameter24 inch diameter

Most houses have the 18-inch pit, where one inch of water is one gallon. In a 24-inch pit an inch is roughly two.

System capacity

6.0 US gal/min

That is 360 gallons an hour, or 22.7 litres a minute. Pump curves in North America are published in US gallons, so that is the figure to shop with.

2. How hard it is to push out (total dynamic head)

Vertical lift

From the pump up to where the pipe turns horizontal.

feet

Discharge pipe length

The real run, horizontal included.

feet

90-degree elbows

elbows

There is a check valve in the lineThere should be, or the water in the pipe falls back into the pit every time the pump stops. It adds 11.5 feet of equivalent pipe.

Friction loss, in feet per 100 feet of pipe

This one you look up, because it depends on your pipe diameter AND your flow rate. Read it off your pump maker's friction table using 6.0 gal/min. Pre-filled with the published worked example: 5.25 for 18 gal/min through 1¼-inch pipe.

ft per 100 ft

Total dynamic head

11.73 feet

Equivalent pipe from fittings3.5 ft per elbow, 11.5 ft per check valve: 22.0 ft
Friction head: 2.73 ft
Vertical lift (static head): 9.0 ft

What to shop for: a pump that still moves 6.0 US gal/min at 12 feet of head. Every pump publishes a curve; the advertised headline flow is at almost no head and is not the number that matters.

Every rule here is quoted from one published method and none of it is invented. The friction-loss figure is the one number this tool will not look up for you: it depends on pipe diameter as well as flow, and the source’s own tables are published as images. Guessing a constant inside a formula is the same defect as guessing a price.

Method

Where this method comes from

Every rule in the calculator is quoted from a single published trade method, “Sizing Up a Sump Pump” in WATERPROOF! Magazine, and WaterproofingIndex has not added a step to it. The one-minute test, the gallons-per-inch figures for both basin sizes, the 14 and 8 gallons per 1,000 square feet planning rates for sandy and clay soil, the 30-gallon threshold for a wider pit, the head formula and the equivalent lengths for elbows and check valves are all its numbers.

The one thing this page will not do for you is look up the friction loss. That figure varies with pipe diameter as well as flow, and the article's own tables are published as images rather than text. Rather than substitute a constant from somewhere else and present it as part of a cited method, the field asks for the figure and is pre-filled with the article's own worked example, labelled as such. A fabricated constant inside a formula is harder to spot than a fabricated price and does the same damage.

One local note the source does not make: much of the GTA sits on clay, which is why the clay planning rate rather than the sandy one is usually the relevant one for a house here that has not been built yet.

Record

Doing the one-minute test properly

Do it in the rain. The number you want is the wet-day number. A test in August tells you what the pump needs to handle in August.
Start from the shutoff level. Run the pump out first, then time from the moment it stops. Measuring from a half-full pit measures the wrong thing.
Use a real minute. Sixty seconds on a phone. The whole calculation scales off this one measurement.
Never let the water reach the inlet pipe. If it rises above the bottom of the pipe coming in from the drain tile, stop the test and run the pump.
Measure the pit too. Eighteen inches and twenty-four inches look similar in a dark corner and the answer differs by a factor of two.

Frequently asked questions

What size sump pump do I need?: It is not a horsepower question, it is two numbers: how many gallons a minute arrive in the pit, and how many feet of head the pump has to push against. A pump is then chosen off its published curve, which shows what it still moves at that head. The advertised headline flow on the box is measured at almost no lift and is not the figure to buy on. A pump rated at 60 gallons a minute may only move 35 at ten feet.
How do I do the one-minute test?: Wait for a properly wet day, because the answer on a dry week is meaningless. Run the pump until the water drops to its shutoff level, switch it off, wait exactly one minute, then measure with a ruler how far the water rose. In an 18-inch pit each inch is one gallon, and in a 24-inch pit each inch is roughly two. Six inches of rise in an 18-inch pit is six gallons a minute.
What is total dynamic head, in plain words?: How hard the pump has to work to get the water out. It is the vertical lift from the pump to where the pipe turns horizontal, plus the friction of pushing water along the pipe and round every corner. The published formula is static head plus friction head, where friction head is the actual pipe length plus the equivalent length of the fittings, multiplied by the friction loss per hundred feet and divided by a hundred.
Why does the calculator ask me to look up the friction loss?: Because that one number depends on pipe diameter as well as flow rate, and no readable published table covers both without ambiguity. Inventing a constant to sit inside a formula is the same defect as inventing a price: it looks authoritative and nobody can check it. The field is pre-filled with the source's own worked example, 5.25 feet per 100 feet for 18 gallons a minute through inch-and-a-quarter pipe, and labelled as exactly that. Your pump maker publishes the real table.
Is a bigger pump always better?: No, and oversizing has its own failure mode. A pump too large for the pit empties it in seconds, shuts off, refills and starts again, and that short-cycling wears out the switch and the motor faster than steady running does. The published advice is that above 30 gallons a minute you want a wider pit rather than simply a stronger pump, precisely so the cycle has room to be a cycle.
Does the discharge pipe size matter?: A great deal, because friction rises steeply as the pipe narrows. The published method notes that going up a pipe size can lower the friction head enough to let you use a smaller, cheaper pump. If your total dynamic head is coming out awkwardly high, widening the discharge line is often a better answer than buying more horsepower.

Sources

Where these numbers came from

This is a sizing tool, so it carries a method rather than prices. The whole method comes from one published trade source, read on 26 August 2026, and every constant on this page is quoted from it.

WATERPROOF! Magazine, Sizing Up a Sump Pump (the method, the one-minute test, the soil rates and the head formula)

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