SumpPumpSizer: free sumppumpsizer planning calculators
Quick answer: Most basements need a sump pump that moves your measured peak inflow, plus a safety margin, at your real total dynamic head — not at zero feet. Time a full pit cycle in heavy rain, convert it to gallons per hour, add vertical lift and pipe friction, then pick a pump whose curve clears that point. A plumber confirms.
Start with the sump pump size calculator
SumpPumpSizer is a small set of free planning tools for homeowners who want to understand what their basement actually asks of a pump before they talk to a plumber or walk into a store. The sump pump size calculator turns a simple field measurement — how fast the water rises in your pit during a storm — into a target flow in gallons per hour (GPH). The total dynamic head calculator tells you how hard the pump has to push, and the pit volume calculator shows how many gallons sit in each inch of your basin. Together those three numbers describe the job. Everything else on the site explains how to choose, back up, test and maintain the pump that does it.
We built the tools around one idea that box labels tend to hide: a pump rated at 3,000 GPH is usually rated at zero feet of lift. Your basement is never zero feet. Water has to climb out of the pit, travel through a check valve and elbows, run along the joists and exit the rim joist several feet above the floor. Every foot of lift and every fitting eats into the flow the pump can deliver. Sizing is therefore a matching exercise between two curves — what your house sends in and what the pump can push out at your head.
The three numbers that size a sump pump
- Peak inflow (GPH). Measure during or right after heavy rain or snowmelt. Note the water level, unplug the pump for one minute (or time a natural fill between cycles), measure the rise in inches and multiply by your pit's gallons per inch. Multiply gallons per minute by 60 for GPH.
- Total dynamic head (feet). Vertical lift from the water surface in the pit to the highest point of the discharge, plus an allowance for friction in the pipe, the check valve and each elbow. Most homes land between 8 and 15 feet.
- Safety margin. Inflow measured on a single storm is a sample, not a maximum. Many planners add 25–50% so the pump is not running flat out on the worst night of the year.
Worked example
A family in a 1970s split-level measures an 18-inch-diameter pit. Our pit calculator gives roughly 1.1 gallons per inch. During a spring thaw the water rises 6 inches in one minute, so inflow is about 6.6 gallons per minute, or about 396 GPH. They add a 50% margin and get roughly 600 GPH. The discharge climbs 9 feet to the rim joist and runs 25 feet of 1½-inch pipe with a check valve and three elbows; the head calculator estimates about 11–12 feet of total dynamic head. They now shop for a pump whose published performance curve shows at least 600 GPH at 12 feet — and they discover that a common 1/3 HP model delivers comfortably more than that, so a 1/2 HP unit is not automatically necessary.
Illustrative note (2026-10-29): The figures in this example were built on 2026-10-29 for teaching purposes. They are not measurements from a real home, a real pump test, or a specific brand. Use your own pit, your own storm and the manufacturer's curve.
Backup power and what happens when the lights go out
Storms that fill sump pits are often the same storms that knock out power. That is why the site includes a battery backup runtime calculator (amp-hours × volts ÷ pump watts, adjusted for duty cycle) and a generator size estimate that accounts for motor start surge. Read battery backup sump pumps and water-powered backups to compare the two main approaches, including the municipal water pressure and backflow requirements that water-powered units bring with them.
Choosing, installing and keeping it running
Once you know the duty point, the guides help you narrow the field. Horsepower explained shows why HP is a rough proxy and the curve is what matters. Submersible vs pedestal compares noise, lifespan and pit fit. The check valve guide covers short-cycling and water hammer. Cost pages for replacement and new installation break down the labour pieces that drive a quote. After it is in, test it each season, follow a maintenance routine, add a high-water alarm, and learn the warning signs of failure and typical lifespan.
Who this site is for
We write for homeowners in Canada and the United States with a basement, crawlspace or slab-on-grade home that relies on a sump. You might be replacing a pump that died, planning a finished basement, buying a house with a pit you do not understand, or trying to decide whether a battery backup is worth it. You do not need engineering training. Each calculator labels its outputs as planning estimates and shows the arithmetic so you can check it.
What we will not do
We do not promise that any pump, backup or alarm will keep your basement dry. Water tables, clogged footing drains, frozen discharge lines, sewer backups and overland flooding are all outside what a sump pump controls. Electrical work — dedicated circuits, GFCI protection, generator transfer switches — belongs to a licensed electrician. Plumbing changes, discharge routing and any connection to municipal water belong to a licensed plumber who knows your local code. Our job is to help you arrive at that conversation with good numbers and good questions.
How we keep the numbers honest
The rules of thumb on this site — gallons per inch of pit, friction allowances per fitting, start-surge multipliers, battery derating — are labelled as estimates and explained on the page where they are used. Where manufacturers publish real data, such as performance curves or battery reserve ratings, we tell you to trust that data over our defaults. Every page shows when its illustrative examples were prepared, and nothing on the site is a product endorsement.
Frequently asked questions
What size sump pump do most houses need?
Many homes are served by a 1/3 HP pump, and wetter lots or higher discharge points often need 1/2 HP. The honest answer is whatever clears your measured inflow at your head; see what size sump pump.
Is bigger always safer?
No. An oversized pump in a small pit short-cycles — it starts and stops so often that the switch and motor wear early. Matching the pump to the pit and adding backup is usually better than buying the largest motor.
Can I use these tools for a sewage ejector?
No. Sewage ejectors, grinder pumps and greywater systems have different code rules, venting and solids handling. Speak to a licensed plumber.
Do the calculators store my numbers?
No. They run in your browser and nothing is sent to us.