A sump pump only has one job: get water out of the pit faster than it comes in, and lift it high enough to leave the house. Sizing comes down to two numbers: how much water flows into the pit during a heavy storm, and how much head (lift) the pump has to work against. Horsepower matters, but only as a rough guide to which pump can deliver the flow you need at your head height.
The three numbers on a sump pump
- Horsepower (HP): the motor size. Common residential sizes are 1/4, 1/3, 3/10, 1/2 and 3/4 HP.
- Gallons per hour (GPH): how much water the pump moves. It is always listed at a specific head height.
- Head height (feet): how high the pump has to push the water. Every pump has a "shut-off head," the height at which flow drops to zero.
Two pumps with the same horsepower can have very different flow at 10 feet of head. Always compare GPH at your head height on the manufacturer's performance curve, not the headline horsepower.
Step 1: Measure how fast water enters the pit
The most reliable sizing number comes from your own pit during a heavy rain:
- Measure the inside diameter of the basin in inches.
- Unplug the pump (or watch right after it shuts off) and time how long the water takes to rise a set number of inches.
- Convert inches to gallons with this formula: gallons per inch = 3.14 × (radius in inches)² ÷ 231.
For a common 18-inch basin, the radius is 9 inches. So 3.14 × 81 ÷ 231 ≈ 1.1 gallons per inch of depth.
Example: In an 18-inch basin, the water rises 10 inches in one minute. That is about 11 gallons per minute, or about 660 gallons per hour. Choose a pump that moves comfortably more than that at your head height. Many installers add a margin of 50% or more for the worst storms.
Plug the pump back in as soon as you finish. Do not let the pit overflow during the test.
Step 2: Work out your total head
Total head is the vertical lift plus an allowance for friction in the pipe:
- Vertical lift: measure from the bottom of the pit (or the pump outlet) to the highest point of the discharge pipe, usually where it exits the rim joist or wall.
- Horizontal run: a common rule of thumb is to add 1 foot of head for every 10 feet of horizontal pipe.
- Fittings: elbows and the check valve add friction too. Long runs with several elbows can add a few more feet.
Example: 9 feet of vertical lift plus 20 feet of horizontal pipe gives about 9 + 2 = 11 feet of total head, plus a little for fittings. Look up each pump's GPH at about 11–12 feet.
Step 3: Match horsepower to the job
Once you know the flow and head, horsepower becomes a shortcut for narrowing the list. These are general guidelines. Confirm them against the pump curve.
| Situation | Typical choice |
|---|---|
| Light inflow, lift under about 10 ft, crawl space or occasional water | 1/4–1/3 HP |
| Average basement with moderate inflow, lift around 10 ft | 1/3 HP |
| High water table, frequent cycling, or lift of about 10–15 ft | 1/2 HP |
| Very high inflow or lift over about 15 ft | 1/2–3/4 HP or larger; check the curve carefully |
Bigger is not automatically better. An oversized pump in a small basin can run very short cycles, which wears the switch and motor. If you need a large pump, consider a larger basin as well.
Other choices that affect performance
Switch type
- Vertical float: moves up and down on a rod. It works well in narrow basins.
- Tethered float: needs a wider basin, but gives a longer pumping cycle.
- Vertical magnetic float (VMF) and electronic switches: have fewer moving parts to snag. Liberty's 257 (1/3 HP) and 287 (1/2 HP) use a VMF switch.
Housing material
Cast iron housings shed motor heat into the water and are common on pumps meant for frequent cycling. Thermoplastic and aluminum housings cost less and weigh less. Zoeller's Model 53 (3/10 HP) and Model 98 (1/2 HP) are examples with a 1-1/2" NPT discharge.
Discharge pipe and check valve
Most residential sump pumps use a 1-1/2" discharge. Reducing the pipe size adds friction and cuts flow. A check valve stops pumped water from falling back into the pit. Drill a small relief hole in the discharge between the pump and the check valve if the manufacturer recommends it, to prevent air lock.
Backup protection
A primary pump cannot help during a power outage or a stuck switch. A battery back-up sump pump installed beside the primary pump adds a second line of defense.
Replacing an existing sump pump
- If the old pump kept up but simply wore out, a pump with the same or slightly higher GPH at your head is a safe choice.
- If the old pump ran constantly or the pit nearly overflowed, use the inflow test in Step 1 and size up.
- Check the basin size before buying. Tethered floats need room to swing.
- Replace the check valve if it is old or noisy.
Compare submersible, crawl space and battery back-up models in our sump pumps collection.
Safety note: Sump pumps run on 115V power next to standing water. Plug them into a properly grounded outlet, and use GFCI protection where your local code requires it. Never handle a pump or reach into the pit while it is plugged in. Have a licensed electrician add or modify circuits, and a licensed plumber handle discharge connections where local rules require one.
Frequently asked questions
What size sump pump do I need for an average basement?
A 1/3 HP submersible is a common choice for an average basement with about 10 feet of lift. Confirm by checking the pump's GPH at your actual head height against your measured inflow.
Is a 1/2 HP sump pump better than 1/3 HP?
It moves more water and handles more head, which helps with a high water table or a tall lift. In a small basin with light inflow, it can short-cycle. Size to the job.
How many gallons per hour should a sump pump move?
Enough to stay ahead of your heaviest inflow at your total head, with a margin. Measure your inflow with the pit test in Step 1.
Can I use a sump pump for sewage?
No. Sump pumps are built for clear groundwater. Wastewater with solids needs a sewage or effluent pump rated for it.