Sump pump sizing: HP, gallons per hour, and how high is your head?
Match the pump to your basin's total dynamic head and inflow rate — not the gallons-per-hour number on the box, because that number is only measured at one test height.
Basement already flooding right now? Stop and go to power-off safety first — do not step into standing water while the power is on. Confirm the panel is dry, clear of water and safely reachable before switching off basement circuits; if it's wet or unreachable without stepping in water, call your electric utility to cut power at the meter. See first-60-minutes and sump pump failure for that sequence. This page is for choosing a replacement or backup pump once the water is under control.
The number on the box isn't the number that matters
Every sump pump is printed with a gallons-per-hour (GPH) rating and a horsepower (HP) figure, and it's tempting to shop by the biggest GPH number. But GPH is measured at one specific total dynamic head — the vertical lift from the water surface in your basin to the point the discharge pipe exits above ground, plus the friction loss added by pipe length and every elbow in the run. Raise the head — a deeper basement, a longer or bendier discharge line — and the same pump pushes measurably less water per hour than the number on the box. A pump's real output at your basin's actual head is printed on its performance chart (sometimes called a pump curve), which manufacturers publish alongside the headline GPH figure — that chart, not the box, is what tells you what the pump will actually do in your basement.
What water are you moving?
Before sizing, know what's in the pit. Groundwater that seeps into a sump basin picks up soil and floor grime, so basement sump water is typically at least Category 2 “grey water” (IICRC S500) under the industry's water-restoration standard — not clean. If the pit is tied to a sewage ejector, or water is arriving up a floor drain rather than through the basin itself, treat it as Category 3 “black water” (sewage-contaminated) and see who pays when the sewer backs up before you do anything else — that's a different problem than an undersized pump.
Sizing by horsepower and head
| Pump | Handles | Best for |
|---|---|---|
| 1/3 HP submersible | Shallow-to-moderate vertical lift, a shorter, straighter discharge run | The common case: a single-level basement with a standard basin depth |
| 1/2 HP submersible | Deeper lift, a longer discharge run, or more elbows | Walkout or multi-level foundations; basins that fill fast in heavy rain |
| 3/4 HP+ submersible | High total dynamic head — a long, high, elbow-heavy discharge run | Unusually deep basements or long horizontal runs to daylight; verify against the pump's own performance chart before buying |
These are starting points, not a formula — read the specific pump's performance chart at your basin's measured head before buying. A pump sized well past what your basin actually needs cycles more than necessary, which wears the motor and float switch faster without moving water any faster.
How to measure your own head before you shop
- Measure straight up from the normal water line in your basin to the point the discharge pipe exits the house above grade — that's your vertical lift.
- Count the elbows and estimate the total horizontal pipe run — each elbow and extra foot of pipe adds friction loss on top of the vertical lift.
- Add the two together for your rough total dynamic head, then compare it against the performance chart (not the headline GPH) of any pump you're considering.
- If your basin fills unusually fast in a heavy storm, size toward the higher end of the horsepower range above rather than the low end — inflow rate matters as much as head.
The float switch and the check valve matter as much as the motor
A float switch (the mechanism that turns the pump on as water rises) that's tethered loosely or sits crooked in a narrow basin can jam against the wall and never trigger — a vertical or wide-angle float switch is worth the extra cost in a tight basin. A check valve installed in the discharge line just above the pump stops water already in the pipe from draining back into the basin every time the pump shuts off, which otherwise makes the pump short-cycle (turn on and off far more often than the actual inflow requires) and wears it out early.
The outage case: why a backup pump is a separate decision
Sizing solves how much water a pump moves; it doesn't solve what happens when the power goes out mid-storm — the single most common time a sump pump is needed and fails at once, because the storm causes both. A correctly sized primary pump paired with a battery-backup or water-powered backup pump covers that gap; the choice between those two backup types is its own decision with a real trade-off (battery runtime and maintenance versus municipal water pressure and ongoing water use), which we cover in full in sump pump failure.
Who this doesn't fit
If you rent, the sump pump and its maintenance are the landlord's responsibility — report a failing or undersized pump in writing so there's a record, rather than buying a replacement yourself. In a condo, the association's master policy typically covers building systems like a shared sump pit, while your own policy covers what's inside your unit — confirm who owns the fix before you spend. A pump you install yourself in a rented or shared building may not be yours to keep, and won't transfer responsibility for a system the building manages.
Common questions
What size sump pump do I need?
Start from two numbers, not the box's gallons-per-hour (GPH) figure alone: your basin's total dynamic head (the vertical lift from the water surface to the discharge point outside, plus friction loss from pipe length and elbows), and how fast water actually enters your pit during a heavy rain. A 1/3 HP pump handles the vertical lift and inflow rate most single-family basements see; step up to 1/2 HP or larger for a deep lift (a walkout or multi-level foundation), a long or elbow-heavy discharge run, or a pit that fills fast during storms. The GPH printed on the box is only measured at one reference head — read the pump's own performance chart at your actual lift, because the same pump moves less water per hour as the lift and pipe run get longer.
Why does my pump's gallons-per-hour rating not match what I'm seeing?
Because that number is measured at a single test height, and your basin almost certainly isn't at that height. Total dynamic head is the vertical distance the pump lifts water plus the friction loss added by pipe length and each elbow in the discharge line — and the same pump pushes less water per hour as head increases. A pump rated for 3,000 GPH at a 10-foot head might move meaningfully less at 15 feet with two 90-degree elbows. Manufacturers publish a pump curve (a chart of GPH at different head heights) — that chart, not the headline number, tells you what your pump actually does in your basement.
Is a bigger horsepower pump always better?
No. A pump sized well past your basin's actual inflow and head cycles on and off more often than it needs to, which wears the motor and the float switch faster, and it doesn't empty a pit any safer than a correctly matched pump. Horsepower should match your total dynamic head and expected inflow rate — a 1/3 HP pump that's correctly sized for a shallow-lift, moderate-inflow basement will outlast an oversized 3/4 HP pump doing the same job. The exception is a basin with a long, high-head, elbow-heavy discharge run, where more horsepower is genuinely needed to reach the same real-world GPH a smaller pump gets at a shallower lift.
My basement is already flooding — should I read this page first?
No — stop and go to the safety steps first. Do not step into a flooding basement while the power is on: the pump, the panel, outlets, and a furnace or water heater may be in or near the water. If your panel is dry, clear of water and safely reachable, switch off the basement circuits or the main breaker; if the panel itself is wet or you'd have to stand in water to reach it, call your electric utility to cut power at the meter and wait for confirmation. Only once power is confirmed off do you go down to extract. Our first-60-minutes checklist and sump-pump-failure guide walk through that order — this sizing page is for choosing a replacement or backup pump once the immediate water is handled.
How to shop for a sump pump (what spec to look for)
Disclosure: each link below runs an Amazon search for that type of item — we're pointing you to the right category to look in, not a specific product we've tested or endorse. What the search returns is Amazon's own ranking for that moment and can include sponsored placements and off-category listings we neither control nor vouch for, so read the notes below on what to look for and check reviews before you buy. They're affiliate links: as an Amazon Associate, Restoration Answers earns from qualifying purchases if you buy through them, at no extra cost to you, and you're always free to buy the same items anywhere else.
- 1/3 HP cast-iron submersible sump pump (Amazon search) - the common size for a shallow-to-moderate lift with a shorter discharge run — check the listing's performance chart against your measured head, not just the headline GPH
- 1/2 HP submersible sump pump (Amazon search) - the step up for a deeper lift, a longer discharge run, or a basin that fills fast in heavy rain
- Vertical or wide-angle float switch (Amazon search) - less prone to jamming against the wall of a narrow basin than a tethered float, so the pump actually triggers when water rises
- Sump pump discharge check valve (Amazon search) - stops water in the pipe from draining back into the basin between cycles, which otherwise short-cycles and wears the motor early
- Battery backup sump pump system (Amazon search) - covers the outage case a primary pump's sizing can't — the storm that floods the basement is often the same one that cuts the power
Keep reading
Sources & methodology
Every published figure on this page comes from the references below. Where we adjust a national figure for local costs we say so and show the multiplier — we never present our own estimate as if it were sourced. Costs are national ranges; your local market, access, and materials move real quotes in both directions.
- IICRC — S500 Standard for Professional Water Damage Restoration (water categories)
- Insurance Information Institute — Homeowners & renters insurance (water backup/sump overflow endorsement)
- FEMA — Protecting Your Home (basement pump-down and flood protection guidance)
- CDC — What to Do to Protect Yourself During a Power Outage (generator carbon monoxide safety)
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This is general information, not insurance, legal, or engineering advice. Estimates are planning ranges, not quotes — always get on-site assessments, and confirm coverage against your own policy wording or with your insurer.