No source gives one correct sump pump size, so this tool checks a pump against your own pit and your own water: the pit volume, how fast it fills, the pump flow at your lift and a horsepower rule of thumb.
What no source can tell you
North Dakota State University Extension answers the question “What size pump should I have for my house?” with “there is no correct size.” It says the horsepower depends on several factors: the length of drain tile connected to the sump, the lift from the sump to the discharge pipe and the length of hose or pipe outside the house. One extension note adds the area of drainage connected to the sump, the depth to groundwater and the depth of the basement. None of them gives a formula that turns those into gallons per minute, so this tool does not invent one. It checks a pump you are considering, using numbers you can measure.
What the sources do give
- Rule of thumb. A one-third horsepower pump is satisfactory for most houses, and with more than 40 feet of pipe outside the house the extension suggests a pump with more horsepower: one-half, three-quarters or even one.
- Flow versus lift. Most new pumps come with a chart or graph showing flow versus height of lift. Flow is given in gallons per minute or gallons per hour (multiply gpm by 60 to get gph), and lift is the vertical feet.
- Half versus third. A one-half horsepower pump will pump more water and lift it higher than a one-third horsepower pump in similar conditions, and where flow becomes rapid it may keep up when a one-third may not.
- The pit. The extension describes a pit about 2 feet in diameter, with room for the pump and piping and for about 15 to 25 gallons between pumping events.
- Discharge. Preferably at least 20 feet from the house, draining away from it, with a check valve on the outlet pipe so water does not flow back.
Water held (gal) = gallons per inch × rise between pump off and on
Inflow (gpm) = water held ÷ minutes between pump off and on
Run time (min) = water held ÷ (pump flow − inflow)
Cycles per hour = 60 ÷ (fill minutes + run minutes)
Worked example
A 24 inch pit where the water rises 10 inches between pump off and pump on, refilling in 4 minutes during a storm. The pump chart shows 30 gpm at the lift
Gallons per inch = π × 12² ÷ 231 = 1.96. Water held = 1.96 × 10 = 19.6 gallons.
Inflow = 19.6 ÷ 4 = 4.9 gpm. Pump flow minus inflow = 30 − 4.9 = 25.1 gpm.
Run time = 19.6 ÷ 25.1 = 0.78 minutes, about 47 seconds, every 4.8 minutes, or about 12.6 cycles per hour.
Reading the result
A pump whose flow at your lift does not beat the inflow cannot keep the pit from rising, and the tool says so. A pump that runs for a small fraction of each cycle has room to spare. How much room is enough is not published, so the tool shows the ratio and leaves the judgment to you and your plumber. Time the cycle in the heaviest rain you see, not on a dry day.
What else to check
The pump needs a working check valve and an outlet that is not frozen or plugged. Extension guidance says to test it by pouring about 5 gallons of water into the pit, and a power failure is when a battery backup matters.
Frequently asked questions
What size sump pump do I need?
There is no single correct size. North Dakota State University Extension says a one-third horsepower pump is satisfactory for most houses, and suggests more horsepower when there is more than 40 feet of pipe outside the house. The pump chart for flow at your lift matters most.
How do I calculate sump pump flow?
Time how long the pit takes to refill between pump cycles, work out the gallons held from the pit diameter and the water rise, and divide. Then compare that inflow with the gallons per minute your pump delivers at your lift, from its chart.
How big should a sump pit be?
Extension guidance describes a pit about 2 feet in diameter, with room for the pump and piping and about 15 to 25 gallons of storage between pumping events.
Is a half horsepower sump pump better than a third?
In similar conditions a half horsepower pump moves more water and lifts it higher, and where flow becomes rapid it may keep up when a third horsepower pump may not, according to the extension.
Related guides
- Sump pumps explained
- What size sump pump do I need
- Sump pits
- Battery backup sump pumps
- Sump pump check valves
Sources
The horsepower rules, the flow chart advice and the pit size come from North Dakota State University Extension and the federal Building America program. The cycle arithmetic is geometry and unit conversion.
- Time to Check Your Sump Pump | North Dakota State University Extension and Ag Research News
- Check Your Sump Pumps Now | North Dakota State University Extension
- Sump Pump Questions | North Dakota State University Extension (copy hosted by the State of North Dakota)
- Drain or Sump Pump Installed in Basements or Crawlspaces | Building America Solution Center (U.S. Department of Energy)