Do I Need a Sump Pump? Signs, Sizing and When to Call a Pro

What this page cannot tell you: whether your basement needs a pump depends on the water table, the soil and whether the house has foundation drains, and nobody can judge those from a calculator. The numbers below come from measurements you take in your own pit, a published rule of thumb for new homes, and our tank volume calculator. Picking a model needs the maker's pump curve as well.

Signs a basement needs a sump pump

• Water comes up through the floor or the joint where the floor meets the wall, especially after heavy rain or snowmelt. That is groundwater, the job a sump pump is for.
• The house already has a pit, often with drain tile feeding it. If the water in it rises after rain, the pump needs to work, and a failed pump is a flood waiting to happen.
• The basement has flooded before when the ground was saturated, or neighbors with similar houses run pumps.
• Water runs in at one spot on a wall after rain: check the gutters, downspouts and the grading on that side first. FEMA lists cleaning gutters and downspouts and improving the lot grading so storm water drains away from the building.
• Walls sweat and the air smells musty, but no water comes in: that is humidity, and a dehumidifier is the fix, not a pump.
• Water comes up through a floor drain or the toilet: that is a sewer backup. FEMA recommends drain plugs for basement floor drains and sewer backflow valves fitted by a licensed plumber. A sump pump does not help.

Size it from the water you measure

The method from the University of Illinois Extension: during heavy rain, let the pump empty the pit, switch it off, and measure how many inches the water rises in one minute. Then turn the pump back on. In a round basin, every inch of rise is a fixed number of gallons: 1.10 gallons in an 18 in basin and 1.96 gallons in a 24 in basin. The rise in one minute times that figure is the inflow in gallons per minute.

If more than 30 gallons a minute come in, a rise of over 27 in a minute in an 18 in basin, the article recommends a 24 in basin, which holds 1.8 times as much per inch, so the pump starts less often. The water should never rise above the bottom of the drain tile inlet pipe while you measure. Measure in the wettest weather you can; a dry-season reading will be far too low.

For a new house, before there is a pit to measure

The same article plans the capacity from the size of the home: about 14 gallons per minute per 1,000 sq ft on sandy soil and 8 on clay, because water moves through sand much faster.

Then the head, then the pump curve

A pump moves fewer gallons the higher it has to push. The total dynamic head is the vertical lift, from the pump to where the discharge pipe turns horizontal, plus the friction in the pipe and its fittings, which comes from friction tables for your pipe size and flow. Look up your gallons per minute at that head on the maker's pump curve, not the box's headline figure. A pump much bigger than the inflow short-cycles, switching on and off too often, which wears it out early. We cannot pick a model for you: that needs the curve for that pump and your pipe run. A plumber or a waterproofing contractor can do the head calculation if you are unsure.

Where the water goes

• NDSU Extension advises running the discharge several feet away from the house to a well-drained area that slopes away. Outlet too close to the foundation, or on flat ground, and the water can soak back down into the drain tile and come straight back to the pit.
• Do not run sump water into a septic system: it can saturate the drain field. In town, piping it into the sanitary sewer can overload the system and cause sewage backups, and it may be illegal. Ask your city or county where it may go.
• Fit a check valve on the discharge pipe so the water in the pipe does not run back into the pit each time the pump stops. The Illinois article suggests a swing-type valve the same size as the pipe, and a dedicated circuit and breaker for the pump.
• If the yard water needs carrying further, a French drain can take it: a 50 ft run in a 12 × 18 in trench takes 4.0 tons of washed stone, 50 ft of perforated pipe and 250 sq ft of filter fabric, and needs 6.3 in of fall. Work out yours with the French drain materials calculator.

Backup power and testing

Pumps are needed most in storms, when the power is most likely to go out. FEMA advises choosing a battery-operated sump pump in case the power fails. A generator is the other way: our generator size calculator allows 800 running watts and 2,100 starting watts for a 1/3 hp sump pump alone, and with a refrigerator, furnace blower, microwave, lights and a computer as well it comes to 3,550 running watts, and the calculator suggests a generator of 5,000 running watts. Check the label on your own pump. The generator wattage chart lists other appliances.

NDSU Extension suggests testing the pump before the wet season by lifting the float switch or pouring water into the pit, cleaning the pump and the pit, and keeping a spare submersible pump. A battery-powered water alarm in the pit warns you if the water rises and the pump has not started.

The air still needs a dehumidifier

A pump removes water that reaches the pit; it does nothing for damp air. Size a dehumidifier for the basement by its floor area and how damp it is, on the current pint rating:

The dehumidifier size calculator works it out for your basement, and what size dehumidifier do I need explains the ratings.

When to call a pro

• There is no pit yet. Installing one means cutting and digging through the slab, and often laying drain tile to it: a job for a waterproofing contractor or plumber. Ask your building department whether it needs a permit.
• The pump needs a new outlet or circuit: an electrician.
• Water comes up floor drains or toilets: a licensed plumber for a backflow valve.
• The walls are cracked, bowing or leaning, or the water keeps coming after you have fixed the gutters and grading: have a waterproofing contractor or a structural engineer look at the foundation.
• The pump runs constantly, or the pit floods even with the pump working: the pump may be too small, failing, or more water is coming in than one pump can handle.

See also French drain materials list, how much gravel for a French drain and slope and grade for drainage.

Frequently asked questions