Dehumidifier Size Calculator

Start with the rule of thumb, then correct it

The old rule is simple to remember: about 10 pints a day for every 500 square feet of damp space. That gives 20 pints for a 1,000 square foot basement and 30 pints for 1,500. It is a decent starting point and a poor stopping point, because it ignores three things that change the answer by more than the area does.

The first is how wet the space is. A basement that smells musty in July is not the same problem as a crawl space with water marks on the joists. The second is what the space is used for, since a crawl space sits over bare soil and a finished room does not. The third is temperature, because every dehumidifier sold in the United States is rated at 80 F and 60% relative humidity, and almost nobody runs one in those conditions.

The pint number on the box comes from that 80 F test, run under the AHAM standard. A 50-pint unit is guaranteed to pull 50 pints in a day at 80 F and 60% humidity. Drop the room to 55 F and the same machine pulls closer to 37 pints, not because it is broken but because cold air holds less moisture for the coil to condense. That is the single biggest reason people buy a unit that turns out to be too small.

Size by area and dampness

The calculator starts at one pint per 50 square feet, then multiplies by a dampness factor. Dampness is judged by what you can see and smell, not by a number, so the table below pairs each level with the signs that go with it.

A 1,200 square foot basement that smells musty lands at 1,200 divided by 50, times 1.0 for damp, times 1.3 for a basement, which is 31.2 pints a day. The nearest standard size above that is a 35-pint unit. Buy the 30-pint model on sale and you are 1.2 pints short before the temperature drops, which is how a "deal" turns into a machine that runs all day and never gets ahead.

Round up, not down. Dehumidifiers are most efficient when they cycle on and off, and a unit running at 100% of capacity around the clock wears out faster and holds the room at 55% instead of 45%. One size up costs $30 to $60 more and usually pays that back in the first summer of run time.

Basements and crawl spaces are not the same job

Two spaces with the same square footage can need units a size apart, because the moisture load is different. A crawl space sits over exposed soil, and soil releases water vapor into the air above it all year. A basement has a concrete floor and walls that slow that flow, and a finished room has drywall, furniture and a heating system that keeps the air moving.

A 1,200 square foot crawl space at the wet level needs 1,200 divided by 50, times 1.4, times 1.6, which is 53.8 pints a day. That jumps the answer from a 35-pint unit to a 70-pint unit, and the 70-pint model pulls about 8.8 gallons a day out of the air.

Crawl spaces also need a different installation. A unit sitting on the dirt will suck in soil dust and clog its filter in weeks, so it belongs on a concrete pad or hung from the joists with the drain line running to a sump. Encapsulating the crawl space with a vapor barrier cuts the moisture load enough to drop one size, and it is usually the better fix if the space is accessible.

Cold air cuts capacity, and the box does not say so

The 80 F rating is the number on the box, and it is the number almost nobody experiences. Basements sit at 55 to 65 F for much of the year, and crawl spaces run colder. A compressor dehumidifier condenses water on a cold coil, and as the air gets colder the coil ices up and the machine has to spend time defrosting instead of drying.

The drop is not small. Between 50 and 65 F, expect about 75% of the rated capacity. Under 50 F, expect about half, and below roughly 41 F many compressor units stop collecting water altogether and just run their fans. The calculator applies those factors, so a 1,200 square foot damp basement at 55 F needs 41.6 pints rather than 31.2, which pushes the answer up to a 45-pint unit.

That 45-pint unit delivers only 33.8 pints a day at 55 F. It is doing its job and it still looks undersized if you compare the box number to the water in the tank. This is the most common misunderstanding in dehumidifier reviews, where a buyer reports that a 50-pint machine "barely fills the tank" in a cold basement that was never going to get 50 pints.

If the space stays under 50 F, a desiccant dehumidifier is the right tool. It uses a rotor coated with silica gel and works down to freezing, at the cost of more electricity per pint. For a cold crawl space in the northern states, that trade is usually worth it.

Worked example: 1,200 square feet, three ways

Same floor area, three different answers, which is the whole point of running the numbers instead of reading a chart.

• Damp basement, 65 F and up. 31.2 pints needed, 35-pint unit, 4.4 gallons a day, 450 watts, about $0.92 a day at 17 cents per kWh.
• Wet crawl space, 65 F and up. 53.8 pints needed, 70-pint unit, 8.8 gallons a day, 780 watts, about $1.59 a day.
• Damp basement, 55 F. 41.6 pints needed, 45-pint unit that delivers 33.8 pints in that air, 4.2 gallons a day, 520 watts, about $1.06 a day.

The crawl space needs twice the machine and 73% more electricity than the basement, for the same 1,200 square feet. A homeowner who buys on square footage alone would pick a 30-pint unit for all three and be wrong twice.

There is a second use for these numbers. If the calculated need comes out above 70 pints, one residential unit will not keep up, and the calculator tells you how many you need. Two 70-pint units in a 1,500 square foot soaked basement with a 9-foot ceiling deliver 140 pints a day between them, which covers a need of 79 pints with room for a cold snap.

Pints, tanks and drains

The pint rating tells you how much water the machine can remove, not how often you have to empty it. Most portable units carry a 12 to 20 pint tank, and a 35-pint machine at full output fills a 14-pint tank in under 10 hours. Emptying a tank twice a day gets old by the second week, and a unit that shuts off on a full tank stops drying the room.

Gravity drain is free and needs a floor drain, a sump pit or a window the hose can reach. If the drain is uphill from the unit, you need a model with a built-in condensate pump, which adds $40 to $80 and saves you from carrying water up a basement stair. A garden hose adapter is a $10 part and most units accept one.

Check the tank size before buying, not after. A 70-pint unit with an 18-pint tank is a machine that needs attention every six hours at full output, which is why the larger models are almost always sold with a hose port.

What it costs to run

Electricity is the ongoing cost, and it scales with the pint rating. A 35-pint unit draws about 450 watts, so 12 hours of run time is 5.4 kWh. At the national average of roughly 17 cents per kWh that is $0.92 a day, or about $28 a month. A 70-pint unit at 780 watts runs 9.4 kWh over the same 12 hours and costs $1.59 a day, or about $48 a month.

Those figures assume 12 hours of compressor run time, which is realistic in a damp basement in summer and optimistic in a wet crawl space, where the unit may run 20 hours. At 20 hours a 35-pint unit costs $1.53 a day. The real number depends on how well the space is sealed, and sealing is cheaper than any dehumidifier.

Two habits cut the bill without buying anything. Keep windows and doors closed while the unit runs, because drying air that is being replaced with humid outside air is a losing fight. Set the target humidity to 50% rather than 30%, since every 5% lower costs roughly 10% more run time and 30% is uncomfortable for people and hard on wood floors.

ENERGY STAR lists certified dehumidifiers with their tested efficiency in pints per kWh, and the difference between the best and worst models in a size class is about 15%. Over five summers that is more than the price gap between them. The ENERGY STAR dehumidifier list is the place to check before buying.

Mistakes that leave the room damp anyway

A correctly sized unit still fails if the setup works against it. These are the ones that show up again and again.

• No hygrometer. A $12 humidity gauge tells you whether the unit is winning. Without one, you are guessing, and guessing usually means running the machine longer than needed or not long enough.
• Blocked airflow. A unit pushed into a corner with 2 inches of clearance cannot move air through its coil. Give it a foot on every side and keep it off the floor on a low shelf if the floor stays wet.
• Drying an open space. A dehumidifier in a basement with an open stairwell to the upstairs is drying the whole house. Close the door at the top of the stairs if you only need the basement dry.
• Ignoring the source. A downspout dumping water next to the foundation, a cracked slab or a leaking pipe will beat any dehumidifier. Fix the water first, then size the machine.
• Running it in a cold room. Under 50 F a compressor unit may run without collecting anything, which looks like a broken machine and is not.

Mold needs moisture, warmth and a food source, and it starts growing on porous surfaces within 24 to 48 hours of a leak. The EPA's guide to mold and moisture covers the cleanup and the prevention side, and the AHAM standard behind the pint rating is published by the Association of Home Appliance Manufacturers.

If the space smells musty after a week at 45% humidity, the dehumidifier is not the problem. Look for a leak, a blocked vent or a floor that stays wet, and fix that before buying a bigger machine.