Dehumidifier Size Calculator

How to Use This Dehumidifier Size Calculator

Enter the length, width and ceiling height of the space. Pick the damp level that matches what you see and smell, then pick how leaky the space is. Set the room temperature and your electricity rate, and the calculator returns the moisture load, the unit class to buy, the daily water volume and an estimated monthly bill.

The default example is a 30 by 20 foot basement with an 8 foot ceiling, a damp smell and average sealing. That is 600 square feet and about 27 pints of moisture per day. At 70 F a compressor unit rated at 35 pints delivers roughly 30 pints, so it covers the load with a small margin. The unit would run about 22 hours a day, pull 350 watts and cost near $35 a month at 17 cents per kilowatt hour.

The load model starts from a pints-per-square-foot rate for the damp level, then scales with ceiling height and air leakage. That is the same logic the common sizing tables use, expressed as a number you can check. It is an estimate for a residential space, not a stamped mechanical design.

Change one field at a time to see what moves the answer. Dropping the damp level from damp to moderate cuts the load to about 18 pints. Dropping the temperature from 70 F to 60 F raises the required class even though the load has not changed, because a compressor unit loses capacity as the air cools.

Why the Pint Rating Is Not What You Get

Every dehumidifier sold in the United States carries a pint rating, and almost every buyer treats it as a constant. It is not. The rating is measured at 80 F and 60 percent relative humidity under the AHAM test procedure. That is a hot, sticky day, and it is not the condition of a basement in April.

As the air cools, a compressor unit pulls less water. The evaporator coil runs colder, frost forms sooner, and the compressor cycles on its defrost control. The drop is not small. A unit rated at 35 pints might deliver about 30 pints at 70 F, 26 at 65 F and 22 at 60 F. Below 55 F many units barely work at all, and below 40 F most compressor models are out of their range.

That is why a dehumidifier that looks oversized on paper can run all day and still leave the basement at 65 percent humidity. The label was earned at 80 F. Your space is not at 80 F.

The derating factors in that table are industry rules of thumb, not a single published curve. Different manufacturers publish their own charts, and a unit with a larger coil or a better defrost strategy holds capacity longer. Check the spec sheet for the model you are considering if the room runs below 65 F.

The Load Math Behind the Recommendation

Moisture load is the amount of water vapor the dehumidifier has to remove each day to hold a stable humidity. In a basement, that water comes from three places: humid outdoor air leaking in, moisture moving through the concrete or stone walls, and water evaporating from the floor or from stored materials.

The calculator uses a rate per square foot for the damp level, then multiplies by a height factor and a leakage factor.

Load (pints/day) = area × base rate × (height ÷ 8) × leakage factor

The base rates are 0.02 for a dry space, 0.03 for moderate, 0.045 for damp and 0.06 for wet. Those numbers come from the common sizing charts that assign about 10 pints to 500 square feet of a dry basement, 15 to a moderate one, 22 to a damp one and 30 to a wet one.

The leakage factor is 0.85 for a tight, sealed space, 1.0 for average and 1.25 for a leaky one. A basement with open rim joists and unsealed sill plates exchanges air with the outside all day, and every exchange brings in new humidity. A sealed and insulated basement with a vapor barrier and closed vents loads the unit far less.

The height factor is simply the ceiling height divided by 8. A 10 foot ceiling holds 25 percent more air over the same floor, and the walls present more surface for moisture migration. The factor is a proxy, not a physical model of wall flux, but it keeps tall spaces from being undersized.

Once the load is known, the required rated capacity is the load divided by the derating factor at your room temperature.

Rated pints = load ÷ derating factor

A 27 pint load at 70 F needs a unit rated about 32 pints. The calculator rounds up to the next common class, which is 35. That is the whole sizing decision. Everything else, including drainage and the humidity setpoint, is about how the unit is installed.

Compressor vs Desiccant: Temperature Picks the Type

Most residential dehumidifiers are compressor units. They work like a refrigerator: a fan pulls moist air over a cold coil, water condenses, and the dried air passes back over the warm condenser coil. They are efficient in warm, humid air and cheap to buy.

Desiccant units use a rotating wheel coated with silica gel or a similar material. A heater dries the wheel and the moisture leaves with a small exhaust stream. They cost more to run per pint in warm air, but they keep removing water at 40 F and below. For a cold crawl space, an unheated garage or a basement that sits near 55 F all winter, a desiccant unit is the honest choice.

There is a third option. A whole-home dehumidifier ties into the HVAC ductwork and dries the entire house. It is the right answer when the air conditioner is oversized and short cycles, because a short cycle cools the air without removing much moisture. Those systems are sized by cubic feet per minute and by the latent load, not by the pint chart, and they need a contractor.

Basement, Crawl Space and Sealed vs Vented

A finished or open basement is the simple case. Size the unit to the floor area and volume, close the windows, and run a drain hose to the floor drain or a condensate pump. If the basement has a bathroom fan or dryer venting into it, fix that first, because a dehumidifier cannot outrun an appliance that dumps moisture into the same room.

A crawl space is harder. A vented crawl space exchanges air with the outdoors, and in a humid climate that means the unit is fighting the weather. The better approach is to seal the vents, cover the soil with a heavy vapor barrier, insulate the rim joists and then size the dehumidifier to the now-sealed volume. That is encapsulation, and it changes the math from an impossible load to a manageable one.

A sealed crawl space still needs drainage. Condensate cannot run uphill, so plan a gravity path to a low point or install a condensate pump with a float switch. A pump failure in a sealed crawl space shows up as standing water under the liner, which is the exact condition the encapsulation was meant to prevent.

For a space with visible seepage, fix the water first. A dehumidifier removes vapor, not liquid. If water is running down a wall after rain, the fix is grading, gutters, a French drain or interior waterproofing. Sizing a bigger unit to dry a wet wall is a monthly bill with no end.

Drainage: Tank, Gravity Hose or Pump

Every pint the unit removes has to go somewhere. A 35 pint unit removing 25 pints per day fills a 14 pint tank in about 13 hours. Emptying a tank twice a day gets old fast, and a unit that shuts off on a full tank stops drying the space at exactly the moment it is needed.

A gravity hose is the simplest fix. Most units have a threaded drain port, and a garden hose or a vinyl tube can run to a floor drain, a sump pit or a daylight outlet. The hose has to slope continuously, with no trap and no high spot. Keep the run short, because a long flat hose collects debris and grows biofilm.

A condensate pump is the answer when no gravity path exists. The pump sits in a small reservoir, kicks on at a set level and pushes water up and out through a flexible tube. Choose a pump with a float switch that can shut the dehumidifier off if the pump fails, and check the tube for kinks once a season.

If you must use the tank, pick a unit with a large tank and an auto shutoff, and set a reminder to empty it. In a damp basement in summer the tank will fill faster than the manufacturer's marketing implies.

Setting the Humidity Target

A dehumidifier with a humidistat will run until the space reaches the setpoint, then cycle. The setpoint matters as much as the size. Setting it to 30 percent in a basement is a losing game: the unit runs constantly, the walls stay cold, and the air never gets that dry.

For a basement, 45 to 50 percent is a practical target. Mold growth slows below about 60 percent on most materials, and 50 percent keeps stored boxes, tools and furniture dry without turning the space into a desert. For living space, 50 to 55 percent is comfortable. In a crawl space, 55 to 60 percent is often the best a vented or partly sealed space can hold, and encapsulation should come before a bigger unit.

A cheap hygrometer is worth buying. Place it in the middle of the space, away from the dehumidifier's exhaust and away from the cold wall. If the unit's built-in sensor disagrees with the hygrometer by more than a few points, trust the separate meter and adjust the setpoint to match.

Expect seasonal variation. A basement that holds 50 percent in July may drop to 35 percent in February with the same setting. That is normal. The unit will simply run less, and the electricity bill will follow.

Common Sizing Mistakes

• Using the pint rating as if it were constant. The rating is earned at 80 F. A cool basement gets less, sometimes much less. Apply the temperature correction before choosing a class.
• Sizing by floor area alone. A 10 foot ceiling, a leaky rim joist and an open stairwell all raise the load. The calculator includes both factors for that reason.
• Buying a compressor unit for a cold crawl space. Below 55 F it will frost, cycle and underperform. A desiccant unit costs more to run but actually works.
• Ignoring the water source. A dehumidifier handles vapor. A leaking wall, a backed-up gutter or a wet floor needs a drainage fix first.
• Forgetting drainage. A unit that shuts off on a full tank is not drying anything. Plan the hose or pump before the unit arrives.
• Oversizing and short cycling. A unit that is too large blasts through the humidity in bursts, then sits idle while the space rebounds. A right-sized unit runs longer and holds a steadier number.
• Setting an impossible target. Thirty percent in a basement is not a goal, it is a way to run the compressor continuously. Set 45 to 50 percent and let the space settle.
• Venting a dryer or bathroom fan into the basement. That adds pints per hour the unit was never sized for. Vent those appliances outdoors.

Worked Example: A 1,200 Square Foot Basement

A homeowner has a 1,200 square foot basement with an 8 foot ceiling, a musty smell and average sealing. The base rate for damp is 0.045, so the load is 1,200 × 0.045, or 54 pints per day. At 68 F the derating factor is about 0.80, which means the unit needs a rating near 68 pints. A 70 pint class fits.

The daily water removal is 54 divided by 8, or 6.75 gallons. A 70 pint unit draws roughly 700 watts. If it runs 18 hours a day, the monthly energy is 0.7 × 18 × 30, or 378 kilowatt hours. At 17 cents that is about $64 a month, and that is before any air conditioning.

Now suppose the homeowner seals the rim joists, closes the vents and covers the floor with a vapor barrier. The leakage factor drops from 1.0 to 0.85, and the load falls to about 46 pints. That is still a 70 pint class at 68 F, but the unit runs fewer hours and the bill drops. Sealing is cheaper than capacity, and it works every month.

If the same basement were held at 60 F, the derating factor falls to 0.62 and the required rating rises to about 74 pints. That pushes past the 70 pint class and into either a larger unit or a desiccant model. This is why the temperature input matters more than most buyers expect.

What to Buy and What to Check

Match the class first, then compare the details that affect daily use. Look for a humidistat with a numeric setpoint, an auto restart after a power outage, a continuous drain port and a washable or easily replaced filter. A unit with a built-in pump saves a separate purchase in a crawl space.

Check the amp draw against the circuit. A 70 pint compressor unit can draw 7 to 8 amps at 115 volts, which is fine on a dedicated 15 amp circuit but not on a circuit shared with a freezer and a shop vacuum. If the basement circuit is old and ungrounded, fix that before adding a motor load.

Finally, read the capacity chart in the manual, not just the box. A good manual shows pints per day at 80 F, 70 F and sometimes 60 F. That chart tells you what the unit will really do in your space, and it is the number that matters.