Start with your home's total square footage and humidity level

The number of dehumidifiers you need depends on two things: the size of the space you're treating and how wet it is. A single dehumidifier rated for 2,000 square feet won't work well if you're trying to dry out a 4,000-square-foot basement, and it won't work at all if the humidity is above 80 percent. Most homes need one dehumidifier, but larger homes, basements, or very damp spaces often need two or more.

Start by measuring the square footage of the room or rooms where you notice moisture problems. Humidity tends to concentrate in certain areas—basements, crawl spaces, bathrooms, laundry rooms, and kitchens are the most common. If moisture is only in one room, you may only need one unit. If it's spread across multiple rooms or your whole basement is damp, you'll need more.

Next, check your current humidity level with an inexpensive hygrometer (a humidity meter that costs $10 to $30). Ideal indoor humidity is between 30 and 50 percent. If your space is above 60 percent, you have a moisture problem. If it's above 75 percent, you have a serious one, and a single standard dehumidifier may not keep up.

Key Takeaways

  • Most dehumidifiers are rated for a specific square footage—check the label and match it to your room size, not your whole house.
  • A space above 60 percent humidity needs a dehumidifier; above 75 percent humidity usually needs a larger unit or a second one.
  • Basements and crawl spaces often need their own dehumidifier because they stay damper than the rest of the house.
  • If you're treating multiple rooms, one unit per 2,000 square feet is a practical starting point, adjusted for how wet the space actually is.
  • Dehumidifier capacity is measured in pints per day—higher numbers mean the unit can remove more moisture and work in damper conditions.

Match dehumidifier capacity to room size and moisture level

Dehumidifiers are labeled with a pint capacity—usually 30, 50, 70, or 100 pints per day. This number tells you how much water the unit can pull from the air in 24 hours. A 30-pint dehumidifier works for a small, mildly damp room. A 70-pint or larger unit is needed for a large basement or a space that's very wet.

The manufacturer's square footage rating assumes normal humidity—around 60 percent. If your space is much damper, the unit won't perform as well. A 50-pint dehumidifier rated for 2,000 square feet at 60 percent humidity may only handle 1,200 square feet if the humidity is 80 percent. This is why very damp basements often need a larger unit or a second one.

Use this as a rough guide: a 30-pint unit covers roughly 500 to 700 square feet; a 50-pint covers 1,000 to 1,500 square feet; a 70-pint covers 1,500 to 2,500 square feet; and a 100-pint covers 2,500 to 4,000 square feet. These numbers assume the space is moderately damp. If it's very wet or has standing water, go up one size.

Basements and crawl spaces usually need their own unit

Basements stay damper than the rest of your house because they're below ground level and surrounded by soil. Even if your basement is only 800 square feet, it may need a 50-pint dehumidifier instead of a 30-pint one, because basement humidity is naturally higher. If you have a finished basement and an unfinished one, each should have its own dehumidifier—they don't share air well.

Crawl spaces are even wetter than basements. If you have a crawl space, it almost always needs a dedicated dehumidifier, even if it's small. Crawl space dehumidifiers are often smaller units (30 to 50 pints) but run continuously. Some homeowners install a dehumidifier in the crawl space and another in the basement above it, because moisture rises.

If your basement is very large—say, 2,000 square feet—one dehumidifier may not be enough. A single 70-pint unit can handle about 2,000 square feet at moderate humidity, but if the basement is actively wet or has had water damage, two 50-pint units placed at opposite ends will work better than one large one. They'll also run less constantly and last longer.

Multiple rooms mean multiple units

Dehumidifiers work best in a closed space. If you put one dehumidifier in your basement and close the door, it treats only the basement. If you leave the door open, the unit has to work much harder to dry out the basement plus the hallway and other connected rooms. For this reason, a dehumidifier in one room won't solve a moisture problem in another.

If you have moisture problems in your basement, your bathroom, and your laundry room, you need at least three dehumidifiers—one for each space. You can use smaller units (30 to 50 pints) in smaller rooms like bathrooms and laundry rooms, and a larger one (50 to 70 pints) in the basement. Running them all at once will cost more in electricity, but it's the only way to bring humidity down across your whole home.

Some homes have a central dehumidifier that connects to the HVAC system and treats the whole house. These are expensive (often $1,500 to $3,000 installed) and make sense only if humidity is a problem everywhere. For most homes, spot dehumidifiers in the wettest rooms are cheaper and more practical.

Test one unit first, then add more if needed

If you're unsure whether you need one dehumidifier or two, start with one that matches your largest problem area. Run it for a week and check your humidity level with a hygrometer. If humidity drops to 50 percent or below, one unit is enough. If it stays above 60 percent, you need a larger unit or a second one.

Keep in mind that a dehumidifier takes time to work. In a very damp space, it may take several days or a week to bring humidity down to normal. Don't judge the unit's performance after just one day. Also, if the space has an active moisture source—like a water leak, poor drainage, or a wet crawl space—no dehumidifier will fix it permanently. You'll need to find and stop the source first.

Once you've run the dehumidifier for a week, empty the water collection bucket or check the drain line. If the bucket fills up every day, the space is very wet and you may need a second unit or a larger one. If the bucket fills up every few days, one unit is probably working.

Dehumidifier placement matters as much as size

Where you put your dehumidifier affects how well it works. Place it in the center of the room or the dampest area, not in a corner or against a wall. The unit needs air to flow around it. If you block the intake or exhaust, it won't pull moisture out of the air efficiently.

In a basement, place the dehumidifier away from the furnace, water heater, or other appliances that produce heat. Heat makes the dehumidifier work harder. If you have a very large basement, two smaller units placed at opposite ends will dry it more evenly than one large unit in the middle.

Keep the door closed to the space you're treating. If you're drying out a basement, close the basement door. If you're drying out a bathroom, close the bathroom door. An open door means the dehumidifier has to work on a much larger area and will never catch up.

Frequently Asked Questions

Can one dehumidifier handle my whole house?

No, unless your house is very small or humidity is only slightly elevated. A dehumidifier works best in a closed room. If you leave doors open, it has to work on a much larger area and won't bring humidity down. Most homes with a moisture problem need at least two units—one for the basement and one for the main floor, or one for each problem area.

What if my dehumidifier runs all day and humidity is still high?

The unit is too small for the space or the humidity is too high. Check the manufacturer's rating and compare it to your room size and humidity level. If the unit is rated for 1,500 square feet but your basement is 2,000 square feet, it won't keep up. You need a larger unit or a second one. Also check for active moisture sources like leaks or poor drainage—no dehumidifier will work if water is still coming in.

Do I need a dehumidifier in every room?

No. Most homes only need dehumidifiers in the wettest areas—usually the basement, crawl space, or bathroom. Other rooms stay dry naturally. If humidity is a problem throughout your whole house, you may need multiple units, but start with the basement and see if that brings the whole-house humidity down.

How often should I empty the dehumidifier bucket?

It depends on how wet the space is and the unit's capacity. A 50-pint dehumidifier in a very damp basement may fill a 5-gallon bucket every day. A 30-pint unit in a mildly damp room may fill it every few days. Most dehumidifiers have a drain hose option so you can run water directly into a floor drain or outside, which saves you from emptying the bucket constantly.

Will a dehumidifier fix a wet basement?

A dehumidifier removes moisture from the air, but it won't stop water from coming in through cracks, poor drainage, or a high water table. If your basement is actively leaking or has standing water, you need to fix the source first—seal cracks, improve drainage around the foundation, or install a sump pump. A dehumidifier is a tool to manage humidity after the water problem is solved.