What Size Dehumidifier for a Basement?

Most basements need a 50-pint dehumidifier rated under the 2019 Department of Energy standard. That covers a moderately to very damp basement up to about 2,000 square feet, and it is the largest size most portable units come in. Drop to a 35-pint unit only for a small or finished basement under roughly 800 square feet that is damp rather than wet. Go to two 50-pint units or a 70–120 pint whole-house dehumidifier once the space passes about 2,000 square feet, is genuinely wet, or sits cold for much of the year.

The reason so much sizing advice disagrees is that the pint labels changed in 2019. A machine that used to be sold as a 70-pint is now labelled roughly 50 pints, without any change to what it actually removes. Square footage is also a weaker guide than most charts admit: a wet 700 square foot basement can need more water removal than a dry 1,800 square foot one, and a single habit like hanging laundry to dry can move you up a size on its own.

If you already know the size and want to choose the actual machine, our best dehumidifier for a basement guide covers specific models, built-in pumps and drainage. This page is about getting the capacity right first, because that is the decision that is hardest to fix after you have bought.

The 2019 Pint Re-Rating — Old 70 ≈ New 50

Dehumidifier capacity is stated in pints of water removed per day. Until 2019, US portable units were rated at 80°F and 60% relative humidity — warm, humid conditions in which a cold coil pulls water out of the air easily. In June 2019 the Department of Energy's revised test procedure took effect and moved the test to 65°F and 60% relative humidity, which is far closer to an actual basement in summer.

Cooler air holds less water and gives it up less readily, so every machine scored lower on the new test. Nothing about the hardware changed. The result is a roughly 30% drop in the number on the box:

This matters in three practical ways.

Old advice reads one size too large. Forum threads, contractor rules of thumb and buying guides written before 2019 routinely say "get a 70-pint for a basement". They are describing what is now sold as a 50-pint. Searching for a current 70-pint portable leads either to a listing still quoting the old number or to nothing at all, because the biggest mainstream portables now top out at 50 pints on the new test.

Some listings still quote the old figure. A bigger number sells, and older stock and some marketplace listings still advertise the 80°F rating. If a portable claims 60 or 70 pints, look for the words "2019 DOE standard" or "new DOE"; if they are absent, assume the number is on the old scale.

Whole-house units are rated differently again. Ducted and crawlspace dehumidifiers are often specified at warmer test conditions, frequently the old 80°F / 60% point, and sometimes at saturation. A 70-pint whole-house unit and a 50-pint portable are therefore closer in real basement output than the labels imply. Compare them at the same conditions where the spec sheet allows.

Sizing Chart by Area and Dampness

The chart below combines floor area with how damp the space actually is. It uses current 2019 DOE pints and assumes a basement with ordinary 7–8 foot ceilings, closed windows and no unusual moisture sources. The adjustments in the following sections then move you up or down from here.

Two things stand out. First, the 50-pint portable covers most of the chart, which is why it is the default recommendation. Second, the chart does not scale smoothly with area. Moisture enters a basement mainly through its walls and slab, through air leaking in, and from activities inside it — not from the volume of air already there — so a basement twice the size does not automatically need twice the capacity.

Why Manufacturer Charts Disagree

The classic industry sizing chart, written decades ago in old pints, lists minimum capacities that look surprisingly small — figures in the teens and twenties for typical basements. Manufacturer charts, by contrast, often suggest their largest model for almost everything. The honest position sits between the two. A minimum-capacity unit can technically hold a basement at 50% in steady conditions, but it runs almost continuously to do it, takes far longer to dry the space out in the first place, and has no reserve for a wet week. The step from 35 to 50 pints usually costs a modest amount more, and that reserve is worth having.

How Damp Is Your Basement?

Dampness is the column people most often guess wrong, usually by underrating it. A smell you have stopped noticing is still a smell. The categories below follow the long-standing industry definitions, with the relative humidity you would typically see on a hygrometer before any dehumidifier is running.

Do not size from a guess. A digital hygrometer costs less than a tank of fuel and removes most of the uncertainty. Put it in the middle of the basement, away from exterior walls, and leave it for three or four days with the windows and upstairs door closed — ideally during a humid spell, because that is the condition the dehumidifier has to handle. Note the highest reading, not the average.

If you see water as a liquid — tide marks along the base of the walls, a damp band at the wall-floor joint, efflorescence (white, powdery salt deposits), or puddles after rain — the problem is not humid air. That water is coming through the foundation, and a dehumidifier will evaporate it indefinitely at your expense while the wall stays wet. Our guide on how to fix a damp basement walks through finding the source; for seepage through the walls themselves, see how to waterproof basement walls.

Adjusting for Ceilings, Laundry and Climate

The chart assumes an ordinary basement. Real ones have dryers, showers, sump pits and climates, and each of those changes the moisture load more than the floor area does.

Ceiling Height

Most sizing charts assume ceilings of about 8 feet. For taller basements, a common adjustment is to scale the floor area by the ceiling height divided by eight: a 1,200 square foot basement with 10-foot ceilings sizes like 1,500 square feet. Use that as a tie-breaker rather than a rule, for a reason worth understanding.

The air in a basement holds remarkably little water. A 1,000 square foot basement with 8-foot ceilings contains about 8,000 cubic feet of air. At 70°F, taking it from 70% down to 50% relative humidity removes roughly 2 pounds of water — about one pint. Yet a new dehumidifier in that basement will fill its bucket several times over in the first few days. Almost all of that water is coming out of the concrete, the framing, the stored cardboard and the soil behind the walls, plus whatever leaks in with outdoor air. Extra ceiling height adds air, but not much moisture. Surface area and moisture sources matter more, which is why a tall basement with one damp wall can still be well served by the chart's figure.

Laundry, Showers and Other Sources

These are the adjustments that most often catch people out. Move up one size (35 to 50 pints, or 50 pints to two units or a whole-house model) if any of the following apply, and two sizes if several do:

• Laundry hung to dry indoors. A typical washer load still holds several pints of water after the spin cycle — commonly cited at around 4 to 5 pints. Drying two loads a week on a basement line can add the equivalent of a wet wall.
• A dryer that vents into the basement, or whose duct is disconnected, crushed or leaking. This dumps an entire load's water into the space every cycle. Fix it rather than sizing around it; a dehumidifier is a poor substitute for a properly ducted dryer vent.
• A basement bathroom with a shower and no exhaust fan ducted outdoors.
• An uncovered sump pit. It is open water with a large surface area. A sealed, gasketed lid is cheap and often cuts the load noticeably.
• Unsealed block walls or an exposed dirt floor, including a basement open to a vented crawlspace. Bare block wicks moisture readily; a block sealer or vapour barrier reduces what the dehumidifier has to remove.
• Living use — a finished basement that people sleep, exercise or cook in. Occupants, plants, aquariums and firewood all add moisture.

Move down slightly if the basement has an air-conditioning supply register, which removes some moisture in summer, or if it is finished, insulated and well sealed to the outside.

Climate

Where you live sets the background load, mainly through the outdoor dew point — the temperature at which outdoor air would start to condense. In the Southeast, Gulf Coast, Mid-Atlantic and much of the Midwest, summer dew points in the mid-60s to 70s°F are common. When that air leaks into a basement whose walls sit at 60–65°F, it cools to or below its dew point and the walls sweat. That is why opening basement windows "to air it out" on a humid summer day makes things worse, not better.

As a rough guide: in a humid-summer climate, size one step up from the chart if you are near a boundary. In mild, damp maritime climates such as the Pacific Northwest, the chart is about right but the season is longer. In the dry interior West, many basements never need a dehumidifier at all, and those that do usually have a water source rather than a humidity problem. Measure before you buy.

Cold Basements Need More Capacity

The 2019 rating is measured at 65°F. Many unfinished basements in the northern half of the country sit in the high 50s or low 60s even in summer, and that changes the effective size of a portable dehumidifier considerably.

A compressor dehumidifier works by blowing air over a refrigerated coil. The cooler the incoming air, the colder the coil runs, and below roughly 65°F it begins to frost. Frost insulates the coil, so the unit pauses to defrost, and every minute spent defrosting is a minute not removing water. At 60°F a standard portable still works, but below its label. In the low 50s°F, real output can fall to a fraction of the rating. The minimum operating temperature on the spec sheet — often around 41°F — is where the unit will switch on, not where it performs well.

For sizing, that means:

• Basement 62–65°F in the humid season: use the chart as written.
• Basement in the high 50s to low 60s°F: size one step up, or choose a unit that specifically lists effective low-temperature operation.
• Basement in the low-to-mid 50s°F: a standard portable is the wrong tool. Look at a whole-house or low-temperature compressor unit built to keep working in the 40s and 50s°F, or a desiccant dehumidifier, which uses a moisture-absorbing wheel rather than a cold coil and is not slowed by low temperatures.

There is a subtlety that often makes cold basements look worse than they are. Relative humidity is relative to temperature: the same amount of water vapour reads as a higher percentage in cooler air. A basement at 56°F and 70% RH holds less water than one at 70°F and 55%. Raising the temperature a few degrees with a small amount of heat lowers the relative humidity directly and brings an ordinary portable back into its effective range. Before buying a specialist unit, watch a hygrometer and a thermometer through a full year — many cold basements are only humid from late spring to early autumn, when they are warmest anyway.

Oversizing vs Undersizing

With air conditioners, buying too large is a real mistake: an oversized AC cools the room quickly, shuts off before it has removed much moisture, and leaves the space cold and clammy. Dehumidifiers do not have that problem, because removing moisture is their only job. Their built-in humidistat switches the compressor on when humidity rises above the set point and off when it falls below, regardless of how fast the unit got there.

What an oversized dehumidifier costs you: a higher purchase price, a slightly larger and heavier unit, and somewhat more noise while it runs. Energy use is not meaningfully worse, because energy follows water removed, and a larger unit simply removes it in fewer hours. Larger units also tend to be at least as efficient per pint as smaller ones. The main genuine drawback is short cycling in a small, dry, tightly sealed room, where the unit may switch on and off frequently; most models handle this with a built-in delay.

What an undersized dehumidifier costs you: it runs continuously and never reaches the set point in humid weather. Continuous running wears the compressor and fan sooner. It takes weeks longer to pull the initial moisture out of the structure. It has no reserve for a rainy spell or an unusually humid week, which is exactly when mould gets a foothold. And in a cool basement it may spend much of its time defrosting, delivering less than its already-inadequate label.

The asymmetry is clear: oversizing wastes a little money once; undersizing wastes energy, equipment life and possibly the contents of the basement. When torn between two sizes, buy the larger. The one exception is a small finished room — a basement bedroom or office of 300–400 square feet — where a big 50-pint unit is louder than it needs to be and a quieter 35-pint is the better fit.

When to Go Whole-House

Whole-house dehumidifiers are a different class of machine: built for continuous duty, rated for low-temperature operation, more efficient per pint, and filtered with pleated media rather than a washable mesh screen. Capacities typically run from about 70 to 120 pints a day for residential ducted models, with larger units available. They are installed either standalone in the basement or crawlspace with a drain line, or ducted into the air handler or their own ductwork so they treat the whole house from a utility room.

Expect roughly $1,500–$3,500 installed for a ducted system, depending on capacity and how much ductwork is involved. That is several times the price of a good portable, so the case has to be specific. Consider a whole-house unit when:

• The basement is over about 2,000–2,500 square feet and damp throughout, or split into rooms that one portable cannot reach.
• A 50-pint portable runs nearly around the clock through the humid season, a month or more after the initial drawdown, and still struggles to hold 50%.
• The basement stays in the 50s°F during the humid months, where portables lose most of their capacity.
• You would otherwise need two or three portables, each with its own drain, filter cleaning and noise.
• The upper floors are humid too, and you want one machine treating the whole house.
• You have a sealed or encapsulated crawlspace, where building codes recognise dehumidification as one way to condition the space.
• The basement is finished living space and the noise of a portable in the room is unwelcome.

For sizing a whole-house unit, the same chart applies but you size for the whole conditioned area if it is ducted. A 70-pint ducted unit suits a large basement or a moderately humid house up to roughly 2,500–3,000 square feet; 90–100 pints suits a larger or wetter house; 120 pints is for big homes, very humid climates, or a large basement plus crawlspace. Ask any installer which test conditions the capacity is quoted at, so you are comparing like with like.

For an ordinary basement under 2,000 square feet that is damp rather than wet, a good portable remains the better value by a wide margin. The models compared in our basement dehumidifier guide include both portable and whole-house options if you are weighing the two.

Using Two or More Units

Two portables are often the right answer for a basement that is too big or too broken-up for one but does not justify a ducted system. Dehumidifiers dry the air that reaches them; air moves slowly through a basement, and a unit in one room does little for a room behind a closed door or at the far end of a long, L-shaped space.

When two units beat one: basements over about 2,000 square feet; layouts split into separate rooms by walls or doors; a finished section and an unfinished utility section with different moisture loads; or a wet area — near a laundry or a sump — at one end and a dry living area at the other.

How to set them up:

• Put each unit in the middle of the zone it serves, with at least a foot of clear space around it, not tucked into a corner.
• Give each one its own drain route — a floor drain, sump, laundry sink or a built-in pump line. Two buckets is twice the chore, and buckets are the main reason dehumidifiers get switched off.
• Set both to the same target. If one is set much lower, it will run constantly and do most of the work while the other idles. Better still, check each zone with its own hygrometer and adjust until both read about the same.
• Use separate circuits where you can. A 50-pint unit typically draws around 5 amps; two on one 15-amp circuit with other loads can trip a breaker.
• In an open-plan basement, a small fan moving air from the far end towards the unit can sometimes let one unit do the job of two.

Two 35-pint units are not usually better than one 50-pint in a single open space, but they are better in two separate rooms, and they give you redundancy: if one fails in the middle of a humid August, the basement is not left with nothing.

How to Tell It Is the Right Size

Sizing charts get you to a sensible starting point. Whether you got it right shows up in the first few weeks, and two cheap measurements tell you.

1. A Separate Hygrometer

Do not rely on the dehumidifier's own display. Its sensor sits at the intake, in the air the unit is drying, and it is often several points off. Put a separate digital hygrometer away from the unit — in the middle of the room or the dampest corner — and adjust the dehumidifier's set point until that hygrometer reads your target. For most basements that target is around 50%; our guide on what humidity to set a basement dehumidifier covers the seasonal adjustments and the reasons not to chase much lower.

Cheap hygrometers can be 3–5 points out. To check yours, put a small mound of table salt moistened to a damp paste in a bottle cap, seal it in a zip-top bag with the hygrometer for 8–12 hours, and read it. Saturated salt holds the air at about 75%; whatever your hygrometer shows, the difference is its error.

2. Run Time

Run time is the most useful single measure of whether the size is right.

• First two to four weeks: a new unit in a damp basement should run most or all of the time while it pulls moisture out of the concrete, framing and contents. That is normal, and the bucket or drain will collect far more water than you expect.
• After the drawdown, in the humid season: a correctly sized unit typically holds the target while running perhaps 8–14 hours a day. It switches off on its own for meaningful stretches.
• Running 20+ hours a day after a month, and still above target: undersized, too cold for the unit, or there is a water source it cannot keep up with.
• Hitting target within hours and running only an hour or two a day all summer: it is comfortably oversized. That is fine — no action needed.

3. Water Collected

A useful cross-check is how much water the unit actually removes. Eight pints make a gallon, so a 50-pint unit running flat out at its rated conditions removes a little over six gallons a day. If a 50-pint unit in a 65°F basement is collecting four or five gallons a day after the first month, it is working near its limit. If the amount it collects stays the same week after week while the humidity barely moves, water is coming in as fast as it is being removed — look at the walls, floor, gutters and grading rather than buying a bigger machine.

4. The Basement Itself

The physical signs are the final test. Within a few weeks of a correctly sized unit running, the musty smell should fade, pipes and walls should stop sweating, cardboard boxes should feel dry rather than soft, and any mildew on stored items should stop spreading. If the smell persists even though the hygrometer reads 50%, look for something actively wet — a damp carpet pad, a hidden leak, or mould already established on a wall or behind panelling — which needs cleaning or removal, not more dehumidification.

The Short Version

For most basements, buy a 50-pint dehumidifier on the 2019 DOE rating. That is what older guides and forum advice mean by "70 pints". Drop to 35 pints only for a small or finished basement under about 800 square feet that is damp rather than wet. Go to two 50-pint units or a 70–120 pint whole-house unit above roughly 2,000 square feet, in a basement that is genuinely wet, or in one that sits in the 50s°F.

Size up one step for laundry dried indoors, a humid-summer climate, an open sump pit, bare block walls or a cold basement. When in doubt, buy the larger size: an oversized dehumidifier simply runs fewer hours, while an undersized one runs constantly and never catches up.

Then check the result with a separate hygrometer and the unit's run time. A correctly sized unit holds about 50% while switching itself off for part of each day. If a week of continuous running barely moves the reading, the issue is liquid water coming through the foundation, and the fix is at the wall or the gutter, not a bigger machine. When you are ready to choose a specific model, our best basement dehumidifier picks are organised by exactly these situations.

Frequently Asked Questions

Related Guides