Dehumidifier Size Calculator
Pint capacity by room size and how damp the space is.
You need
30 pints/day
Post-2019 DOE rating. Older labels read about 40% higher for the same machine.
- Space area
- 600 sq ft
Estimates only. Verify quantities before ordering.
How this is calculated
Capacity in pints per day = area / 500 sq ft x a dampness factor (20 for musty or humid air, 25 for visibly damp walls or floors, 30 for wet spots or seepage), rounded up to the next standard size. Based on post-2019 DOE-rated capacities.
Enter the space's dimensions and how damp it gets, and this calculator recommends a capacity in pints per day under the current (post-2019) DOE test standard. That standard change matters when comparing: a unit rated 35 pints today removes about as much water as one labeled 50 pints under the old test, so ignore any sizing chart that predates it.
Dampness level matters as much as area. A basement that merely smells musty in summer needs far less capacity than one with visible condensation or seepage after rain. When in doubt, round up one size: a larger dehumidifier cycles less to hold the same humidity, and most units shut off at their target rather than over-drying.
Dehumidifier sizes for common spaces
Rows use post-2019 DOE pint ratings at the stated dampness.
| Space | Pints/day | Area |
|---|---|---|
| 15 x 20 ft room, musty | 15 pints/day | 300 sq ft |
| 30 x 20 ft basement, musty | 25 pints/day | 600 sq ft |
| 30 x 20 ft basement, damp | 30 pints/day | 600 sq ft |
| 40 x 25 ft basement, damp | 50 pints/day | 1,000 sq ft |
| 40 x 30 ft basement, wet | 75 pints/day | 1,200 sq ft |
The 2019 rating change, and why old charts mislead
In 2019 the Department of Energy changed the test conditions for dehumidifier capacity, lowering the test temperature from 80 to 65 degrees Fahrenheit. Removing moisture from cooler air is harder, so the same machine now carries a smaller pint rating than it did before. A unit labeled 50 pints under the old standard is roughly a 35 pint unit today, with no change to the hardware.
This makes any sizing chart written before 2019 actively misleading, and a great many still circulate online. If a chart recommends 70 pints for a damp basement, it is almost certainly using old-standard numbers and you would be buying substantially more capacity than you need. The figures here use the current standard.
When comparing models, check that both are quoting the current rating, which most reputable listings now do. Integrated pump models and units sold for crawlspaces sometimes still quote saturation-condition capacity, a third figure that flatters the machine further.
Placement, drainage, and airflow
A dehumidifier needs air moving through it, so it wants to stand clear of walls and furniture with unobstructed intake and exhaust, ideally somewhere central rather than tucked in a corner. It will not effectively dry a separate room through a closed door: humidity migrates slowly, and closed interior doors mean multiple units or a whole-house solution.
Emptying a bucket daily is the main reason a dehumidifier ends up switched off, so plan continuous drainage from the start. Most units accept a garden hose fitting and will drain by gravity to a floor drain. If the drain is higher than the unit, you need a model with a built-in pump or a separate condensate pump, since gravity models cannot push water upward.
In an unheated space, check the operating temperature range. Standard compressor units frost up and stop working efficiently below about 65 degrees unless they have an auto-defrost cycle, and in a cold crawlspace or garage a desiccant model, which works differently, may be the only thing that functions through winter.
What humidity level to target
Aim for somewhere between 30 and 50 percent relative humidity, and around 45 to 50 percent in a basement. Below roughly 30 percent the air is uncomfortably dry and wood furniture and trim shrink; above 60 percent, mold and dust mites thrive. A cheap hygrometer is worth having, because perceived dampness is a poor guide and basements often read far higher than people assume.
In winter, the target drops. Warm indoor air holding 50 percent humidity will condense on cold window glass and inside exterior walls when it is well below freezing outside, so 30 to 40 percent is more appropriate in cold weather. Persistent window condensation is the practical signal to lower the setting.
If a dehumidifier runs constantly and never satisfies, the problem is usually a moisture source rather than machine capacity. Bulk water entering through foundation walls, a missing vapor barrier over a dirt crawlspace, downspouts discharging next to the foundation, or an unvented dryer will overwhelm any unit. Fixing the source is cheaper than running a larger machine indefinitely.
Common questions
What size dehumidifier for a basement?
A typical 600 to 1,000 square foot basement that is moderately damp wants a 30 to 50 pint unit under today's ratings. Whole-basement coverage from a single portable unit works best with an open layout; closed-off rooms may need the unit moved or a second small one.
Why do dehumidifier sizes seem smaller than they used to be?
The DOE changed the test conditions in 2019 to cooler, more realistic temperatures, and measured capacities dropped about 30 to 40% on the same hardware. A modern 35 pint unit roughly equals an old 50 pint unit. Compare only within the same rating era.
What humidity should I aim for?
30 to 50% relative humidity is the comfort and mold-prevention range; 50% is the common setpoint for basements. Below 30% the air gets uncomfortably dry, and above 60% mold and dust mites thrive. A cheap hygrometer tells you where you stand before and after.
Does a dehumidifier need a drain?
Not necessarily: units collect into a bucket you empty, typically once or twice a day in damp spaces. But any unit running constantly (a basement in summer) is far more practical with its hose port draining to a floor drain or a condensate pump, and continuous-drain setup takes five minutes.