Dehumidifier sizing

Researched from the AHAM AC-1 test method, EPA and CARB guidance, and published manufacturer specifications

Quick answer

A 1,000 square foot moderately damp basement needs about a 14 pint a day dehumidifier, and the same space needs about 23 pints if it is extremely wet. Those are 2019 DOE test condition ratings, which are roughly thirty percent lower than the pre-2019 numbers for the same physical machine, so an old sizing chart read against a new label will oversize by a whole class.

Dehumidifier sizing Worked answer below
Capacity needed 17 pints per day
Floor area1,000 sq ft
Machine class to look for20 to 22 pint class
If it were one class damper20 pints at 1,000 sq ft
Rating basis2019 DOE test conditions
Target relative humidity30 to 50 percent

Aim for relative humidity between 30 and 50 percent, and reliably under 60. Above 60 percent is where mould and dust mites do well.

A bucket holding a few pints needs emptying more than once a day at this rate. Continuous drainage to a floor drain, or a model with a pump, is what decides whether the machine actually gets used.

How it decides: floor area and dampness class, read against the published capacity table for the 2019 DOE test conditions

Values between the published rows are interpolated in a straight line, which is ours rather than the table's. Standing water is not a dampness class: it is a drainage failure, and no dehumidifier repairs one.

Equipment this calculator applies to

BLACK+DECKER 20-Pint Portable Dehumidifier, BDT20WTB, White

A machine that publishes a figure you can actually compare against the answer above, rather than a coverage area with no air change rate behind it.

$182.09Check pricePrices change often

Prices and specifications change, so check the listing against the number this calculator gave you.

Dehumidifiers and a hygrometer to check the result

Common questions

Why did dehumidifier pint ratings change?

The DOE test conditions changed in 2019 and now run at a lower chamber temperature, which is harder on the machine. The same physical unit that was sold as 70 pint before the change is labelled around 50 pints after it. Nothing about the machine changed, only the test it is measured under, which is why old sizing charts oversize.

Is a bigger dehumidifier better?

Not usually. An oversized machine satisfies the humidistat quickly, switches off, and cycles repeatedly, which dries unevenly and uses more power than a right sized unit running steadily. Oversize deliberately only where the space is very cold or the moisture load is extreme and seasonal.

Will it work in an unheated basement?

A standard compressor model loses capacity as the air gets colder and can ice up below roughly 65 degrees Fahrenheit without automatic defrost. Models that state low temperature operation handle it, and desiccant machines work down to much lower temperatures at the cost of using more power for the same water removed.

How low should I set the humidistat?

Between 45 and 50 percent relative humidity is the usual setting for a basement. Lower than that costs power for no benefit, and the aim is to stay reliably under 60 percent rather than to make the space arid. Put a separate hygrometer somewhere other than beside the machine to check what is actually happening.

Do I need a dehumidifier or an air purifier for a musty basement?

The dehumidifier, first and by a wide margin. Musty smells come from moisture supporting growth, and drying the space is what stops it. A purifier captures spores already airborne, which helps with the symptom while the cause continues. Dry the space, clean what is already growing, then filter.

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