What size dehumidifier do I need for my room?

How to size a dehumidifier

In the US the calculator starts from the current ENERGY STAR chart, which gives a capacity range for each condition, rated the way dehumidifiers are tested since 2019:

Condition without a dehumidifier	Up to 2,000 sq ft	Over 2,000 sq ft
Moderately damp: musty at times, 50-75% RH	20-30 pints	30+ pints
Very damp: always smells damp, damp spots, 75-90% RH	25-40 pints	40+ pints
Wet: walls or floor sweat, seepage, laundry drying, 90-100% RH	30-50 pints	50+ pints

The chart does not say where a given room falls inside the range, so the calculator moves from the low end for a small room to the high end at 2,000 sq ft, in proportion to the floor area. Above 2,000 sq ft it keeps the same pints per square foot. The chart gives no ceiling height either: the calculator assumes 8 ft and adds capacity in proportion for higher ceilings. Laundry drying puts the room in the wet row, as ENERGY STAR does.

The result is a range of ±10%, half the step between two rows. The calculator then picks the smallest size on sale that covers the need, since ENERGY STAR advises oversizing rather than undersizing.

A worked example

In feet (US). The calculator opens with a 30 × 25 ft basement, 8 ft ceiling, moderately damp, no laundry.

• Area: 750 sq ft, so 20 + 10 × 750 ÷ 2,000 = 23.75 pints per day
• Range: 21 to 26 pints per day
• A 22-pint unit falls short, so the size to buy is a 35-pint dehumidifier

If the basement is very damp the need rises to 30.6 pints and the 35-pint unit still covers it. If it is wet, or laundry dries there, it reaches 37.5 pints and you need a 50-pint dehumidifier.

In metres (UK and elsewhere). UK and European dehumidifiers are rated in litres per day at 30 °C and 80% RH, so in metres the calculator uses the earlier AHAM chart, graded every 500 sq ft and rated at 26.7 °C and 60% RH. A 4 × 4 m room with a 2.7 m ceiling is 172 sq ft, below the first column: 10 pints, plus 11% for the ceiling, 5.2 litres per day. At 30 °C and 80% RH that becomes 9.2 litres (× 1.75, see below), so the size to buy is a 10 litre dehumidifier.

Dehumidifier size chart by square feet

The size to buy with an 8 ft ceiling, in pints per day (DOE 2019 rating):

Area	Moderately damp	Very damp	Wet
200 sq ft	22 pints	35 pints	35 pints
500 sq ft	35 pints	35 pints	35 pints
1,000 sq ft	35 pints	35 pints	50 pints
1,500 sq ft	35 pints	50 pints	50 pints
2,000 sq ft	35 pints	50 pints	50 pints

For a basement over 2,000 sq ft that is wet, the need goes past 50 pints: two units, or a whole-home dehumidifier sized and installed by an HVAC contractor.

Pint ratings, the 2019 change and litres in the UK

Since 2019 the Department of Energy (DOE) tests portable dehumidifiers at 65 °F instead of 80 °F, at the same relative humidity. Cooler air holds less water, so ratings dropped: by the ENERGY STAR comparison, an old 30-pint unit is about 20 pints today, an old 50-pint about 30 and an old 70-pint about 40 to 45. Portable units now sell mostly as 20, 22, 35 and 50 pints. If you are replacing an old dehumidifier, the calculator also shows the need on the pre-2019 scale.

In the UK the box shows litres per day, usually at 30 °C and 80% RH (10, 12, 16, 20, 25 and 30 litres). Meaco publishes its 20 litre model at several conditions: 20.38 litres at 30 °C and 80%, 13.27 at 30 °C and 60%, 8.48 at 20 °C and 60%. In proportion between 20 and 30 °C, 26.7 °C and 60% works out at 11.67 litres, and 20.38 ÷ 11.67 = 1.75 is the factor the calculator uses.

Common mistakes

• Comparing old and new ratings. A 50-pint unit from before 2019 and a 50-pint unit today are not the same size.
• Buying too small. An undersized unit runs nonstop and never reaches the humidity you set; go one size up.
• A cold basement. Below 65 °F frost can form on the coils, ENERGY STAR warns: look for a model rated for low temperatures.
• Doors and windows open. Keep the room closed while the dehumidifier runs, or it dries the outdoors too.
• Setting the wrong humidity. ENERGY STAR puts the right indoor humidity between 30% and 50%, and 30% to 40% in the heating season in cold climates; mold grows more easily above that.