How big a dehumidifier do you need? Pints per day.

Short answer

An 800 sq ft basement that is very damp (75 to 90% relative humidity) needs a portable dehumidifier rated about 25 to 40 pints per day, which is 11.8 to 18.9 litres per day. ENERGY STAR gives separate ranges for rooms under and over 2,000 sq ft, and says it is better to oversize than undersize.

How it works, with a worked example

ENERGY STAR publishes the minimum portable dehumidifier capacity by two things: how damp the space is, and whether the room is under or over 2,000 sq ft. The tool looks up your row. For a space under 2,000 sq ft the ranges are 20 to 30 pints per day (slightly to moderately damp), 25 to 40 (very damp) and 30 to 50 (wet). Over 2,000 sq ft the guide gives 30+, 40+ and 50+ pints per day. One US pint is 0.4732 litres.

Worked example. An 800 sq ft basement that consistently feels and smells damp, with damp spots on walls and floors, is in the very damp row: 25 to 40 pints per day, or 11.8 to 18.9 litres. A listing showing an old 50-pint rating maps to about 30 pints on the 2020 scale, which is the low end of that range, so a unit rated near 40 new pints gives headroom.

Why ratings changed. ENERGY STAR explains that the Department of Energy changed the test to 65 F from 80 F, so there is less water to remove and the same machine reports a smaller capacity. Its conversion table is built into the tool: 30 to 20, 40 to 25, 50 to 30, 55 to 30-35, 60 to 35, 70 to 40-45 and 90 to 55 pints per day. Actual changes vary by model.

If the space regularly falls below 65 F, ENERGY STAR suggests a unit specified for lower temperatures because frost can form on the coils.