The short answer
A 1,000 sq ft basement that is very damp needs about 12 pints a day at the current 2019 rating, so a 20-pint dehumidifier. It was a 30-pint unit on older labels. If it's wet, it needs a 20-pint unit, and if it's extremely wet, a 20-pint one.
Dehumidifier size chart
The AHAM sizing chart, converted to the 2019 rating. Each cell is the pints a day you need at the 2019 rating, with the old-label pints in parentheses. Between rows, the calculator interpolates.
| Area | Moderate | Very damp | Wet | Extreme |
|---|---|---|---|---|
| 500 sq ft | 7 (10) | 8 (12) | 10 (14) | 11 (16) |
| 1,000 sq ft | 10 (14) | 12 (17) | 14 (20) | 16 (23) |
| 1,500 sq ft | 13 (18) | 15 (22) | 18 (26) | 21 (30) |
| 2,000 sq ft | 15 (22) | 19 (27) | 22 (32) | 26 (37) |
| 2,500 sq ft | 18 (26) | 22 (32) | 27 (38) | 31 (44) |
Above 2,500 sq ft, add 2.8, 3.5, 4.2, 4.9 pints (2019) per extra 500 sq ft for the four columns. Then pick the smallest current size that covers it: 20, 22, 30, 35, 50 pints.
How it works
The formula
We start from the AHAM (Association of Home Appliance Manufacturers) sizing chart, which gives the pints per day for a space by its area and dampness. That chart was written for the old test. We add the extras, then convert to the 2019 rating.
pints at the 2019 rating = old pints × 0.7
size = smallest of 20, 22, 30, 35, 50 that is at least that
Assumptions
- The AHAM chart in old pints per day: 500 sq ft 10/12/14/16; 1,000 sq ft 14/17/20/23; 1,500 sq ft 18/22/26/30; 2,000 sq ft 22/27/32/37; 2,500 sq ft 26/32/38/44, for moderately damp, very damp, wet and extremely wet. Above 2,500 sq ft it adds 4/5/6/7 per extra 500 sq ft. Between rows we interpolate. Below 500 sq ft we use the 500 sq ft row.
- Since 2019, DOE rates dehumidifiers at 65°F and 60% humidity, not 80°F, which is closer to a basement. A unit pulls less water out of cooler air, so the same machine now has a smaller number. We multiply by 0.7. A 35-pint unit today is about an old 50-pint unit.
- The extras are our rules of thumb, not AHAM's: a washer or dryer adds 5 old pints, more than 4 people add 5, and a hot, humid climate adds 10% of the chart value.
- Current sizes are 20, 22, 30, 35, 50 pints. Above 50 pints, use a whole-house unit (70+ pints, ducted) or two portable units.
- Sources: AHAM dehumidifier sizing chart; ENERGY STAR dehumidifiers; DOE Energy Saver (archived).
Worked example: 1,000 sq ft, very damp
A basement of 1,000 sq ft that always feels damp and musty. No laundry, few people, no humid climate.
- AHAM chart for 1,000 sq ft, very damp: 17 old pints a day.
- No extras: 17 + 0 = 17.
- Convert to the 2019 rating: 17 × 0.7 = 11.9 pints.
- The smallest size at least 11.9: 20 pints. On an old label, that is about 30 pints.
- With a washer and dryer, many people and a humid climate, a 1,500 sq ft wet space adds up differently: 26 × 1.1 + 5 + 5 = 38.6 old pints; × 0.7 = 27 pints; the size is 30 pints.
Too big or too small
- Too big: costs more. A bigger unit costs more to buy and uses more power while it runs. It is the lesser evil, since it dries the space and shuts off.
- Too big: too dry or noisy. It can overshoot, and the cycling fan is a nuisance in a living space. Set the humidistat at 45-50%.
- Too small: it never catches up. It runs around the clock, the humidity stays above 60% and mold keeps growing.
- Too small: the tank fills. A small unit with a small bucket needs emptying all day. Use a drain hose or a pump.
- Between sizes? Take the larger one. A unit that is one size up works less hard and shuts off sooner.
Questions people ask
What size dehumidifier do I need for a 1,000 sq ft basement?
It depends on how damp it is. At 1,000 sq ft you need a 20-pint unit if it's moderately damp (musty, damp only in humid weather), a 20-pint unit if it's very damp, 20-pint if it's wet (wet walls or floor) and 20-pint if it's extremely wet. Sizes are in pints per day at the 2019 rating.
Why is a 35-pint dehumidifier now about the same as a 50-pint one?
In 2019 the DOE changed the test for dehumidifiers from 80°F and 60% humidity to 65°F and 60%, closer to a real basement. Dehumidifiers pull less water out of cooler air, so every unit's pints per day dropped by about 30%. A new 35-pint unit is roughly an old 50-pint unit. The machines didn't get weaker; the rating got honest. We convert the AHAM chart, written for old pints, by multiplying by 0.7.
What size dehumidifier do I need for a 2,000 sq ft house?
For a 2,000 sq ft space that's very damp, we get about 19 pints a day at the 2019 rating, so 20-pint. If the whole house is damp, a ducted whole-house unit (70+ pints) is the other choice; it dries every room instead of one, and it drains by itself.
What does "pints per day" mean?
It is how much water the dehumidifier removes from the air in 24 hours of running, measured in a test room at a set temperature and humidity. Pints are a volume; 1 pint of water weighs about 1 pound. The number on a unit sold after 2019 is at 65°F; before that it was at 80°F, so the same label means something different.
Is a bigger dehumidifier better?
Somewhat. A larger unit dries the space faster and then cycles off, so it saves running time, but it costs more to buy and draws more power. It can't hurt comfort. A unit that's too small runs all day and never reaches 50% humidity, which is the target for stopping mold. If you're between sizes, take the next one up.
How do I know if my space is damp, very damp or wet?
Moderately damp: a musty smell, damp only when it's humid out. Very damp: it always feels damp and smells musty. Wet: damp spots on walls or floor, or seepage in wet weather. Extremely wet: standing water, or you dry laundry in the space. If it's wet or extremely wet, fix the source first (gutters, grading, a sump pump). A dehumidifier treats the air, not a leak.