Key takeaways
- ENERGY STAR's chart gives 20 to 30 pints per day for a slightly to moderately damp room under 2,000 square feet.
- ENERGY STAR says it's better to oversize than undersize, and calls 30% to 50% relative humidity the optimum range.
- Under DOE's newer 65°F test, an older 70-pint portable correlates to about 40 to 45 pints, so ratings from different years don't compare directly.
Enter the room's length and width, or its area, and pick the description that best matches how damp it is. If you have a hygrometer reading, use the humidity ranges in each description. The calculator gives ENERGY STAR's recommended capacity range in pints per day.
How the calculator works
- Area = length × width
- Size column: under 2,000 square feet, or over
- Condition row: slightly to moderately damp, very damp, or wet
- Result: the pints-per-day range where the row and column meet
ENERGY STAR's dehumidifier page publishes this chart:
| Condition without dehumidification | Room under 2,000 sq ft | Room over 2,000 sq ft |
|---|---|---|
| Slightly to moderately damp: feels damp, musty odor at times, 50% to 75% relative humidity | 20 to 30 pints/day | 30+ pints/day |
| Very damp: always feels and smells damp, damp spots on walls and floors, 75% to 90% | 25 to 40 pints/day | 40+ pints/day |
| Wet: walls or floor sweat or seepage is present, 90% to 100% | 30 to 50 pints/day | 50+ pints/day |
ENERGY STAR's wet row also mentions high-load conditions, such as drying laundry in the space. The chart's columns split at 2,000 square feet and its condition rows share 75% and 90% at their edges. If your reading is right on one of those lines, look at both rows.
Worked example
A 30 × 40 foot basement. A hygrometer reads 80%, and there are damp patches on the floor:
- Area: 30 × 40 = 1,200 square feet, so the under-2,000 column
- Condition: 80% with damp spots matches very damp
- Chart result: 25 to 40 pints per day
ENERGY STAR says "it's better to oversize than undersize", so in a basement like this one you'd look toward the 40-pint end of the range. If the floor were sweating or water were seeping in, the wet row would apply instead, at 30 to 50 pints per day. Seepage is also a sign to look at drainage outside, which a dehumidifier can't fix.
Why old and new ratings don't match
ENERGY STAR explains that the Department of Energy's test procedure now runs portable units at 65°F rather than 80°F, closer to basement conditions. Standards using the new efficiency metric took effect in June 2019, and the new capacity ratings appeared from early 2020. The same machine gets a lower pints-per-day figure at the cooler test.
ENERGY STAR gives approximate correlations for portable units. A few rows:
| Older rating | Newer rating |
|---|---|
| 30 pints/day | 20 pints/day |
| 50 pints/day | 30 pints/day |
| 70 pints/day | 40 to 45 pints/day |
So if you're replacing a 70-pint unit from before 2020, a new one rated about 40 to 45 pints may do a similar job. ENERGY STAR says actual changes vary by model, and that manufacturers may be able to give more reliable comparisons. Check which test a rating uses before you compare two models.
Read the room's humidity first
A hygrometer measures relative humidity, and you can find one at a hardware store. Leave it in the room for a day or so, since readings change with the weather and with activities such as showers and laundry.
What to aim for:
- ENERGY STAR calls 30% to 50% relative humidity generally the optimum, and says anything above that may promote mold growth.
- In colder climates during the heating season, it says 30% to 40% helps prevent window condensation.
- The EPA advises keeping indoor humidity below 60%, ideally between 30% and 50%.
Basements, drains and placement
Basements. ENERGY STAR notes that frost can form on the coils if the air drops below 65°F, and some units have an anti-frost sensor that shuts them off when it's too cold. In a cool basement, check the unit's operating temperature range. ENERGY STAR and the EPA both suggest grading the soil to slope away from the foundation, so water doesn't pool beside it. Gutters and downspouts matter too: see the gutter and downspout calculator.
Drains. A portable unit collects water in a bucket. If yours has a hose connection, its manual says how to set up continuous draining. ENERGY STAR says to keep drain hoses away from electrical cords and connections, and never to set up drainage near electrical circuits or devices. Whole-home units usually have only a drain, which it says should be routed to a floor drain or condensate pump by an HVAC professional.
Placement. Keep the unit away from walls and furniture so air can move around it, close the room's doors and windows while it runs, and keep it away from dust sources such as woodworking tools. ENERGY STAR also says the air right around the unit may feel warmer, which is normal.
Whole-home units. For a home with central air that stays humid, ENERGY STAR says a whole-home dehumidifier may be the right choice, and that it should be sized and installed by a professional. This calculator is for portable units in one space.
ENERGY STAR says certified dehumidifiers use about 20% less energy than similar conventional models.
Common mistakes
- Comparing ratings across test years. A pre-2020 rating and a newer one aren't on the same scale.
- Guessing the condition. A hygrometer reading puts you in the right row.
- Running it in an open space. Closed doors and windows let the unit work on the room it's sized for.
- Ignoring the cause. Seepage and pooling water outside call for drainage work, not just a bigger unit.
- Undersizing to save space. ENERGY STAR says it's better to oversize than undersize.
Frequently asked questions
What size dehumidifier do I need for a basement?
Measure its area and check how damp it is. For a very damp basement under 2,000 square feet, ENERGY STAR's chart gives 25 to 40 pints per day, and 30 to 50 if the walls or floor sweat or seep.
Is a 50-pint dehumidifier too big?
Not by ENERGY STAR's guidance, which says it's better to oversize than undersize. Check whether the 50 pints is an older rating or a newer one, since older ratings ran higher.
Why are dehumidifiers rated lower than they used to be?
DOE's test now runs portable units at 65°F instead of 80°F. ENERGY STAR's correlation puts an older 50-pint rating at about 30 pints on the newer test.
What humidity should a dehumidifier be set to?
ENERGY STAR calls 30% to 50% generally the optimum, with 30% to 40% in cold-climate heating seasons to prevent window condensation.
Can I drain a dehumidifier into a floor drain?
If your unit has a hose connection, follow its manual for running it to a drain. ENERGY STAR says to keep drain hoses away from electrical cords and connections.
Does a dehumidifier work in a cold basement?
Frost can form on the coils below 65°F, according to ENERGY STAR. Check the unit's operating range, and look for an anti-frost feature.
Sources
We use primary sources where we can: building codes, federal rules and manufacturer documents. Read our editorial standards.
- ENERGY STAR: Dehumidifiers (sizing chart, humidity range, installation tips)
- ENERGY STAR: Dehumidifier testing and capacity
- EPA: A brief guide to mold, moisture and your home