Understanding your result
The headline is the dehumidifier size to buy in pints per day under the 2019 DOE test, with the capacity needed and a description of the space in the sub-line. The stats give the capacity needed, the size to buy and the daily electricity. The table builds the figure up step by step, from the base capacity for a slightly damp space of that area, to the dampness adjustment and the climate factor, then gives the size, the same unit’s rating under the pre-2019 test, the running cost and the water removed per day at capacity.
The old-rating row is useful in two ways: it lets you compare with a pre-2019 unit you already own, and it explains why a review from 2017 recommended a “70-pint” unit for a basement that the current chart sizes at 50. The water figure tells you how often the bucket would need emptying, which for any basement unit is a reason to run a drain hose.
The dew point and humidity calculator shows how close the basement air is to condensing on a cold wall or pipe at your humidity; the humidifier size calculator is the winter opposite for the main floors; and the whole-house ventilation calculator covers the fresh air that, in a humid climate, brings much of the moisture in.
How we calculate this
The sizing follows the shape of the AHAM and ENERGY STAR dehumidifier selection chart, which steps up the capacity with area and with how damp the space is, restated for the 2019 test conditions (65 °F, 60% RH). The climate factor nudges the result for outdoor air that is unusually humid or dry. The energy estimate uses an integrated energy factor of 1.9 litres per kWh, typical of ENERGY STAR portable units under the current test, and assumes the unit runs at rated capacity for the hours entered. The ventilation and moisture figures used across the site are on the ventilation and exhaust minimums page.
Metric mode shows litres per day (1 US pint = 0.473 litres) and converts the area to square feet for the chart.
The assumptions behind the numbers
| Assumption | Value | Where it comes from |
|---|---|---|
| Test condition | 65 °F, 60% RH | US DOE dehumidifier test procedure, Appendix X1 (effective 2019) |
| Sizing chart | Area and dampness steps, restated for the 2019 test | AHAM / ENERGY STAR dehumidifier sizing guidance |
| Old-to-new rating | New ≈ old × 0.7 | Typical ratio between the 80 °F and 65 °F test results |
| Sizes sold | 20, 22, 30, 35, 40, 50 pints/day | Portable dehumidifiers on the US market |
| Efficiency | 1.9 L per kWh | Typical ENERGY STAR integrated energy factor under the 2019 test |
| Electricity price | $0.17 per kWh | US residential average, EIA, 2025 |
| Target humidity | 50% RH | Common guidance for basements; below 60% limits mould |
Assumptions last reviewed October 8, 2026.
The calculator does not model the moisture load directly, which depends on seepage, the soil, the ventilation and the season, nor the fall in capacity when a basement is colder than 65 °F, which is why low-temperature models with defrost exist. It does not size whole-house ducted dehumidifiers, which are rated the same way but connected to the air handler. The guide to dehumidifier pints, really explains the 2019 change, and the guide to humidity and comfort covers the indoor humidity targets.
Two worked examples
A damp 1,000 sq ft basement
1,000 sq ft, damp (musty smell, damp spots), average climate, basement, running 10 hours a day at 17 cents per kWh.
- Base capacity: 10 + 1,000 ÷ 100 = 20 pints/day
- Damp: + 5 → 25 pints/day; climate × 1.0
- Size to buy: 30 pints/day (about 42 under the old rating)
- Electricity: 30 × 0.473 ÷ 1.9 × 10/24 = 3.1 kWh/day, $0.53 a day, about $16 a month
A 30-pint unit with a drain hose to the floor drain, set to 50%, is the usual answer for this basement. The note about cool basements applies: at 60 °F the unit removes noticeably less than its rating, so a model rated for low-temperature operation is worth the small premium.
A very wet basement in a humid climate
1,800 sq ft, very wet (seepage), humid climate, basement, 10 hours a day.
- Base capacity: 10 + 1,800 ÷ 100 = 28 pints/day
- Very wet: + 10 → 38; humid × 1.1 = 41.8 pints/day
- Size to buy: 50 pints/day (about 70 under the old rating)
- Electricity: 5.2 kWh/day, about $26 a month
At this size a single portable unit is near its limit and will run many more than 10 hours a day while the seepage continues; the note is blunt that the water problem comes first. With the gutters, grading and wall cracks fixed, the same basement would size as “damp” at about 36 pints and run a fraction of the time.
Where to find your inputs
Area. The floor area of the space the unit will serve. A basement divided into rooms with closed doors needs either a unit per area or the doors left open.
Dampness. “Slightly damp” smells musty only in humid weather; “damp” smells musty most of the time or has damp spots; “very wet” has visible water, wet walls or seepage.
Climate. Humid for the Southeast, Gulf Coast and much of the Midwest in summer; dry for the desert Southwest and mountain West.
Run time. Once the space is dry a unit typically runs 8 to 12 hours a day in summer; the first week runs almost continuously.
Common mistakes
- Comparing old and new ratings. A 70-pint unit from 2017 and a 50-pint unit today are about the same machine.
- Treating a water problem with a dehumidifier. Seepage needs drainage and grading, not a bigger unit.
- Emptying a bucket by hand. The unit stops when the bucket is full; drain it to a floor drain or use a pump.
- Setting it too low. Below 40% RH there is no benefit and the electricity bill grows.
- Leaving doors closed. A unit in one room does little for the room next door.
- Ignoring temperature. In a cold basement a standard unit’s coil frosts; choose one rated for low temperatures.
Questions people ask
- What size dehumidifier do I need for a basement?
- For a 1,000 sq ft basement that smells musty and has damp spots, about 25 pints per day under the 2019 rating, so a 30-pint unit. A slightly damp basement of the same size needs about 20 pints; a very wet one with seepage about 30, or 33 in a humid climate. A 1,800 sq ft very wet basement in a humid climate needs about 42 pints and a 50-pint unit. Fix water coming through the walls first; a dehumidifier cannot keep up with seepage.
- Why did dehumidifier pint ratings change in 2019?
- The Department of Energy changed the test conditions from 80 °F and 60% relative humidity to 65 °F and 60%, because most dehumidifiers run in cool basements where they remove much less water than at 80 °F. The same machine now carries a rating about 30 to 40% lower, so a unit sold as 35 pints today is roughly what was sold as 50 pints before 2019. Compare like with like when reading older reviews and charts.
- What humidity should a basement be?
- Below 60% relative humidity to keep mould and dust mites in check, and 50% is the usual target setting on the dehumidifier. Above 60% for long periods, cool basement walls and stored boxes start to grow mould; below 40% there is no benefit and the unit wastes electricity. A cheap hygrometer tells you where you are, and the dew point calculator shows how close the air is to condensing on cold pipes and walls.
- How much does it cost to run a dehumidifier?
- A 30-pint unit that removes about 14 litres a day at an efficiency of 1.9 litres per kWh uses about 7.5 kWh in a full day of running, or about 3 kWh in the 10 hours a day a typical basement unit runs once the space is dry. At 17 cents per kWh that is about 53 cents a day or $16 a month. ENERGY STAR units use about 15% less than standard ones of the same size.
- Should I buy a bigger dehumidifier than I need?
- One size up is reasonable, and the calculator already rounds up. A larger unit reaches the setpoint faster and runs fewer hours, and it copes better with a wet week; it does not use much more electricity for the same water removed. Much larger is wasteful, and in a small room it cycles on and off. The real constraint is usually emptying the bucket, which is why a drain hose to a floor drain or a unit with a pump matters more than the last 10 pints.
- Can the air conditioner dehumidify instead?
- In summer the central air conditioner removes moisture from the rooms it cools, and a correctly sized one keeps the main floors below 50 to 55% RH. It does little for a basement it barely cools, and an oversized one that short-cycles removes little moisture anywhere. A basement usually needs its own dehumidifier, or a whole-house dehumidifier ducted into the HVAC system for a house that is humid throughout.