Dehumidifier Size Calculator
Work out how many pints per day you need, corrected for the real air temperature of your space.
- Moisture load to remove
- 24.0pints/day
- Water removed per day
- 3.00gallons
- Temperature derate applied
- 1.00×
- Space volume
- 8,000cu ft
- Run the unit on a continuous drain if a floor drain or sink is available. At this load a tank will need emptying at least once a day.
How this calculator works
Dehumidifier capacity is sold in pints per day: how much water the unit can pull out of the air in 24 hours. Picking the right number takes two steps. First, work out how much water your space actually needs removed, which depends on the volume of the space and how damp it is. Second, work out how much of its nameplate capacity the unit will really deliver at your air temperature, because that number is lower than the box suggests.
This calculator estimates the moisture load from floor area, ceiling height, and moisture level, then divides by a temperature correction factor. It adds a 25% margin, because Energy Star's guidance is that oversizing a dehumidifier is safer than undersizing it, and finally rounds up to a size you can actually buy.
Why temperature matters more than most charts admit
Condenser dehumidifiers are rated under a standard test condition of 65°F and 60% relative humidity. Almost every sizing chart online takes that rating at face value and matches it to a square footage. But the water a condenser unit can remove depends on the absolute humidity of the air reaching its coil, and cold air holds far less moisture than warm air.
The correction used here comes from the Clausius–Clapeyron relationship, evaluated with the Magnus formula for saturation vapor pressure. At 60% relative humidity, a unit rated at 50 pints works out to roughly 42 pints in a 60°F basement and only about 30 pints at 50°F. That is the difference between a dry basement and a dehumidifier that runs forever without keeping up.
Quick reference: capacity by area and moisture
| Area (8 ft ceiling) | Slightly damp | Moderately damp | Very damp | Wet |
|---|---|---|---|---|
| 500 sq ft | 20 pints | 20 pints | 25 pints | 35 pints |
| 1,000 sq ft | 20 pints | 30 pints | 45 pints | 70 pints |
| 1,500 sq ft | 30 pints | 45 pints | 70 pints | 100 pints |
| 2,000 sq ft | 40 pints | 60 pints | 90 pints | Two × 70 pints |
Reference figures at 65°F. Colder spaces need a larger nameplate size — use the calculator above for your actual temperature.
AHAM versus DOE ratings
If you are comparing a newer unit against an older one, or reading a review from before 2019, you will see two different numbers for the same machine. AHAM rated capacity at 80°F and 60% relative humidity. The DOE test introduced in 2019 measures at 65°F and 60% relative humidity. Because the cooler test is harder, the same dehumidifier carries a noticeably lower number today — a unit that was sold as 70 pints is generally sold as 45 to 50 pints now.
This matters when you are replacing a unit and reaching for the same number you bought last time. If your old unit was rated under the AHAM method, the equivalent DOE rating is roughly two thirds of it. The calculator above returns DOE-rated sizes, which is what is printed on units sold today.
Choosing the unit once you have the size
Size is the first decision, not the only one. For any space below 60°F, check that the unit is rated for low-temperature operation, since standard models defrost and lose output. For crawlspaces, confirm the unit can drain continuously rather than relying on a tank. And in a space that is genuinely wet rather than merely humid, fix the water source first: a dehumidifier sized for the symptom will run constantly and still lose ground against an active leak or poor drainage.
A hygrometer costs very little and removes the guesswork from the moisture level input. Close the space up for a few hours with no dehumidifier running, then read the relative humidity. That reading, plus the temperature, is all you need.
Frequently asked questions
- What size dehumidifier do I need for a 1,000 sq ft basement?
- For a 1,000 sq ft basement with 8 ft ceilings at a moderate moisture level, you need a 30 pint per day unit at the DOE rating. Drop the temperature to 60°F and the same space calls for a 40 pint unit, because a condenser dehumidifier loses about 15% of its rated capacity in cooler air.
- What is the difference between AHAM and DOE pint ratings?
- AHAM measured capacity at 80°F and 60% relative humidity. Since 2019 the DOE measures at 65°F and 60% relative humidity, which is a colder, harder test. The same physical machine therefore carries a lower number under the DOE method — typically about two thirds of its old AHAM figure. A unit once sold as 70 pints is now sold as roughly 45 to 50 pints.
- Can I use a bigger dehumidifier than recommended?
- Slightly oversized is safer than undersized, which is why this calculator builds in a 25% margin. Beyond that, an oversized unit short-cycles: it satisfies the humidistat quickly and shuts off before it has run long enough to dry the surfaces and framing in the room. It also draws more power and costs more. Stay within the acceptable range shown above.
- Do I need a different dehumidifier for a cold basement?
- Yes. Most condenser dehumidifiers are rated only down to about 65°F and cycle into defrost below roughly 60°F, at which point they stop removing water for part of each cycle. For spaces that stay below 60°F, look for a low-temperature unit rated for operation to 41°F, or a desiccant dehumidifier, which works on a different principle and holds up better in the cold.
- How many pints per day do I need for a crawlspace?
- Crawlspaces have low ceilings, so use the actual height rather than 8 ft. A 600 sq ft crawlspace with 4 ft of headroom is only 2,400 cubic feet, which at a moderate moisture level needs about a 20 pint unit. The bigger issue is drainage: choose a unit with a continuous drain hose, because emptying a tank through a crawlspace access hatch is impractical.
- Is a 20 pint dehumidifier enough for a basement?
- Only for a small, mildly damp one. A 20 pint unit suits roughly 500 sq ft or less at a slight moisture level in a space at 65°F or warmer. For a typical full basement at 55 to 60°F, a 20 pint unit will run continuously and still not hold the humidity down.
- At what temperature do dehumidifiers stop working?
- Compressor dehumidifiers lose capacity steadily as air cools and generally stop being effective below about 60°F, because frost forms on the evaporator coil and the unit spends part of each cycle in defrost rather than removing water. Standard units are usually rated for operation down to 65°F; low-temperature models extend that to 41°F. Desiccant units operate below freezing.
- How do I know if my dehumidifier is too small?
- The clearest signs are a unit that runs continuously without the humidity reading dropping, a tank that fills several times a day, and a space that still feels damp or smells musty. If the target relative humidity is never reached, the unit is undersized for the load, the space is too cold for it, or there is an active water source the unit cannot keep up with.
- Should I get a desiccant or a compressor dehumidifier?
- Compressor units are more energy efficient in warm conditions and are the right choice for basements and living spaces above 65°F. Desiccant units cost more to run but keep working in cold and near-freezing spaces, which makes them the better pick for unheated crawlspaces, garages, and construction drying in winter.
- How often will I need to empty the tank?
- Divide the daily removal figure shown above by your tank size. A unit removing 24 pints a day filling a 20 pint tank needs emptying more than once daily. Because the load is highest overnight when temperatures drop, running on a continuous drain hose to a floor drain, sink, or condensate pump removes the chore entirely.
- Does dehumidifier size depend on humidity level, not just square footage?
- Yes, and it matters as much as area does. Two identical 1,000 sq ft basements need very different units if one sits at 60% relative humidity and the other at 85%. This calculator takes the moisture level as an input for that reason. Measure with a cheap hygrometer after the space has been closed up for a few hours with no dehumidifier running.
- What size dehumidifier for a 500 sq ft room?
- A 500 sq ft room with 8 ft ceilings at a moderate moisture level and 65°F needs about a 20 pint unit. If the room is damp and cold, step up to 25 or 30 pints. Small bedrooms and living spaces rarely need more than 30 pints unless there is a specific damp problem.