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Calculator · Dehumidifier
Dehumidifier size calculator
A dehumidifier has to pull out the water that gets into the space each day. This calculator adds up what leaking outdoor air carries in, what people give off and anything else you can put a number on, then reports the total in pints per day at the temperature and humidity you want to hold.
- Enter the floor area, ceiling height and air changes per hour for the basement, crawlspace or house.
- Enter a humid outdoor temperature and RH, then the temperature the space runs at and the RH you want to hold.
- Count the people and their hours in the space, and add any other moisture you can estimate in pints per day.
- The result shows pints per day to remove and how much comes from each source. Compare it with ratings from the same DOE test.
Example result for the default inputs
1,000 sq ft at 8 ft, 0.18 ACH. Outdoor 85 °F at 65% RH, target 70 °F at 50% RH, sea level.
Moisture to remove
23.7 pints per day
Infiltration share
22.2 pints per day
| Space volume | 8,000 cu ft |
|---|---|
| Infiltration airflow (ACH x volume / 60) | 24.0 CFM |
| Indoor target dew point | 50.5 °F |
| Outdoor dew point | 71.9 °F |
| Indoor humidity ratio | 0.00776 lb water per lb dry air |
| Outdoor humidity ratio | 0.01680 lb water per lb dry air |
| Dry air entering per day | 2,557 lb |
| Infiltration moisture | 22.2 pints per day |
| Occupants (200 BTU/h latent each) | 1.5 pints per day |
| Other moisture sources (entered) | 0.0 pints per day |
| DOE rating condition, portable units | 65 °F dry-bulb, 56.6 °F wet-bulb |
For estimation purposes only. Consult a licensed engineer for final designs.
Free without signup, 3 questions a day
How the moisture load is calculated
The calculator runs a daily moisture balance on the space. Outside air that leaks in carries water, people add some, and anything else you know about gets added on top. Every term is in pounds of water until the last step converts to pints.
- Infiltration airflow
CFM = ACH x floor area x ceiling height / 60ACH is natural air changes per hour. The presets in the field hint are BuildSolver load engine defaults, not published figures.- Humidity ratios
W = 0.621945 x p_w / (14.696 - p_w)Pounds of water per pound of dry air, outdoors and at your target, from the same psychrometric code as the psychrometric calculator (ASHRAE Handbook Fundamentals, Chapter 1, with Magnus-Tetens saturation pressure) at sea level pressure.- Dry air per day
lb dry air = CFM x 60 x 24 / vv is the specific volume of the indoor air in cu ft per lb of dry air, so 1 / v is the dry air density. The calculator treats ACH as a volume of indoor air.- Infiltration moisture
lb water = lb dry air x (W_out - W_in), or 0 if outdoor air is drierWhen outdoor air holds no more water per pound than your target, leakage adds nothing. The calculator does not subtract it as free drying.- People
lb water = people x hours x 200 BTU/h / 1,061 BTU/lb200 BTU/h latent per person is the constant in BuildSolver's Manual J engine, and 1,061 BTU/lb is the latent heat term from the ASHRAE enthalpy equation. Both are assumptions of this calculator.- Pints per day
pints = lb water / 1.04 + other sourcesDOE's dehumidifier test procedure uses 1.04 as the density of water in pounds per pint. Other sources are entered directly in pints per day.
Worked examples for a basement, a crawlspace and a house
Every row uses 85 °F and 65% RH outside and no other moisture sources. The basement and crawlspace are held at 70 °F and 50% RH, the house at 75 °F and 50% RH. Slab and wall moisture is not included in any of them.
| Space | Infiltration | People | Total, pints per day |
|---|---|---|---|
| 1,000 sq ft basement, 0.18 ACH, 2 people 4 h | 22.2 | 1.5 | 23.7 |
| Same basement, leaky at 0.30 ACH | 37.0 | 1.5 | 38.5 |
| 1,500 sq ft crawlspace, 3 ft, 0.30 ACH, empty | 20.8 | 0.0 | 20.8 |
| 2,000 sq ft house, 8 ft, 0.18 ACH, 4 people 16 h | 36.9 | 11.6 | 48.5 |
Going from 0.18 to 0.30 ACH multiplies the basement's infiltration moisture by 1.67, the same ratio as the air change rates, so air sealing pays off directly in pints. The empty crawlspace still needs 20.8 pints per day from leakage alone, before any ground moisture. In the house, people account for 24% of the total, because they spend most of the day inside.
Standards and data sources
- 10 CFR Part 430, Subpart B, Appendix X1 (DOE dehumidifier test procedure) - the 1.04 lb per pint factor (section 5.6) and the rating conditions (section 4.1.1), 65 °F dry-bulb and 56.6 °F wet-bulb for portable units and 73 °F dry-bulb and 63.6 °F wet-bulb for whole-home units. The appendix defines product capacity as pints collected per 24 hours of operation under the specified ambient conditions.
- ENERGY STAR, Dehumidifier Testing and Capacity - the change from an 80 °F to a 65 °F portable test temperature, and ENERGY STAR's explanation that cooler air holds less water to remove, the reason rated capacities dropped.
- ENERGY STAR, Dehumidifiers - the note that frost can form on the coils below 65 °F, and the suggestion to look for a product specified for lower temperatures when a space typically runs colder than that.
- ASHRAE Handbook Fundamentals, Chapter 1 Psychrometrics - humidity ratio and specific volume equations behind the psychrometric code this page shares with the psychrometric and dew point calculators.
Limitations of this calculator
- The result is the moisture the space needs removed at your conditions. A dehumidifier's rated pints per day is measured at DOE test conditions, 65 °F for portable units and 73 °F for whole-home units, so the two numbers are not measured the same way. This page does not convert one into the other.
- Water moving through a basement slab, foundation walls or an exposed crawlspace floor is not modeled. Put your estimate in other moisture sources.
- Infiltration uses one steady ACH and one outdoor condition for the whole day. Real leakage and outdoor humidity swing by hour and by season.
- This calculator uses sea-level pressure and does not model elevation. By this calculator's own estimate the effect on infiltration moisture is small: at 5,000 ft the humidity ratios rise but each CFM carries less dry air, and the default basement changes by about 0.5%.
- The 200 BTU/h per person and the ACH presets are BuildSolver assumptions, not published rates. Use your own numbers when you have them.
- Outdoor air drier than the target is counted as zero, not as drying. Supply ventilation, a dryer vent pulling air in and duct leakage are not modeled separately.
- Targets with a dew point under 32 °F are outside the psychrometric range this calculator solves.
Common dehumidifier sizing mistakes
- Comparing old and new pint ratings. ENERGY STAR lists a unit once sold as 50 pints under the old 80 °F test at about 30 pints under the current 65 °F test. A new box rated at 30 pints can be the same machine that was sold as 50. Compare units rated under the same test.
- Sizing by floor area alone. Two basements the same size can need very different moisture removal. Leakage, the outdoor dew point and water coming through the walls drive the number, and floor area only scales the leakage term.
- ACH50 entered as ACH. A blower door number at 50 pascals is many times the natural air change rate. Entering it here overstates infiltration moisture by the same factor.
- Ignoring a cold basement. ENERGY STAR notes that frost can form on the coils below 65 °F and the compressor can cycle without removing moisture. If the space runs cooler than that, look for a unit specified for low temperature operation.
- Leaving out known wet sources. Clothes drying on a line, a sump without a lid or visible seepage all add water the leakage math cannot see. Put a number on them under other moisture sources before you settle on a unit.
- A target set too low. Every point of RH you pull off the target raises the load and the run time. Set the target at the RH the job calls for, and pull it lower only when the job requires it.
Common questions about dehumidifier sizing
How many pints per day does a basement dehumidifier need to remove?
It depends on leakage, outdoor humidity, your target and any water coming through the walls. For the default 1,000 sq ft basement at 0.18 ACH, 85 °F and 65% RH outside, held at 70 °F and 50% RH with two people for four hours, the balance on this page comes to about 23.7 pints per day before slab and wall moisture.
What conditions is a dehumidifier's pints per day rating measured at?
Under DOE's current test procedure, 10 CFR Part 430 Subpart B Appendix X1, portable dehumidifiers are tested at 65 °F dry-bulb and 56.6 °F wet-bulb, and whole-home dehumidifiers at 73 °F dry-bulb and 63.6 °F wet-bulb. The appendix defines capacity as pints collected per 24 hours of operation under those specified conditions.
Why do new dehumidifiers show fewer pints than older ones?
ENERGY STAR explains that portable units used to be tested at 80 °F and are now tested at 65 °F, and that there is less water to remove in cooler air. A unit once rated 70 pints lists at about 40 to 45 pints under the new test.
Can I convert a rated capacity to my basement conditions?
This calculator does not. Capacity at conditions other than the rating point depends on the specific unit, so check the manufacturer's performance data. Compare your moisture load to ratings from the same test procedure.
Does a cold basement change which dehumidifier I should buy?
ENERGY STAR says frost can form on the condensing coils below 65 °F and can make the compressor cycle without removing moisture. If the space typically runs below 65 °F, it suggests a product specified for use at lower temperatures.
What if the outdoor air is drier than my target?
Then infiltration adds no moisture and the calculator shows zero for that line. It does not count the dry air as removing water, so the result is only people and any other sources you entered.
Where do the ACH presets come from?
They are BuildSolver's load engine defaults for room-by-room cooling, 0.10 for a tight space, 0.18 for average and 0.30 for leaky. They are assumptions, not code or standard values. A measured number from a blower door test converted to natural infiltration is better.
Does this work for a crawlspace?
The balance works for any enclosed space. Enter the footprint and the average clear height, and add an estimate for ground moisture under other sources, since an exposed or poorly covered soil floor is not modeled.
What the rated pints per day means
A rated pints per day number comes from the DOE test conditions listed under Standards and data sources, 65 °F for portable units and 73 °F for whole-home units.
ENERGY STAR notes that portable units used to be tested at 80 °F, and that cooler air has less water to remove, so the same class of machine now carries a lower number. Compare the load above with ratings from the same test, and check the manufacturer's data for a space that runs colder. This page does not convert a rating to your conditions.
Used on the same job
Check the psychrometrics in chat
The chat has no dehumidifier tool, so the moisture load comes from this page. In chat you can run the free whole-house Manual J cooling and heating load, a dew point or a full psychrometric state for the same job. Open the chat to run whole-house Manual J for this job, or use the dew point and psychrometric calculators for the last two without leaving the site.
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