Dehumidifier Size Calculator

How to use this tool

Choose what to work out. “The size to buy” takes floor area, ceiling height, dampness, people, laundry and tank size, and returns pints and litres a day and hours per tank fill. “What area a unit covers” runs the chart backwards from a box rating, and “Old and current rating” converts between the two test standards.

The input people most often get wrong is the dampness. Use the chart’s own words: moderately damp is damp and musty only in humid weather, very damp is always damp and musty with damp spots on walls or floor, wet feels and smells wet with sweating walls or seepage, and extremely wet adds laundry drying or a wet floor. Then check which rating a box quotes. This is general information, not a mould inspection.

What the calculation is doing

The tool starts from the AHAM sizing chart that ENERGY STAR published: pints per 24 hours for five floor areas, 500 to 2,500 sq ft, and four conditions. The tool reads it by band: a space counts as the smallest chart area that holds it, so 800 sq ft uses the 1,000 sq ft column and nothing is interpolated. Past 2,500 sq ft the chart ends and the tool says so. The chart gives no ceiling height, so the tool takes 8 ft as standard and scales the area by ceiling ÷ 8 above it, an adjustment of its own.

Retail rules then add 5 pints when several people use the space and 5 for a washer, dryer or laundry drying. Home Depot states both, and PickHVAC agrees on laundry but adds 5 for people only above five occupants. The sum is an old rating figure, because the chart dates from the 80 F era.

The current rating comes from the ENERGY STAR table, where old 30, 40, 50, 55, 60, 70 and 90 pints become 20, 25, 30, 30 to 35, 35, 40 to 45 and 55. The tool joins the rows with straight lines and takes the middle of each range. Below 30 old pints the table has no rows, so the first row is scaled down to zero, which is an extension and not a published value.

Why one unit has two ratings, and why a cool basement is harder

Until 2019 portable dehumidifiers were rated with air at 80 F dry bulb and 69.6 F wet bulb, which is 60 percent relative humidity. The DOE test in appendix X1 uses 65 F and 56.6 F wet bulb, still 60 percent. Standards using it apply to units made on or after June 13, 2019, and Natural Resources Canada uses the same date and appendix. The machine did not change. The yardstick did.

The air explains it. At 60 percent humidity, 80 F air carries about 13.2 g of water per kilogram of dry air and 65 F air about 7.9 g, which is 60 percent as much. DOE puts the cut in measured capacity at about 35 percent on average, and ENERGY STAR’s rows imply nearer 39, so the water in the air falls by roughly the same fraction as the rating.

Your basement is probably colder than either test room. Air at 55 F and 60 percent holds 5.5 g, 42 percent of the old test air and 70 percent of the new. No source gives a capacity for a cooler room, so treat the current rating as the best case. ENERGY STAR warns that frost can form on the coils below 65 F, making the compressor cycle without removing moisture. The dew point of 65 F air at 60 percent is about 50.8 F by the Magnus relation, checked against the CRC vapour pressure table, so an uninsulated concrete wall colder than that sweats.

Undersizing runs forever and oversizing does not. ENERGY STAR says it is better to oversize than undersize, and its dehumidifiers have a built in humidistat. A large unit reaches the target and rests, while a small one may never reach it and runs around the clock. DOE minimum efficiency also rises with capacity, from 1.30 litres per kWh at 25 pints or less to 2.80 above 50.

Health Canada lists showers, laundry, cooking, water through foundation cracks, plumbing leaks, condensation and poor ventilation as the sources. A dehumidifier treats vapour, not the leak. Aim for 30 to 50 percent, the range EPA, Health Canada and CMHC all give.

The sizing chart on both ratings

Each cell shows the chart figure on the old rating, then the approximate current rating.

ENERGY STAR’s current buying page ranges by dampness for rooms under 2,000 sq ft: 20 to 30 pints slightly to moderately damp, 25 to 40 very damp and 30 to 50 wet, with no rating named. Those run above the AHAM figures. The AHAM chart alone has a cell for every area and condition, so the tool uses it and shows the ENERGY STAR range beside the answer. Read the tool’s figure as a floor.

Retail pairings also differ from ENERGY STAR, which gives old 50 as 30 and old 70 as 40 to 45. AJ Madison gives 35 and 50, TCL 30 and 50, and Whynter says its 50 pint unit was 70 before. DOE’s cut is an average and each maker converts its own models. The tool uses ENERGY STAR and the converter mode shows the rest.

A worked example you can check on paper

An 800 sq ft very damp basement, 8 ft ceilings, one person, no laundry. The chart area is 800 sq ft, so the 1,000 sq ft column applies and the very damp row lists 17 pints, the old rating. Below 30 the table has no row, so 17 × 20 ÷ 30 = 11.33, about 11 on the current rating. Litres: 17 × 0.4731765 = 8.04 and 11 × 0.4731765 = 5.20. A 16.2 pint tank fills in 16.2 ÷ 11 × 24 = 35.3 hours.

A taller case: 1,500 sq ft, wet, 10 ft ceilings, four people, laundry. Chart area = 1,500 × 10 ÷ 8 = 1,875 sq ft, so the 2,000 sq ft column and the wet row give 32. Adding 5 and 5 makes 42 old. Between the 40 and 50 rows, 25 + (42 − 40) × (30 − 25) ÷ (50 − 40) = 26, so 26 pints current, or 12.3 litres a day.

Mistakes that catch people out

Comparing labels from different tests. A 50 pint box from 2018 and one from 2023 are not the same machine. Check whether the number is at 80 F or 65 F.

Choosing the dampness you hope for. A basement that sweats in July is wet, not moderately damp. Go one row up if unsure.

Sizing a cold basement by its label. The rating is a warm air number. Below 65 F a unit removes less and its coils can frost, so the answer is a minimum.

Stacking laundry on the wettest row. The extremely wet row already mentions laundry drying, so adding 5 pints may count that load twice. The extra capacity is harmless.

Treating the machine as the fix. If water enters through the wall or floor, the tank fills whatever the size. Fix the leak first, then size for what is left.

Popular calculations

Each opens the calculator with those figures filled in, and they can be changed once it is open.

Moderately damp rooms by area

Very damp basements by area

Wet rooms by area

Extremely wet spaces by area

Taller ceilings, more people and laundry

Metric examples

What area a unit covers

Old rating to current rating

How this was checked

The engine reproduced all 20 chart cells typed in from three sources: the archived ENERGY STAR page, the AlorAir guide and a brochure printing the chart beside an 80 F rating. Home Depot’s rule rebuilds two rows independently. Band selection matched a loop at 2,600 areas, including the edges at 500, 500.01, 2,500 and 2,500.01. The conversion hits all five exact ENERGY STAR rows, sits inside both ranges and returns to its start to 13 decimal places. The answer never fell as area, dampness, ceiling, people or laundry rose across 113,856 comparisons, and coverage agreed with sizing in 1,216 cases. The vapour pressure formula matched nine CRC values within 0.04 percent.

Sources: ENERGY STAR, the DOE final rules of July 2015 and June 2016, Natural Resources Canada, Home Depot, PickHVAC, NIST, EPA, Health Canada and CMHC. Where they disagree, as on conversion pairs and on ENERGY STAR’s two charts, the article says so.

Common questions

What size dehumidifier do I need for a 1,000 square foot basement?

On the chart a 1,000 sq ft space needs 14, 17, 20 or 23 pints a day on the old rating from moderately damp to extremely wet, which is about 9, 11, 13 or 15 on the current rating. Shelf units start near 20 pints current, and ENERGY STAR’s buying page lists 25 to 40 for a very damp room, so a 30 pint unit suits a cool basement and clears the whole chart before extras.

What changed in dehumidifier ratings in 2019?

The US Department of Energy moved the capacity test from air at 80 F to air at 65 F, both at 60% relative humidity. Standards that use it apply to units made from June 13, 2019, and labels changed in early 2020. Cooler air holds less water, so the same machine scores about 35 percent lower on average.

Is a 50 pint dehumidifier sold today the same as a 50 pint one from before 2020?

No. Whynter and TCL pair a current 50 pint unit with an old 70 pint one, while ENERGY STAR shows old 70 as 40 to 45 and old 90 as 55. The pairings differ, so compare the test conditions printed on each box.

What humidity should I set a dehumidifier to?

The US EPA says to keep humidity below 60 percent and ideally between 30 and 50 percent, and Health Canada and CMHC give 30 to 50. A hygrometer costs $10 to $50, says EPA.

How long does a dehumidifier tank take to fill?

Divide the tank size by the capacity in pints a day and multiply by 24. A 16.2 pint tank on a 30 pint unit running at its test conditions fills in 16.2 ÷ 30 × 24 = 13.0 hours. Cooler real basements mean longer, and a hose to a floor drain saves emptying.

Is it better to buy a bigger dehumidifier than the chart says?

Usually. ENERGY STAR says it is better to oversize than undersize, and a bigger unit reaches the humidity setting sooner and stops. DOE minimum efficiency also rises with capacity, from 1.30 litres per kWh to 2.80.

Related Calculators and Reference

• Square Footage Calculator: measure an irregular basement first.
• Volume and Capacity Converter: pints, litres and gallons of tank water.
• Bathroom Fan CFM Calculator: vent shower moisture outdoors.
• Electricity Cost Calculator: what a unit running all day costs.
• Attic Ventilation Calculator: the other half of moisture control in a house.
• Air Changes per Hour Calculator: how many air changes a room gets from its fans and purifiers.
• ENERGY STAR: Dehumidifier testing and capacity: the old to current conversion table.