Dehumidifier sizing calculator

What size dehumidifier do I need?

The AHAM baseline answers in one sentence: 1 pint per 24 hours per 50 square feet at 50-60 percent relative humidity. That sizing scales linearly with floor area, so a 500 square foot bedroom at moderate damp needs about 10 pints per day, a 1,000 square foot basement needs about 20 pints, and a 2,000 square foot finished basement needs about 40 pints. Everything else is an adjustment on that baseline.

The number that gets printed on the side of every dehumidifier box is the AHAM-rated pint capacity, measured at 80 degrees Fahrenheit and 60 percent relative humidity per the ANSI/AHAM DH-1 test procedure. That rating is the legal capacity claim under DOE 10 CFR 430 Appendix X1. When you size a unit, you size to the AHAM number because that is what the units are sold by. The mistake homeowners make is reading the AHAM rating as the capacity in their actual basement. A 50 pint unit in a 65 degree basement at 50 percent humidity will extract closer to 30 pints per day, because dehumidifier capacity drops sharply as air gets colder and drier. The calculator above bakes this in: it shows recommended capacity (the number to shop on) alongside expected real-world extraction (what you will actually see in the bucket).

The AHAM humidity tier table

ANSI/AHAM DH-1 defines four humidity tiers that drive the sizing multiplier. Each tier roughly doubles the moisture load the dehumidifier has to remove:

• Moderately damp (50-60 percent RH): air feels slightly muggy in summer, no visible condensation. Baseline multiplier of 1.0.
• Very damp (60-70 percent RH): persistent musty smell, damp spots on concrete floors, peeling paint at floor level. Multiplier of 1.3.
• Wet (70-85 percent RH): visible condensation on cold pipes and windows, wet stains on walls, must dry within 24 hours of mopping. Multiplier of 1.6.
• Extremely wet (above 85 percent RH): seepage at the floor-wall joint, standing water, visible mold growth, sweating walls. Multiplier of 2.0.

The right humidity tier matters more than any other input. A 1,000 square foot basement at moderately damp needs 20 pints per day, but the same room at wet needs 32 pints. Buying a hygrometer (any $10 humidity meter) and measuring is more useful than guessing.

Real-world extraction: why a 50 pint unit only pulls 30 pints in a basement

Dehumidifier capacity scales with two factors: air temperature and air humidity at the coil. The AHAM 80 F / 60 percent RH test condition is deliberately warm and wet because that is where compressor-based refrigeration extracts moisture most efficiently. As the room cools and dries, the coil dewpoint drops, condensate forms slower, and extraction rate drops with it. The relationship is roughly:

• 80 F / 60 percent RH (AHAM test): 100 percent of rated capacity
• 70 F / 55 percent RH (typical comfort): ~75 percent of rated
• 65 F / 50 percent RH (typical basement): ~55-65 percent of rated
• 55 F / 50 percent RH (cool basement or crawlspace): ~35-45 percent of rated
• Below 50 F: most consumer portable units stop extracting and may freeze the coil

This is why the calculator above sizes to the AHAM number but reports the expected real-world extraction separately. If you have a cold basement and the math returns "50 pints recommended, 30 pints expected extraction", that is by design. A 30 pint AHAM unit would extract only about 18 pints in the same conditions, which is too small. Always size to the AHAM number, then expect to see roughly two-thirds of that in the bucket.

Basement, crawlspace, above-grade, whole-house: which type fits

Different spaces have different moisture characteristics and need different equipment classes:

Basement. Standard portable dehumidifier with continuous drain hose to a floor drain or condensate pump. Sizes between 30 and 70 pints depending on area, age, and how dry the slab is. The most common install: a 50 pint unit on a stand by the laundry area, draining into the laundry sink. Skip the bucket; nobody empties it consistently and the unit shuts off when full.

Crawlspace. Standard portables fail here because crawlspace air typically runs 50-65 F year-round. The compressor in a consumer unit cannot run efficiently below about 60 F, and the coil ices up. Use a crawlspace-rated low-temperature unit (Santa Fe Compact70, Aprilaire E070, Honeywell TrueDRY DR65) that is engineered for operation down to 40 F. Pair with a sealed vapor barrier on the dirt floor; an unsealed crawlspace will burn through any dehumidifier capacity you throw at it.

Above-grade rooms. Portable units in the 20-35 pint range handle a single room or a small zone (bedroom, home office, sunroom). Empty the bucket every 1-2 days or run a hose to a sink. Most modern units have built-in pumps that lift condensate 12-15 feet vertically.

Whole-house ducted. Once required capacity passes 90 pints or area passes 2,500 square feet, a whole-house unit installed in the return-air plenum is more efficient and quieter than two portables. Models like the Aprilaire 1850, Santa Fe Ultra98, and Honeywell TrueDRY DR90 cost $1,800 to $3,500 installed, run quietly because they are tucked in the mechanical room, and tie into the existing HVAC ductwork. The downside: they require a dedicated 115 V or 230 V circuit and condensate drain plus professional install.

Energy use and operating cost

Current ENERGY STAR dehumidifiers run at roughly 1.85 liters of moisture removed per kWh of electricity (integrated energy factor, IEF) under the DOE test procedure. For a 50 pint unit at typical basement conditions, that comes out to:

• Real-world extraction: ~30 pints/day (14 liters)
• Energy use: ~7.5 kWh/day
• Run cost at $0.13/kWh average US electric rate: ~$0.97/day, $29/month
• Annual cost (June through September peak season): ~$120/year

Older units rated before the current DOE standard run at IEF closer to 1.4, which is about 30 percent more electricity for the same moisture removed. If your dehumidifier is more than five or six years old, the upgrade to a current ENERGY STAR unit pays back in 3-5 years on energy savings alone. Whole-house units are significantly more efficient: a Santa Fe Ultra98 runs at IEF 2.85, roughly half the kWh per pint of a portable.

Setting the humidistat and target humidity

ASHRAE 55 (Thermal Environmental Conditions) and the EPA Mold Course both recommend 30-50 percent indoor relative humidity for comfort and mold prevention. The sweet spot is 45-50 percent in summer (mold suppression starts to fail above 60 percent) and 30-40 percent in winter (window condensation starts to form above 45 percent in cold climates). Most dehumidifiers have a humidistat with a 35-80 percent range; set it to 50 percent and let it cycle on and off. Running it at the lowest setting (30-35 percent) burns extra energy and dries out wood furniture, hardwood floors, and trim until they crack.

A common mistake: leaving the dehumidifier on continuous mode without a humidistat reference. Continuous mode runs the compressor 24/7 regardless of room conditions, which can pull a sealed basement below 30 percent RH and waste 200-300 kWh per month. Use the humidistat. If the unit cannot hold 50 percent at the humidistat setting, the unit is undersized, not the humidistat broken.

When dehumidifier sizing is the wrong question

A dehumidifier treats the symptom. It does not fix the source. If the calculator returns 70+ pints for a moderate-area space, or the unit runs continuously and still cannot hold 50 percent RH, the problem is bulk water entry that no dehumidifier can outrun. Check these first:

• Grading away from the foundation. Soil should slope at least 6 inches over 10 feet away from the house. Negative grade dumps roof runoff into the basement wall.
• Gutters and downspouts. Downspout extensions should carry water 5+ feet away from the foundation. A clogged or short downspout is the single most common cause of basement moisture.
• Foundation cracks and parging. Hairline cracks in poured concrete or step cracks in block walls let water through under pressure. Hydraulic cement and waterproofing membrane fix the cheap ones.
• Vapor barrier on dirt crawlspace floors. An unsealed dirt floor evaporates 10-20 gallons of water per day per 1,000 square feet up into the floor system. Lay 6 mil poly with 12 inch overlaps, sealed at seams and edges. Encapsulation is even better.
• Bathroom and dryer venting. Bath fans and dryers must vent to outside, not into the attic or crawlspace. A dryer that vents into a crawlspace dumps 5 pounds of water per load.

Solve those first, then size the dehumidifier for the residual load. A correctly sized unit on a properly drained basement should cycle on and off, not run continuously, and the bucket or drain hose should never see more than a few gallons a day at peak summer humidity.