Dehumidifier sizingFREE CALCULATOR

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Dehumidifier sizing

Find the water removal needed for your space. Fix air and water leaks first.

First Principles & How This Guide Works

First Principles

A dehumidifier does not fix a leaky building. Fix air leaks, duct leaks, roof leaks, plumbing leaks and drainage first. Seal the attic. In a crawl space, seal it and cover the ground.

How This Guide Works

  1. Find where the moisture comes from. Count air leaks, planned outdoor air, people and moisture from materials or soil. Floor area alone is not enough.
  2. Use humid-weather data. Use your local design dew point and the indoor temperature and humidity you want. Check cool, damp weather too. The hottest attic hour may not be the hardest hour to dry.
  3. Check what the unit can remove. The result allows for the daily run time you enter. Use the unit’s output at your air temperature, humidity and duct pressure. Its 80°F / 60% label rating may be higher.

Tim De Stasio’s sizing guide explains the load and equipment checks. Have a complete latent-load calculation? Use “Enter a known moisture load” below.

Starts with very humid Orlando weather

Starts with a sealed attic and a 79°F outdoor dew point. The other settings are examples. Change them to match the job; they are not worst-case limits for every building.

Find design weather for your location ↓
79°Fsuggested outdoor dew point
Find your local design-weather chart

Open the free ASHRAE design-conditions table (2021, PDF), find a nearby weather station that fits your location, and use Dehumidification → 0.4% → DP for this humid-weather starting point. Enter that dew point in the outdoor field (under “More settings” for attic/crawl). This version uses °F. Use a different weather column if your project designer calls for it.

Orlando-area basis: Orlando Executive is 78.6°F dew point, rounded up to 79°F; Orlando International is 78.0°F. Both are on PDF page 3. The dew point can be higher than this about 35 hours per year. This is not a record high. Check which station and data edition your project needs.

DP means dew point: the temperature where water starts to condense. For a full load calculation, also use MCDB (mean coincident dry bulb): the average air temperature that comes with that dew point. Executive is 82.3°F; International is 81.6°F. This calculator uses only the dew point. Do not substitute the cooling dry-bulb or wet-bulb columns. If your station is absent, use ASHRAE’s Weather Data Center to locate additional station data.

Estimate from the space
1 The space you are drying

Choose the space the unit will dry, even if the unit sits elsewhere. We keep your edits when you switch spaces. A sealed crawl space also needs a ground cover and good drainage.

Use the air temperature expected while the unit runs.

RH means relative humidity: how close the air is to being saturated at this temperature.

More settings

ACH means air changes per hour under normal conditions. Do not enter a blower-door ACH50 reading. Do not count the same air as both leakage and planned ventilation.

We start with 10 pints/day for moisture released by wood and other materials. This is an assumption, not a measurement for your attic. Adjust to the actual moisture sources; zero excludes them.

20 hours leaves some spare run time. A unit may also remove less water when the air is cooler or drier. Check both.

Terms, math & sources

What the terms mean

PPD means pints of water removed per day. CFM means cubic feet of air per minute. ACH means air changes per hour. RH means relative humidity. Latent load is the load from moisture, shown in Btu/h (British thermal units per hour).

How the math works

We add moisture from air leaks, planned outdoor air, people and other sources. We compare outdoor moisture with your indoor target. Drier outdoor air adds zero load; it does not cancel moisture from inside. A known total load replaces this estimate, so nothing is added twice.

Air leakage in CFM = floor area × average height × ACH ÷ 60. For each outdoor-air source, latent load = 0.68 × CFM × positive difference in moisture content. That difference is measured in grains of water per pound of dry air. One pound contains 7,000 grains.

Daily pints = latent Btu/h × 24 ÷ 1,054. Each average person adds 200 latent Btu/h. People and ventilation are full-day averages. For part-time use, multiply the people or airflow by hours of use ÷ 24.

Capacity needed = daily pints × 24 ÷ run hours allowed. At 20 run hours, this leaves a 20% capacity allowance. It does not account for output lost in cooler or drier air.

These are standard-air estimates near sea level. We do not correct for high altitude or model each hour of weather. This common HVAC pints conversion differs by about 4.6% from a direct water-mass calculation.

What the starting settings mean

79°F outdoor dew point comes from Orlando Executive’s 2021 ASHRAE 0.4% design value of 78.6°F. It is a humid-weather starting point, not a record high. Building size, height, leakage, people, extra moisture and run time are examples. Measure or check them for the job.

The 90 CFM home example uses an older 2013 ASHRAE 62.2 formula for a 2,000 sq. ft., three-bedroom home, before any air-leakage credit. It is not a current code check. Count only the air that actually enters the space, and do not count it twice.

Why equipment ratings differ

Check output at the air temperature and humidity entering the unit. AHAM ratings use 80°F / 60% RH. Whole-house DOE ratings use 73°F / 60%; portable DOE ratings use 65°F / 60%. Those numbers do not prove output at 75°F / 50%. Also check airflow, duct pressure, controls and temperature limits. This form does not check mixed-air inlet conditions or pick a unit for you.

Comparison with AprilAire

An AprilAire example for a 2,000 sq. ft. Orlando home on a slab, with three bedrooms at 75°F / 50% RH, gave 61.95 PPD. Its E130 series reference was 84 PPD. Our home example gives 173.3 daily pints, or 207.9 PPD with a 20-hour run allowance. It includes 90 CFM of outdoor air, 26.7 CFM of air leakage, a 79°F outdoor dew point and 18.2 PPD from people.

The inputs differ. Floor area alone does not make the two cases equal. AprilAire does not show all its weather and leakage assumptions. Our tests suggest a lower area-based airflow term in its calculator; that finding is not a ventilation rule. If you have a full manufacturer or Manual J moisture load, enter that known load once.

Seal attic and crawl spaces and fix water problems first. Check soil moisture and drying wood on each job. This guide does not copy a maker’s private sizing method, promise the same unit choice, replace a full load calculation or check code compliance.