Dehumidifier sizingFREE CALCULATOR

KALOS FIELD TOOLS

Dehumidifier sizing

Estimate the daily moisture load and the capacity needed at your indoor temperature and humidity.

First Principles

A dehumidifier does not fix a leaky building. Control air leakage, duct leakage, roof and plumbing leaks, drainage, and ground moisture first. Seal the attic or encapsulate the crawl space before sizing.

How This Guide Works

  1. Define the space and moisture sources. Use measured leakage, actual ventilation, people, and material or ground moisture. Square footage alone cannot size a dehumidifier.
  2. Use peak moisture weather. Choose the local dehumidification design dew point, then enter the temperature and RH you need to maintain. Check cooler, humid periods too; the hottest attic hour is not necessarily the hardest condition for moisture control.
  3. Match real equipment capacity. The result includes your operating-hour allowance. Compare capacity at the actual inlet temperature, RH, and duct resistance—not just the warmer 80°F / 60% nameplate rating.

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

Conservative Orlando-area moisture weather

Starts with a sealed attic and a 79°F outdoor dew point. Building size, leakage, and material-moisture values are editable examples, not worst-case limits for every property.

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 representative nearby station, and use Dehumidification → 0.4% → DP for this conservative approach. Enter that dew point in the outdoor field (under “Adjust assumptions” for attic/crawl). This version uses °F. A project designer may specify a different design percentile.

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 0.4% condition is exceeded about 35 hours/year; it is not an absolute maximum. Check the station and applicable edition for your project.

For a full load calculation, use the MCDB beside that DP: Executive is 82.3°F, International 81.6°F. This moisture-only calculator needs DP alone. 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 space details
1 The space you are drying

Starts with the sealed attic only. Select the space being dried, even if the unit is mounted elsewhere. Your edits are kept when switching spaces.

Adjust assumptions

Natural air changes per hour, not blower-door ACH50. Do not count the same outdoor air as both leakage and ventilation.

The starting 10 pints/day is an illustrative allowance for material and roof-deck moisture, not a measured Central Florida value. Adjust to the actual moisture sources; zero excludes them.

20 hours allows operating reserve. This is separate from capacity loss at cooler or drier conditions.

How to use the result & source references

Start with the moisture load

Central Florida suggestions: the 79°F outdoor dew point rounds Orlando Executive’s 78.6°F 0.4% dehumidification condition in the public 2021 ASHRAE table. This is conservative Orlando-area moisture weather, not an absolute maximum or a guarantee of the total building load. The 2,000 sq. ft. home, four average occupants, 75°F / 50% RH target, 0.10 natural ACH and 20 operating hours are editable design examples, not measured regional averages. The 90 CFM example follows the 2013 ASHRAE 62.2 gross-flow equation for 2,000 sq. ft. and three bedrooms before any infiltration credit; it is not a current code determination. Height, leakage and other moisture suggestions change with the space served. Verify actual ventilation and remaining leakage so the same air is not counted twice.

The source estimate combines outdoor-air moisture with occupant and additional moisture sources. Outdoor air is compared with your indoor target; its dew point matters more than outdoor relative humidity alone. The known-load path uses your complete latent-load calculation instead.

For each outdoor-air source, latent load in Btu/h = 0.68 × CFM × positive humidity-ratio difference in grains/lb. Daily removal in pints = latent Btu/h × 24 ÷ 1,054; the ventilation input is a 24-hour average. Infiltration uses floor area × height × natural ACH ÷ 60. Occupant moisture uses 200 Btu/h per average occupant for 24 hours. For intermittent ventilation or occupancy, multiply the on-period flow or people by hours per day ÷ 24.

These are conventional sea-level, standard-air approximations; no high-altitude correction is applied. Outdoor design conditions are held for the relevant calculation hours rather than modeled hour by hour. Drier outdoor air contributes zero outdoor-air moisture load; it does not cancel internal moisture sources.

Required 24-hour equipment capacity = net pints/day × 24 ÷ available operating hours/day. This operating allowance does not replace an equipment performance correction.

Outdoor dew point determines outdoor moisture content; outdoor dry-bulb temperature is not needed for this moisture-load calculation. Equipment airflow and duct static pressure affect installed performance and must be checked separately. This simplified form does not calculate mixed-air inlet conditions or automatically select equipment. The customary HVAC pints conversion differs by about 4.6% from a direct water-mass calculation.

Match capacity to operating conditions

The required removal rate is a capacity at the conditions you entered. Manufacturer ratings at 80°F / 60% RH (AHAM) or 73°F / 60% RH (whole-house DOE) do not establish capacity at 75°F / 50% RH. Portable DOE ratings use 65°F / 60% RH and should not be equated with whole-house ratings. Check the specific model’s performance data, duct resistance, operating limits, and ventilation configuration before selecting equipment.

Manufacturer comparison

A verified AprilAire example for an existing 2,000 sq. ft. Orlando home, slab foundation, three bedrooms, 75°F and 50% RH returned 61.95 PPD. Its E130 series reference was 84 PPD. This tool’s Central Florida home example uses 90 CFM of ventilation, 26.7 CFM of residual infiltration, a 79°F outdoor dew point and 18.2 PPD from occupants: 173.3 PPD before the 20-hour operating allowance, or 207.9 PPD of required capacity. These are different load assumptions. Identical floor area alone does not make an equivalent sizing case.

Aprilaire does not expose all climate and infiltration assumptions. Benchmark testing found behavior consistent with a lower area-based airflow term; that is an empirical observation, not a current ventilation requirement. If you already have a complete manufacturer or Manual J moisture load, use the known-load path and enter it once.

Account for the enclosure

Use attic and crawl-space estimates for controlled enclosures with a defined moisture load. Ground vapor, roof-deck drying, outdoor openings, and liquid-water problems need their own assessment. Square footage cannot capture those effects by itself.

This planning tool expands on manufacturer sizing approaches. It does not reproduce their proprietary selection methods, guarantee an equivalent model recommendation, replace a complete load calculation, or establish code compliance.