Dehumidifier sizingFREE CALCULATOR

KALOS FIELD TOOLS

Dehumidifier sizing

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

Estimate from space details
1 The space you are drying

Choose the space being dried, even if the unit is mounted elsewhere. Changing spaces loads editable example values.

2 Moisture entering the space

Use your local humid-weather design dew point. The starting value is an example.

Use the 24-hour average: 90 CFM for 12 hours = 45 CFM. Enter 0 if none.

For example, 4 people for 12 hours = 2 average occupants. Include activities separately under assumptions.

Adjust assumptions

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

Add activities, materials, or ground vapor. Zero omits these sources; it does not confirm they are absent.

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

No A/C moisture-removal credit is assumed.

How to use the result & source references

Start with the moisture load

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.

No A/C removal credit is assumed. 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 reference capacity was 84 PPD. Our illustrated home uses explicit inputs: 90 CFM of ventilation, 26.7 CFM of residual infiltration, and 18.2 PPD from occupants, yielding 115.2 PPD before an operating-hours reserve. Those are different load assumptions. Do not treat identical floor area as 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.