Dehumidifier Capacity Sizing Calculator
Estimate the dehumidifier capacity (in pints per day) needed for your space based on room dimensions, current humidity level, and environmental conditions.
Formulas Used
1. Saturation Vapor Pressure (Magnus / Tetens formula):
es(T) = 6.1078 × exp(17.27 × TC / (TC + 237.3)) [hPa]
where TC = (TF − 32) × 5/9
2. Absolute Humidity:
AH = (RH / 100) × es × 216.7 / TK [g/m³]
where TK = TC + 273.15, and 216.7 = Mw/R (g·K/J)
3. Moisture to Remove per Day:
Moisture (g/day) = (AHcurrent − AHtarget) × Volume (m³) × ACHday
ACHday = 12 (equivalent to 0.5 air changes per hour, typical residential)
4. Base Capacity (pints/day):
Base = Moisture (g/day) / 473.176 × Condition Factor
(1 US pint of water ≈ 473.176 g)
5. Total Capacity:
Total = Base + (Windows × 0.5) + (Occupants × 0.5) [pints/day]
Assumptions & References
- Air change rate assumed at 0.5 ACH (12 changes/day), a standard residential infiltration rate per ASHRAE 62.2.
- Saturation vapor pressure calculated using the Magnus–Tetens formula, accurate to ±0.1% for 0–60°C (WMO No. 8).
- Absolute humidity conversion uses the ideal gas law approximation: AH = (RH × es × Mw) / (R × T), where Mw = 18.015 g/mol, R = 8.314 J/(mol·K).
- Condition severity multipliers (1.0–1.8×) are based on ENERGY STAR® Dehumidifier Sizing Guidelines.
- Each window/door adds ~0.5 pints/day for infiltration; each occupant adds ~0.5 pints/day for respiration and perspiration (ASHRAE Fundamentals Handbook).
- Target humidity of 45–55% RH is recommended by the EPA and ASHRAE for comfort and mold prevention.
- Dehumidifier ratings are per DOE 2019 test standard (AHAM Standard DH-1).
- This calculator provides an estimate; actual performance varies with unit efficiency (EF), temperature, and specific building characteristics.