Humidity and Cooling Load Calculator
Calculate sensible and latent cooling loads for HVAC systems based on indoor/outdoor conditions, airflow rate, and occupancy. Results are given in BTU/hr and tons of refrigeration.
Outdoor Conditions
Indoor Design Conditions
Airflow & Space
Lighting & Equipment
Formulas Used
Saturation Pressure (Magnus Formula):
Pws = 0.6108 × exp(17.27 × TC / (TC + 237.3)) [kPa]
Humidity Ratio:
W = 0.62198 × (Pws × RH/100) / (Patm − Pws × RH/100) [lb water / lb dry air]
where Patm = 14.696 psia (standard atmosphere)
Enthalpy of Moist Air:
h = 0.240 × TDB + W × (1061 + 0.444 × TDB) [BTU/lb dry air]
Air Density (Ideal Gas):
ρ = Patm / (0.3704 × Tabs) [lb/ft³], where Tabs = T°F + 459.67 °R
Sensible Cooling Load — Ventilation:
Qs,vent = ṁair × Cp × ΔT = (CFM × ρ × 60) × 0.240 × (Tout − Tin) [BTU/hr]
Sensible Cooling Load — Lighting:
Qs,light = W/ft² × Area × 3.412 [BTU/hr] (1 W = 3.412 BTU/hr)
Latent Cooling Load — Ventilation:
Ql,vent = ṁair × hfg × ΔW = (CFM × ρ × 60) × 1061 × (Wout − Win) [BTU/hr]
where hfg ≈ 1061 BTU/lb is the latent heat of vaporization at ~60°F
Sensible Heat Ratio:
SHR = Qsensible / Qtotal
Tons of Refrigeration:
Tons = Qtotal / 12,000 [BTU/hr per ton]
Assumptions & References
- Specific heat of dry air Cp = 0.240 BTU/(lb·°F); latent heat of vaporization hfg = 1061 BTU/lb at 60°F (ASHRAE Handbook of Fundamentals, 2021).