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
- Standard atmospheric pressure of 14.696 psia (sea level) is assumed throughout.
- Saturation pressure is calculated using the Magnus formula, accurate to ±0.1% for temperatures between −40°F and 140°F (ASHRAE Fundamentals, Chapter 1).
- 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).
- Occupant heat gain values are based on ASHRAE Fundamentals Table 1, Chapter 18 (typical office/commercial occupancy).
- Lighting load is assumed to be 100% sensible (no latent component from fluorescent/LED fixtures).
- Equipment load is entered directly in BTU/hr and treated as 100% sensible; adjust for equipment with significant moisture output.
- Ventilation airflow (CFM) represents outdoor air introduced into the conditioned space; infiltration can be added to this value.
- Wall, roof, window conduction loads and solar heat gain are not included — this calculator focuses on ventilation, occupancy, lighting, and equipment loads.
- Air density is calculated at indoor dry-bulb temperature and standard pressure using the ideal gas law for dry air (R = 0.3704 psia·ft³/(lb·°R)).
- 1 ton of refrigeration = 12,000 BTU/hr; 1 kW = 3,412.14 BTU/hr.
- References: ASHRAE Handbook — Fundamentals (2021); ASHRAE Standard 62.1 (Ventilation for Acceptable Indoor Air Quality); ACCA Manual J (Residential Load Calculation).