Psychrometric Calculator — Moist Air Properties
Enter the dry-bulb temperature, relative humidity and atmospheric pressure to compute all key psychrometric properties of moist air: dew point, wet-bulb temperature, humidity ratio, specific enthalpy and specific volume — following ASHRAE conventions.
°C
%
kPa
Temperature at which condensation begins — the lower the dew point relative to dry-bulb, the drier the air
- 1
Saturation vapour pressure
611.2 × exp(17.67 × 25 ÷ (25 + 243.5)) = 3,167.4 PaMagnus formula — accurate to 0.05 % between −10 °C and +50 °C. - 2
Actual vapour pressure
(60 ÷ 100) × 3,167.4 = 1,900.5 Pa - 3
ln(P_v ÷ 611.2)
ln(1,900.5 ÷ 611.2) = 1.1344 - 4
Dew point temperature
243.5 × 1.1344 ÷ (17.67 − 1.1344) = 16.71
How does this calculator work?
Psychrometrics computes moist-air properties from dry-bulb temperature and relative humidity. Key outputs: dew point = temperature at condensation onset; wet-bulb = evaporative cooling equilibrium; humidity ratio W = kg water vapour per kg dry air; enthalpy h = 1.006T + W(2501+1.86T) kJ/kg. Results follow ASHRAE 2021 conventions and are accurate to ±0.2 % for standard atmospheric conditions.
Formula
How this is calculated
Psychrometrics is the study of thermodynamic properties of moist air — a mixture of dry air and water vapour. The starting point is the saturation vapour pressure P_sat, computed here via the Magnus formula P_sat = 611.2 × exp(17.67T/(T+243.5)) Pa (T in °C). This approximation is accurate to better than 0.05 % for temperatures between −10 °C and +50 °C. Multiplying by relative humidity gives the actual vapour pressure P_v = (RH/100) × P_sat.
From P_v the humidity ratio W = 0.621 98 × P_v / (P_atm − P_v) kg/kg dry air is derived from Dalton's law of partial pressures, using the ratio of molar masses (0.621 98 = 18.015/28.966). The dew point is the inverse of the Magnus formula at P_v; it is the temperature at which the current vapour pressure equals saturation. The wet-bulb temperature uses Stull's (2011) empirical polynomial, which is accurate to ±1 °C for T ∈ [−20, 50] °C and RH ∈ [5, 99] %.
Specific enthalpy h = 1.006T + W × (2501 + 1.86T) kJ/kg dry air comes from ASHRAE Fundamentals (2021): 1.006 kJ/(kg·K) is the specific heat of dry air at constant pressure, 2501 kJ/kg is the latent heat of vaporisation at 0 °C, and 1.86 kJ/(kg·K) is the specific heat of water vapour. Specific volume v = 287.058 × (T+273.15) / (P_atm−P_v) m³/kg dry air applies the ideal gas law to the dry-air partial pressure. Accuracy is better than 0.2 % at typical atmospheric conditions; significant errors occur only at pressures below ≈ 30 kPa or above 200 kPa.
Frequently asked questions
The dew point is the temperature at which air becomes saturated and condensation begins — it depends only on moisture content, not on dry-bulb temperature. The wet-bulb temperature is the equilibrium temperature of a wetted surface in contact with the air, incorporating both temperature and evaporation rate; it always lies between the dew point and dry-bulb temperature.
Relative humidity (%) is the ratio of actual vapour pressure to saturation vapour pressure — it changes as temperature changes even if the moisture content stays constant. The humidity ratio W (g/kg or kg/kg) is the actual mass of water vapour per unit mass of dry air — it is independent of temperature and is the conserved quantity used in HVAC load calculations.
Yes — just enter the correct local atmospheric pressure. At 2 000 m altitude, P_atm ≈ 79.5 kPa; at 4 000 m, ≈ 61.6 kPa. Lower atmospheric pressure shifts all properties: the same dry-bulb and RH produce a higher humidity ratio, lower enthalpy per kg, and larger specific volume. The Magnus and Stull formulas remain valid regardless of altitude.
Also known as
TG we-Calculate Editorial Team. (2026). Psychrometric Calculator — Moist Air Properties [Online calculator]. TG we-Calculate. https://we-calculate.com/calculator/psychrometric-calculator
TG we-Calculate Editorial Team. "Psychrometric Calculator — Moist Air Properties." TG we-Calculate. 2026. https://we-calculate.com/calculator/psychrometric-calculator.
TG we-Calculate Editorial Team, "Psychrometric Calculator — Moist Air Properties," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/calculator/psychrometric-calculator
@misc{wecalculate_psychrometric_calculator, title = {Psychrometric Calculator — Moist Air Properties}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/calculator/psychrometric-calculator}}, year = {2026}, note = {TG we-Calculate} }
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