Formula and Method for the Psychrometric Calculator
Psychrometrics is the branch of thermodynamics that studies the physical and thermal properties of moist air — mixtures of dry air and water vapor. Air's capacity to hold water vapor depends strongly on temperature, so a single pair of readings, dry-bulb temperature and relative humidity, is enough to derive every other common moist-air property: the actual mass of water vapor present (humidity ratio), the temperature at which it would start to condense (dew point), the temperature a wetted thermometer would read in that air (wet-bulb temperature), and the total heat energy stored in it (enthalpy). This calculator derives all four from dry-bulb temperature, relative humidity, and atmospheric pressure using standard psychrometric equations.
How the calculation works
The calculator first finds the saturation vapor pressure Pws at the dry-bulb temperature using the Buck equation, Pws = 0.6112 × e^[(18.678 − T/234.5)(T/(257.14 + T))] kPa, an accurate empirical fit to the Clausius-Clapeyron relation for water vapor over liquid water. Multiplying Pws by the relative humidity gives the actual vapor pressure, Pv = (RH/100) × Pws. The humidity ratio follows from Dalton's law of partial pressures applied to an ideal-gas mixture: W = 0.622 × Pv ÷ (P − Pv), where 0.622 is the ratio of the molar mass of water (18.02 g/mol) to dry air (28.97 g/mol) and P is the total atmospheric pressure. Inverting the same saturation curve at the actual vapor pressure Pv gives the dew point — the temperature at which that amount of vapor would saturate the air. Wet-bulb temperature has no closed-form solution from first principles (it comes from an energy balance between evaporative cooling and sensible heat transfer at a wetted thermometer bulb), so this calculator uses Stull's (2011) empirical regression, which is accurate to within about 0.3°C of the iterative psychrometric solution across typical conditions. Finally, specific enthalpy combines the sensible heat of the dry air with the latent and sensible heat carried by its water vapor: h = 1.006T + W(2501 + 1.86T) kJ per kilogram of dry air, with T in °C.
Reading the results
Humidity ratio (also called mixing ratio) is reported in grams of water vapor per kilogram of dry air — it is the quantity that stays constant as air is heated or cooled without adding or removing moisture, which makes it the right number for sizing dehumidification or humidification loads. Dew point tells you the surface temperature at which condensation (fogging windows, dripping ducts, or forming dew) will begin. Wet-bulb temperature is what evaporative cooling can realistically achieve, and it sets a practical lower bound for cooling-tower and evaporative-cooler performance. Enthalpy is the total heat content per unit mass of dry air and is used directly in HVAC load calculations because it captures both the temperature change and the moisture change of an airstream.
Limitations and valid ranges
The Buck saturation-vapor-pressure equation used here is accurate to within about 0.05% over liquid water for temperatures from roughly −20°C to 50°C; below freezing, saturation vapor pressure over ice is slightly lower than over supercooled water, a distinction this calculator does not make. Stull's wet-bulb formula was fit for relative humidity between 5% and 99% and pressures near the standard sea-level value of 101.325 kPa, so it becomes less reliable at very low or very high humidity, or at pressures far from that reference — treat the wet-bulb result as approximate at high altitude. For applications where a few tenths of a degree matter, such as precision HVAC design, laboratory work, or cooling-tower sizing, validate against an iterative psychrometric solver or a certified psychrometric chart for your exact site elevation.