About Psychrometric Calculator
The psychrometric calculator returns the full set of moist-air properties used in HVAC design: humidity ratio, enthalpy, dew-point and wet-bulb temperature, vapour pressure, specific volume, and density. These properties drive cooling-coil loads, dehumidification, condensation risk, and air-mixing calculations.
Enter a dry-bulb temperature and one secondary measurement — relative humidity, wet-bulb temperature, or dew-point — together with the atmospheric pressure (default sea-level 101 325 Pa). The tool fixes the air state and reports every derived property, and plots your state on a mini psychrometric chart of constant relative-humidity lines.
How It Works
- Enter the dry-bulb temperature and choose the secondary input: relative humidity, wet-bulb, or dew-point.
- The tool finds the saturation vapour pressure Pws at the dry-bulb temperature, then the actual vapour pressure Pw from the secondary input.
- It computes the humidity ratio W = 0.62198·Pw/(P − Pw) and the enthalpy h = 1.006·T + W·(2501 + 1.86·T).
- It derives dew point (inverse Magnus), wet-bulb (psychrometer relation), specific volume, and density, and marks the state on the chart.
Worked Example
For air at 25°C dry-bulb and 50% relative humidity at sea level (101 325 Pa): the saturation pressure from the Magnus form Pws = 610.94·exp(17.625·25/(25+243.04)) = 3162 Pa, so the vapour pressure is Pw = 0.50·3162 = 1581 Pa. The humidity ratio is W = 0.62198·1581/(101325 − 1581) = 0.00986 kg/kg (9.86 g/kg). The enthalpy is h = 1.006·25 + 0.00986·(2501 + 1.86·25) = 50.3 kJ/kg. The dew point is 13.9°C and the wet-bulb is 17.9°C — all matching a standard ASHRAE psychrometric chart reading at this point.
Formulas
- Saturation vapour pressure (Magnus-Tetens form)
Pws = 610.94 * exp(17.625 * T / (T + 243.04))- Vapour pressure and humidity ratio
Pw = (RH/100) * Pws ; W = 0.62198 * Pw / (P - Pw)- Moist-air enthalpy
h = 1.006 * T + W * (2501 + 1.86 * T)- Specific volume and density
v = 287.042 * (T+273.15) * (1 + 1.6078*W) / P ; rho = (1 + W) / v
Standards & References
- ASHRAE Handbook of Fundamentals — Psychrometrics chapter
- Magnus-Tetens / Alduchov & Eskridge (1996) saturation-pressure form
- ASHRAE psychrometric chart (sea-level and altitude)
Frequently Asked Questions
Which saturation-pressure formula does this calculator use?
It uses the Magnus-Tetens form Pws = 610.94·exp(17.625·T/(T+243.04)) in Pascals, with T in °C. This agrees with the ASHRAE Handbook of Fundamentals saturation tables to within a few tenths of a percent across the 0–60°C range used in building services.
What is the humidity ratio and why is enthalpy expressed per kg of dry air?
The humidity ratio W is the mass of water vapour per kilogram of dry air. Psychrometric properties are referenced to dry air because its mass is conserved as the air is heated, cooled, humidified, or dehumidified, whereas the total moist-air mass changes. Enthalpy h is therefore given in kJ per kg of dry air.
How are wet-bulb and dew-point different?
Dew point is the temperature at which the air would become saturated if cooled at constant pressure and moisture; it depends only on the actual vapour pressure. Wet-bulb is the temperature reached by evaporative cooling and always lies between the dew point and the dry-bulb temperature. They are equal only at 100% relative humidity.
Does atmospheric pressure (altitude) affect the results?
Yes. Saturation pressure depends only on temperature, but the humidity ratio, specific volume, and density all depend on total pressure. At higher altitude the lower pressure raises the humidity ratio for the same vapour pressure, so set the pressure field to your site value for accurate results.