Psychrometrics — Dew Point & Humidity
Dew point, wet-bulb, relative humidity, humidity ratio, enthalpy, specific volume and vapour pressure of moist air from dry-bulb, pressure and one measure.
All the psychrometric properties at once — RH, dew point, wet-bulb, g/kg and grains/lb, vapour and saturation pressure, enthalpy, specific volume, density, absolute humidity — with °F readings.
Example: 25 °C and 50 % RH at sea level: dew point 13.9 °C, wet-bulb 17.9 °C, 9.86 g/kg (69 gr/lb), enthalpy 50.3 kJ/kg, specific volume 0.858 m³/kg, vapour pressure 1.58 kPa.
One state,
eight numbers.
How the properties of moist air are linked, which formulas are used, and their accuracy.
The chain
Everything hangs on the vapour pressure p_v. Relative humidity is p_v ÷ the saturation pressure at the dry-bulb; the dew point is the temperature whose saturation pressure equals p_v; the humidity ratio is W = 0.621945·p_v/(p − p_v) kilograms of water per kilogram of dry air. Enthalpy h = 1.006·T + W·(2501 + 1.86·T) kJ/kg and specific volume v = 0.287042·(T + 273.15)·(1 + 1.607858·W)/p follow from the ideal-gas relations in ASHRAE Fundamentals.
Saturation and wet-bulb
Saturation pressure uses the Magnus formula with the Alduchov–Eskridge (1996) coefficients, e_s = 6.1094·exp(17.625·T/(243.04 + T)) hPa, within about 0.4 % from −40 to 50 °C over water. The wet-bulb temperature has no closed form: the page solves the ASHRAE relation between W, dry-bulb and wet-bulb by bisection. A wet-bulb or dew point as input is converted to p_v first.
Accuracy and limits
The formulas are good to a fraction of a percent; a typical humidity sensor (±2–3 % RH) is the larger error. Below 0 °C the curve over ice differs (frost point); at altitude the entered pressure is used throughout, which changes W, h and the wet-bulb. Nothing leaves the browser; the same four anonymous usage counts as the rest of the site apply.
SOURCES
- Magnus formula with Alduchov–Eskridge (1996) coefficients for saturation pressure; ASHRAE Fundamentals ideal-gas relations for humidity ratio, enthalpy, specific volume and the wet-bulb equation
Last reviewed 21 September 2026. How results are checked: How we verify.