Absolute Humidity Calculator

How it is calculated

Three steps, with temperature T in °C and relative humidity RH in percent (0 to 100):

Saturation vapour pressure: es = 6.112 × exp(17.67 T / (T + 243.5)) hPa

Actual vapour pressure: e = es × RH / 100 hPa

Absolute humidity: AH = 2.1674 × (e × 100) / (273.15 + T) g/m³

The constants are the Magnus approximation for saturation pressure; the last step applies the ideal gas law for water vapour, with e converted from hPa to Pa. A relative humidity below 0 or above 100 gives no result. Both the absolute humidity and the vapour pressure are shown with two decimals.

Common mistakes

• Entering relative humidity as a fraction such as 0.6: the field expects percent, so 0.6 means 0.6 % and gives a tiny result.
• Entering Fahrenheit or Kelvin: temperature must be in °C.
• Comparing two rooms by relative humidity alone: warm air at 50 % RH holds far more water than cold air at 50 % RH, which is why absolute humidity differs.
• Assuming the value stays constant when the air is heated: absolute humidity stays the same only until water is added or removed, while relative humidity falls.