Relative Humidity Calculator
Find relative humidity from air temperature and dew point.
About the Relative Humidity Calculator
Enter the air temperature and the dew point, in °C or °F. The calculator gives the relative humidity and whether it's in the comfortable indoor range.
Relative humidity is the actual water vapour in the air as a percentage of the most the air could hold at that temperature. The Magnus approximation for saturation vapour pressure gives RH from temperature and dew point. Indoors, 30–50% is usually comfortable and limits mould and dust mites.
The formula
RH = 100 × exp(a Td ÷ (b + Td) − a T ÷ (b + T))
Worked example
77 °F (25 °C) with a dew point of 60 °F (15.6 °C)
- Magnus formula
- Relative humidity 55.8%
- Within the 30–60% comfort range
Questions people ask
Why is winter air so dry indoors?
Heating cold outdoor air raises its temperature but not its moisture.
Does RH change during the day?
Yes, it falls as the temperature rises even with the same moisture.