Relative humidity calculator
If you have the temperature and dew point from a forecast or a meter, enter them to get the relative humidity.
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How to use it
- Enter air temperature, dew point and temperature unit.
- The result appears straight away and updates as you edit.
- Copy the result, or copy a link that keeps the same values to share it.
How it is calculated
RH = 100 × exp(aTd ÷ (b + Td) − aT ÷ (b + T)), with a = 17.625 and b = 243.04°C.
Example
Inputs: Air temperature = 30, Dew point = 20, Temperature unit = Celsius (°C). Result: Relative humidity: 55.1%; Actual vapour pressure: 23.33 hPa; Saturation vapour pressure: 42.37 hPa.
Limits and caveats
- The Magnus formula is very close in normal weather and slightly less so in deep cold.
- Calculated over water; instruments that use saturation over ice may read a little differently.
Questions about this tool
Why does relative humidity drop at midday?
Warmer air can hold more vapour, so the percentage falls even if the vapour stays the same.
What indoor humidity is comfortable?
Roughly 30 to 60% is usually recommended for comfort and to limit mould.
What does 100% humidity mean?
The air is saturated and the temperature equals the dew point, so condensation begins.