Relative Humidity Calculator

Relative humidity (RH) is the amount of water vapour in the air compared with the most it could hold at that temperature. Enter the temperature and dew point to calculate it.

How to use the Relative Humidity Calculator

Enter the air temperature.
Enter the dew point (it must not be higher than the temperature).
Choose °C or °F and read the relative humidity.

How it works

Relative humidity is calculated from the ratio of the actual vapour pressure (set by the dew point) to the saturation vapour pressure (set by the air temperature), using the Magnus approximation:

RH = 100 × exp( aT_d/(b + T_d) − aT/(b + T) )

with a = 17.625 and b = 243.04 °C. Indoor humidity of about 30–50% is generally recommended for comfort and to limit mould.

Relative humidity at 25 °C (77 °F)

Dew point	Relative humidity
0 °C / 32 °F	19%
5 °C / 41 °F	28%
10 °C / 50 °F	39%
12 °C / 54 °F	44%
15 °C / 59 °F	54%
18 °C / 64 °F	65%
20 °C / 68 °F	74%
22 °C / 72 °F	83%
25 °C / 77 °F	100%

Frequently asked questions

What is a good indoor humidity?

Around 30–50% relative humidity is comfortable and helps limit dust mites and mould.

Why does relative humidity fall during the day?

As the air warms, it can hold more water vapour, so the same amount of moisture is a smaller percentage of the maximum.

What does 100% humidity mean?

The air is saturated: the temperature equals the dew point, and fog or condensation can form.