Calculator

Dew point calculator

Type the room temperature and relative humidity from a thermometer and hygrometer. The dew point is the surface temperature below which condensation forms.

In short

Uses the Magnus formula (a = 17.62, b = 243.12 °C). Any wall, window or bottle colder than the dew point gets condensation. Valid roughly from -45 to 60 °C.

Work out the dew point

Room temperature (°C)Relative humidity (%)

Dew point

12.0 °C

Walls and windows colder than 12.0 °C get condensation.

Surface temperature (°C)For example a window pane or an outside wall, measured with a surface thermometer.

To keep a surface at 12.0 °C dry, keep the room below 60.0 % RH at 20.0 °C.

Magnus formula, a = 17.62, b = 243.12 °C. Accurate to a few tenths of a degree in normal room conditions.

Why windows stream

At 20 °C and 60 % RH the dew point is about 12 °C: surfaces colder than that get condensation.

Dew points at a glance

Room	50 % RH	60 % RH	70 % RH
16 °C	5.6 °C	8.2 °C	10.5 °C
18 °C	7.4 °C	10.1 °C	12.4 °C
20 °C	9.3 °C	12.0 °C	14.4 °C
22 °C	11.1 °C	13.9 °C	16.3 °C

Magnus formula, a = 17.62, b = 243.12 °C, rounded to 0.1 °C.