WattNumbers
Home › Dew point calculator

Dew point calculator

Enter the air temperature and relative humidity to see the dew point: the temperature at which surfaces get wet.

Measure with a room hygrometer; a surface thermometer helps for windows and wall corners.

Dew point12.0 °C
Window glasswet when colder than 12.0 °C

The drawing is an example house. The scale shows which surface temperatures collect water (hatched) at the air you entered.

Dew point12.0 °C
Water vapour in the air
10.3 g/m³
Surface check
Enter a surface temperature

Magnus formula, WMO-No. 8 (2023), Annex 4.B. Calculated in your browser; rounded to 0.1. How it works

Short answer: room air at 20 °C and 60 % relative humidity has a dew point of 12.0 °C (53.6 °F). Any surface colder than that, such as a window pane or an outside-wall corner in winter, collects water.

How we calculate

The dew point is the temperature to which air must cool for its water vapour to condense. We work it out from the saturation vapour pressure over water.

NeutralWe sell nothing and pass on no enquiries. No sign-up, no phone number.
Worked in your browserThe dew point is calculated on your device; what you type is not sent to us.
Method shownMagnus formula with the coefficients of the World Meteorological Organization (WMO-No. 8, 2023, Annex 4.B), valid from −45 °C to +60 °C.
CheckedOur test file checks the result against the definition (saturation at the dew point equals the vapour pressure of the air) and against a second formula.

Deutsch: Taupunkt-Beispiel und deutsche Rechner

Coming next

Heat load planned

How much heating power a home needs.

U-value planned

How well a wall or roof keeps heat in.

Solar yield planned

What a roof or balcony system makes in a year.

Questions

What is the dew point?

The temperature at which air, cooled at constant pressure, becomes saturated with water vapour. Surfaces at or below it collect condensation.

Why does my window get wet?

The glass is colder than the dew point of the room air. Lower the humidity (airing, extractor fans) or keep the surface warmer (heating, better glazing, insulation).

Which humidity is normal indoors?

Often quoted guidance is 40 to 60 % relative humidity for living rooms. What matters for damp is whether a surface is colder than the dew point.

How accurate is it?

The Magnus formula with WMO coefficients is accurate to about 0.1 °C in normal room conditions. Your result is only as good as your hygrometer; cheap ones can be several percent off.

Does it work in Fahrenheit?

Yes. Switch to °F; we convert your input, calculate and show the result in °F.

Created by Saad Danish · Formula checked on 12 November 2026