Dew Point Calculator

About Dew Point Calculator

The dew point is the temperature to which air must be cooled for the water vapour in it to start to condense. When a surface is colder than the dew point, water forms on it: a cold glass in summer, a window on a winter morning, or a cold wall in a damp room. It is a more direct measure of how much moisture is in the air than relative humidity, because relative humidity changes with temperature.

This calculator uses the Magnus formula with the Alduchov and Eskridge constants, accurate to a fraction of a degree between minus 40 and 50 degrees Celsius. Enter the temperature and the relative humidity to get the dew point, or the temperature and the dew point to get the relative humidity. It also shows the vapour pressure. As a guide, a dew point above about 16 degrees feels muggy, and walls colder than the dew point are at risk of condensation and mould.

How to use Dew Point Calculator

1. Enter the air temperature and choose Celsius, Fahrenheit or kelvin.
2. Choose whether you know the relative humidity or the dew point.
3. Enter that value.
4. Read the dew point, humidity, the gap to the air temperature and the vapour pressure.

Common uses

• Judging the risk of condensation and mould on cold walls
• Working out when fog or frost will form
• Setting a heating or ventilation strategy for a damp room
• Gardening and weather questions