Dew point calculator
Dew point from temperature and humidity, or humidity from the dew point.
About this calculator
This dew point calculator finds the temperature at which dew forms, from the air temperature and relative humidity. It can also work back from the dew point to the humidity.
It shows how muggy the air will feel, the absolute humidity and the gap to condensation, which matters for damp and mould.
How it works
Uses the Magnus formula with the Alduchov and Eskridge constants: γ = ln(RH ÷ 100) + 17.625T ÷ (243.04 + T), then dew point = 243.04γ ÷ (17.625 − γ), with T in °C. It holds well across everyday weather.
Last reviewed:
Worked example
- Work out: Dew point
- Temperature in: °F
- Air temperature: 80
- Relative humidity: 60%
Dew point: 64.9 °F (Becoming sticky, with muggy evenings)
Common questions
What is the dew point?
The temperature the air must cool to for dew to form, when the relative humidity reaches 100%.
What dew point is comfortable?
In summer, 55 °F (13 °C) or below feels dry and comfortable, 55 to 65 °F (13 to 18 °C) feels sticky, and above 65 °F (18 °C) feels oppressive.
Is the dew point better than relative humidity?
For comfort, yes. Relative humidity changes with the temperature, while the dew point shows how much moisture is actually in the air.
Sources and further reading
The story behind it
Where the idea came from: The chemist John Frederic Daniell built one of the first dew point hygrometers in 1820, cooling a polished bulb until dew appeared on it.
Relative humidity changes as the day warms and cools, but the dew point stays steady while the moisture in the air does. That is why a 30 °C day with a 20 °C dew point feels muggy, whatever the humidity reading.
Why you can trust this calculator
- It runs in your browser. Nothing you type into it is sent anywhere.
- The method is explained on this page, so you can check it yourself.
- Free to use, with no sign-up.
- Results are estimates, so please double-check anything important.
- Spotted a mistake? Tell us and we'll look into it.
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