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Dew Point Calculator

Find the dew point temperature using the Magnus formula.

Result

16.70 °C

How it is calculated

The tool uses the Magnus approximation with constants a = 17.625 and b = 243.04. First it computes γ = ln(RH/100) + a·T / (b + T), where T is the air temperature in °C and RH is the relative humidity in percent. The dew point is then Td = b·γ / (a - γ), also in °C. Humidity must be above 0 and at most 100, otherwise there is no result. At 100% humidity the dew point equals the air temperature. The Magnus formula is an approximation, and the result is rounded to two decimals; treat it as accurate to a fraction of a degree rather than exact.

Worked example

Air at T = 25 °C and RH = 60%. ln(0.60) = -0.5108 and 17.625·25 / (243.04 + 25) = 1.6438, so γ = 1.1330. Then Td = 243.04 · 1.1330 / (17.625 - 1.1330) = 275.37 / 16.492, which gives a dew point of 16.70 °C. Surfaces cooler than about 16.7 °C would start to collect condensation in that air.

Try this example →

Common mistakes

  • Entering humidity as a fraction (0.6) instead of a percent (60); the tool would read it as 0.6% and give a very low dew point.
  • Entering the temperature in °F. The formula expects °C.
  • Expecting the dew point to be above the air temperature. It can equal it at 100% humidity but never exceeds it.
  • Using indoor humidity with outdoor temperature (or the reverse), which mixes two different air masses.

About this calculator

This calculator finds the dew point — the temperature air would need to cool to for the water vapor in it to condense into dew or fog — from the current air temperature and relative humidity, using the Magnus formula, a well-established approximation used in meteorology.

A dew point close to the air temperature signals muggy, uncomfortable conditions (since sweat cannot evaporate efficiently) and a rising risk of fog, while a low dew point means dry, comfortable air; it is used by pilots checking fog risk before flights, gardeners and farmers watching for frost or dew formation overnight, and weather-watchers who find dew point a more reliable comfort indicator than humidity percentage alone.

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Comments (1)

  • Editorial team

    Dew point tells you at what surface temperature water will condense. At 25 °C and 60% humidity it is about 16.7 °C, so a glass colder than that will sweat and a window or wall at that temperature invites mould. The tool works in Celsius only, so convert Fahrenheit readings first. Humidity must be above 0 and at most 100. Where do you use dew point: weather, home moisture or something else?

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