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Dew Point Calculator

The dew point is the temperature at which air becomes saturated and water vapour starts to condense. Use it to judge how humid it feels and whether condensation or fog is likely.

How it works Report a problem

Dew Point Calculator

Magnus formula — dew point, humidity, and frost point

Td = (243.04 × [ln(RH/100) + 17.625T/(243.04+T)]) / (17.625 − [ln(RH/100) + 17.625T/(243.04+T)])
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Dew Point
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Temp − Dew Point
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Frost Point
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Abs. Humidity
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About the Dew Point Calculator

The dew point is the temperature at which air becomes saturated and water vapour starts to condense. Use it to judge how humid it feels and whether condensation or fog is likely.

How to use the Dew Point Calculator

  1. Enter the Air Temperature and choose °C or °F.
  2. Enter the Relative Humidity (%).
  3. Press Calculate.

Formula and method

The Magnus approximation: γ = ln(RH ÷ 100) + (a × T) ÷ (b + T) and Dew point = b × γ ÷ (a − γ), with a = 17.62 and b = 243.12 °C.

Worked example

At 25 °C and 60% relative humidity the dew point is about 16.7 °C.

When to use the Dew Point Calculator

  • Judging how humid the air feels.
  • Predicting fog, frost and condensation.
  • Checking indoor humidity to prevent mould.

Common mistakes to avoid

  • Confusing dew point with relative humidity. Dew point is an absolute measure of moisture in the air.

Tips

  • A dew point above about 20 °C usually feels muggy; below 10 °C feels dry.
  • Surfaces colder than the dew point will collect condensation.

Frequently asked questions

What is the difference between humidity and dew point?
Relative humidity depends on temperature. The dew point is an absolute measure of moisture in the air.
What dew point feels uncomfortable?
A dew point above about 18 °C (65 °F) feels muggy, and above 21 °C (70 °F) feels oppressive to most people.