Dew Point Calculator from Temperature and Humidity
Find the dew point from the air temperature and relative humidity with the Magnus formula, in °F or °C, to one decimal. Valid from -45 °C to 60 °C.
- Dew point
- 56.9 °F
About the Dew Point Calculator
The Dew Point Calculator works out the temperature at which the moisture in the air would start to condense into dew, from the air temperature and the relative humidity. The dew point is a better guide to how muggy it feels than the relative humidity, because it does not change as the air warms and cools during the day. Type the air temperature in °F or °C and the relative humidity, and the calculator shows the dew point to one decimal, using the Magnus formula. The same page also works out the Heat Index and the Wind Chill.
The calculator covers air temperatures from -45 °C to 60 °C (-49 °F to 140 °F) and relative humidity above 0% and up to 100%, the range in which these constants are accurate to a fraction of a degree. Outside it, the field shows a message instead of a figure. The result is an estimate, not safety advice.
How to use it
Steps
- Choose Dew point.
- Type the Air temperature and pick °F or °C.
- Type the Relative humidity (%).
The Magnus formula
With the air temperature in °C, the relative humidity in percent and the constants and °C,
Readings in °F are converted to °C first, and the dew point is converted back for display.
Worked examples
- 25 °C at 60% humidity: 16.7 °C.
- 77 °F at 50%: 56.9 °F.
- 20 °C at 100%: 20.0 °C, because saturated air is already at its dew point.
- -10 °C at 70%: -14.4 °C.
What the dew point tells you
Most people find a dew point under 55 °F (13 °C) comfortable, 60 to 65 °F (16 to 18 °C) sticky and above 70 °F (21 °C) oppressive. When the overnight temperature falls to the dew point, dew or fog forms; below freezing, frost. Builders and painters check it too, because surfaces colder than the dew point get damp, and paint or sealant applied to a damp surface may not bond. Pilots and farmers watch the gap between the temperature and the dew point: when it narrows to a few degrees in the evening, fog, low cloud or frost is likely by morning. To convert temperatures, use the Temperature Converter.
Privacy
Nothing is uploaded; every calculation runs in your browser and never leaves your device. The readings you type stay on this page and are gone when you close or reload it.
Frequently asked questions
What is the dew point?
It is the temperature to which air must cool for its water vapour to condense into dew. The closer the dew point is to the air temperature, the more humid the air feels.
How is the dew point calculated?
With the Magnus formula: γ = ln(RH/100) + bT/(c + T) and dew point = cγ/(b − γ), using b = 17.62 and c = 243.12 °C. At 25 °C and 60% humidity it gives 16.7 °C.
What dew point feels uncomfortable?
Many people find a dew point under 55 °F (13 °C) comfortable, 60 to 65 °F (16 to 18 °C) sticky, and above 70 °F (21 °C) oppressive.
Why does 100% humidity give a dew point equal to the air temperature?
At 100% relative humidity the air is already saturated, so it does not need to cool at all before water condenses. At 20 °C and 100% the dew point is 20.0 °C.
What range does the calculator cover?
Air temperatures from -45 °C to 60 °C (-49 °F to 140 °F) and relative humidity above 0% and up to 100%, the range for which these Magnus constants are accurate.