Dew Point Calculator
Find the dew point, the temperature the air has to cool to before it reaches 100% relative humidity. Enter the air temperature in °C or °F and the relative humidity, above 0% and up to 100%. The calculator uses the Magnus formula and shows a comfort label for the result.
How to use the Dew Point Calculator
- Enter your numbers in the Dew Point Calculator above.
- Press the Calculate button to see the result.
- To try other numbers, change any value and press the button again.
The Magnus formula
γ(T, RH) = ln(RH/100) + (b·T)/(c+T) · · · Dew point = c·γ / (b − γ)
T is the air temperature in °C and RH the relative humidity in percent. The calculator uses b = 17.27 and c = 237.3 °C. Other sets of constants are in use, such as b = 17.625 and c = 243.04 °C, so another calculator can give a slightly different dew point. In °F mode the calculator converts the temperature to °C, applies the formula and converts the dew point back. The calculator rounds the result to one decimal.
Worked example
At 25°C (77°F) with 60% relative humidity, the dew point is 16.7°C (62.1°F), labelled Slightly humid. At 30°C and 80% it is 26.2°C (79.1°F), labelled Oppressive / uncomfortable.
Frequently asked questions
Why is dew point important?
Relative humidity changes with temperature even when the amount of water vapor in the air stays the same. The dew point depends only on that water vapor, so it tells you how much moisture the air holds. A higher dew point means more moisture.
Can the dew point be higher than the air temperature?
Not in this calculator. At 100% relative humidity the dew point equals the air temperature, and below 100% it is lower. The calculator takes humidity above 0% and up to 100%, so its dew point is never above the temperature you enter.
What do the comfort labels mean?
The calculator picks them from the dew point in °C. Below 10°C (50°F) it shows Dry and very comfortable. From 10°C to under 16°C (60.8°F) it shows Comfortable, then Slightly humid up to 21°C (69.8°F) and Muggy up to 24°C (75.2°F). At 24°C and above it shows Oppressive / uncomfortable. These bands are the calculator's own, not an official scale.