Dew Point Calculator
Use this free Dew Point Calculator to calculate your Dew point result instantly and accurately.
Conditions
Dew point
What is a Dew Point Calculator?
A Dew Point Calculator computes the dew point temperature from the current air temperature and relative humidity. It tells you how much moisture is actually in the air and helps you understand comfort levels, predict fog, and assess humidity for weather monitoring or HVAC applications.
How to calculate dew point manually (the formula)
The Magnus formula for dew point is: Td = (b × α(T,RH)) / (a − α(T,RH)) where α = (a × T) / (b + T) + ln(RH/100), with constants a = 17.27 and b = 237.7°C. T is the air temperature and RH is relative humidity as a percentage.
Example calculation
If the air temperature is 25°C and relative humidity is 60%: α = (17.27 × 25) / (237.7 + 25) + ln(60/100) = 1.596 + (−0.511) = 1.085. Then Td = (237.7 × 1.085) / (17.27 − 1.085) = 258 / 16.185 ≈ 15.9°C. This feels comfortable since it is below 18°C.
Common mistakes
- Confusing dew point with relative humidity — RH is a percentage; dew point is a temperature. 50% RH at 10°C feels very different from 50% RH at 35°C, but dew point tells the real story.
- Using wrong units — The Magnus formula requires Celsius. If you enter Fahrenheit, convert first or use the calculator’s unit selector.
- Forgetting natural log — The formula uses ln(RH/100), not a plain division. This is a common manual calculation error.
- Ignoring that dew point cannot exceed temperature — If your calculation shows Td > T, check your inputs. RH cannot exceed 100%.
Frequently asked questions
What is dew point?
Dew point is the temperature at which air becomes saturated with moisture and water vapor begins to condense into liquid. It is a direct measure of atmospheric humidity.
How is dew point calculated?
Dew point is calculated using the temperature and relative humidity. The Magnus formula is: Td = (b × α(T,RH)) / (a − α(T,RH)) where α = (a × T) / (b + T) + ln(RH/100), with constants a = 17.27 and b = 237.7°C.
What is a comfortable dew point?
A dew point below 13°C (55°F) feels dry and comfortable. Between 13–18°C (55–65°F) it feels pleasant. Above 18°C (65°F) it starts feeling sticky, and above 21°C (70°F) it feels muggy or oppressive.
What is the difference between dew point and humidity?
Relative humidity is a percentage that compares current moisture to the maximum possible at that temperature. Dew point is an absolute temperature measure of moisture content. Dew point above 15°C always feels humid regardless of the relative humidity percentage.
Can dew point be higher than temperature?
No, dew point cannot exceed the air temperature. When dew point equals air temperature, relative humidity is 100% and condensation forms. If temperatures drop below dew point, fog or dew appears.
Why does dew point matter for weather forecasting?
Dew point helps meteorologists predict fog, frost, precipitation, and how comfortable conditions will feel. A rising dew point often indicates increasing moisture and potential storm development.