Dew Point from Humidity Calculator
The dew point temperature is the temperature at which air becomes saturated with water vapor and condensation begins. It depends on both the air temperature and the relative humidity. This calculator uses the Magnus formula, a standard meteorological approximation with coefficients recommended by Alduchov and Eskridge (1996, as cited by Lawrence 2005 in the Bulletin of the American Meteorological Society). The calculation uses gamma = (a T)/(b + T) + ln(RH/100), then dew point Td = b gamma / (a - gamma), where a = 17.625 and b = 243.04 degrees C.
Magnus formula for dew point
gamma = (a T)/(b + T) + ln(RH/100) Td = b × gamma / (a - gamma)
Where a = 17.625 and b = 243.04 degrees C (Alduchov and Eskridge 1996 coefficients). T is air temperature in degrees C, RH is relative humidity in percent (0 to 100). Td is dew point in degrees C. Error less than 0.1 degrees C for -40 to 60 degrees C range.
Dew point comfort guide
- Below 10 degrees C (50 F): very dry, comfortable for most activities.
- 10 to 13 degrees C (50 to 55 F): comfortable, similar to typical spring mornings.
- 13 to 18 degrees C (55 to 65 F): comfortable for most people.
- 18 to 21 degrees C (65 to 70 F): becoming uncomfortable, noticeable humidity.
- Above 21 degrees C (70 F): oppressive; sweating is less effective at cooling.
Frequently asked questions
What is the dew point?
The dew point is the temperature to which air must be cooled (at constant pressure and humidity) for water vapor to begin condensing into dew or frost. When air temperature equals the dew point, relative humidity is 100%.
What formula does this calculator use?
This calculator uses the Magnus formula approximation: dew point Td = b * gamma / (a - gamma), where gamma = a * T / (b + T) + ln(RH/100), a = 17.625, b = 243.04 degrees C. This formula is widely used in meteorology (Lawrence 2005, AMS Bulletin).
What is the difference between dew point and relative humidity?
Relative humidity (RH) is the ratio of actual water vapor pressure to the saturation vapor pressure at the current temperature. Dew point is an absolute measure of moisture content that does not depend on temperature. Dew point is more informative for human comfort.
What dew point feels comfortable?
A dew point below 13 degrees C (55 F) feels dry and comfortable. Between 13 and 18 degrees C (55 to 65 F) is comfortable for most people. Above 21 degrees C (70 F) is oppressively humid. Above 26 degrees C (79 F) is extremely dangerous.
How accurate is the Magnus formula?
The Magnus formula with coefficients a = 17.625 and b = 243.04 (Alduchov and Eskridge 1996, as cited by Lawrence 2005) has an error of less than 0.1 degrees C for temperatures from -40 to 60 degrees C, sufficient for all practical applications.
Official sources
- NOAA/NWS: Why Dew Point vs Relative Humidity (NWS).
- NOAA: National Oceanic and Atmospheric Administration.
Reviewed by the CalculatorHub team, edited by James Graham, 15 June 2026. See our methodology.