Dew Point Temperature Calculator
The dew point temperature tells you how much water vapour is in the air. Unlike relative humidity, which varies with temperature, the dew point is a fixed measure of moisture content. When air temperature drops to the dew point, condensation forms as fog, dew or frost. This calculator applies the August-Roche-Magnus approximation, which is the standard formula recommended by the World Meteorological Organization (WMO) for meteorological applications. Enter the air temperature in degrees Celsius and the relative humidity as a percentage to get the dew point temperature and the temperature-dew point spread.
August-Roche-Magnus formula
gamma = 17.625 × T / (243.04 + T) + ln(RH / 100)
Td = 243.04 × gamma / (17.625 - gamma)
Where T is air temperature in deg C and RH is relative humidity in percent. This approximation is accurate to within 0.35 deg C for temperatures between -40 and 60 deg C, per Lawrence (2005) in the Bulletin of the American Meteorological Society.
Dew point comfort scale
- Below 10 deg C (50 deg F): Dry and comfortable; air feels crisp.
- 10 to 13 deg C (50 to 55 deg F): Slightly humid but pleasant.
- 13 to 16 deg C (55 to 60 deg F): Comfortable for most people.
- 16 to 18 deg C (60 to 65 deg F): Noticeably humid; some discomfort.
- 18 to 21 deg C (65 to 70 deg F): Muggy and uncomfortable.
- Above 21 deg C (70 deg F): Oppressively humid; heat illness risk is significant.
Dew point calculator: frequently asked questions
What is the dew point temperature?
The dew point is the temperature at which air must be cooled (at constant pressure and humidity) for water vapour to condense into liquid water (dew). When air is cooled below its dew point, condensation occurs on surfaces. Dew point is a direct measure of the amount of moisture in the air, unlike relative humidity which changes with temperature.
What formula is used to calculate dew point?
This calculator uses the August-Roche-Magnus approximation: Td = (243.04 x (ln(RH/100) + 17.625 x T / (243.04 + T))) / (17.625 - (ln(RH/100) + 17.625 x T / (243.04 + T))), where T is air temperature in deg C and RH is relative humidity as a percentage. This is the standard WMO-recommended approximation.
What is a comfortable dew point?
Dew points below 10 deg C (50 deg F) feel dry and comfortable. Dew points of 13 to 16 deg C (55 to 60 deg F) are comfortable for most people. Dew points above 18 deg C (65 deg F) begin to feel muggy or humid. Above 21 deg C (70 deg F) the air feels oppressively humid and heat illness risk rises rapidly.
How does dew point differ from relative humidity?
Relative humidity is a ratio: the amount of moisture in the air relative to the maximum it can hold at that temperature. It changes as temperature changes even if the actual moisture content stays the same. Dew point is an absolute measure of moisture content that does not change with temperature, making it a better indicator of how humid the air actually is.
Why does the dew point matter for aviation?
In aviation, the dew point spread (difference between air temperature and dew point) is used to estimate cloud base height and fog risk. A small spread (under 2 to 3 deg C) indicates high risk of fog or low cloud. The spread is also used with the lifted condensation level formula to estimate cloud base height in feet.
Official sources
- Lawrence, M.G. (2005). The relationship between relative humidity and the dew point temperature in moist air. Bulletin of the American Meteorological Society: journals.ametsoc.org.
- World Meteorological Organization, Guide to Meteorological Instruments and Methods of Observation (WMO-No. 8): library.wmo.int.
Reviewed by the CalculatorHub team, edited by James Graham, 14 June 2026. See our methodology.