Dew Point Calculator
Calculate the dew point from air temperature and relative humidity using the widely-used Magnus approximation.
What dew point means
The dew point is the temperature to which air must be cooled, at constant pressure and moisture content, for water vapor to condense into dew or fog. A higher dew point means more moisture in the air and generally a muggier feel.
The formula
This calculator uses the Magnus-Tetens approximation: dew point = (b × α) / (a − α), where α = ln(RH/100) + (a × T) / (b + T), T is air temperature in Celsius, RH is relative humidity as a percentage, and a = 17.625, b = 243.04 are standard Magnus constants. If you enter Fahrenheit, the calculator converts to Celsius internally and converts the result back.
This approximation is widely used in meteorology and is accurate to within about ±0.4°C over typical temperature and humidity ranges. This tool is for general educational and planning purposes, not for certified weather forecasting.