Relative Humidity Calculator

Relative humidity is the amount of water vapor in the air expressed as a percentage of the maximum the air could hold at that temperature. It is the number you see on most weather reports and thermostats, and it drives comfort, condensation and mold risk. This calculator derives relative humidity from the air temperature and the dew point, the two quantities a thermometer and a hygrometer or weather station give you most directly. It works by computing the saturation vapor pressure at both temperatures using the Magnus equation and taking their ratio, the standard method used in meteorology. Enter the air temperature and the dew point, choose Celsius or Fahrenheit, and read the relative humidity instantly. This is handy when a data source reports dew point but not humidity, or when you want to sanity check a sensor. All calculations run locally in your browser for private, immediate results.