About this tool
The Relative Humidity Calculator derives relative humidity (RH) from the air temperature and the dew point. Relative humidity is the ratio of the actual water-vapor pressure in the air to the saturation vapor pressure at the same temperature, expressed as a percentage. It is the number weather reports quote, and it drives comfort, condensation, mold growth, and how quickly things dry.
The tool applies the August–Roche–Magnus relation with constants a = 17.625 and b = 243.04°C: RH = 100 · exp( a·Td/(b+Td) − a·T/(b+T) ), where T is the air temperature and Td is the dew point, both in °C. This is mathematically equivalent to RH = 100 · es(Td)/es(T), the ratio of saturation vapor pressures at the dew point and the air temperature. The result is clamped to the physical 0–100% range.
Everything is computed locally in your browser. Enter the air temperature and dew point in °C or °F; the calculator returns the relative humidity along with the actual and saturation vapor pressures (hPa). Because the dew point can never exceed the air temperature, a dew point above the temperature is rejected as invalid input.