The Vaisala Humidity Calculator app for the Apple iPhone allows users to calculate several humidity parameters from one known value. Unit conversions can be made anywhere and users can see the effects of changing ambient conditions, such as temperature and pressure, right from their smartphone. Users can calculate or convert relative humidity, dew point and frost point, absolute humidity, water content, mixing ratio, vapor pressure, parts per million, and wet bulb temperature. The free Humidity Calculator app can be downloaded at the Apple iTunes store, or from the Vaisala website at www.vaisala.com/iphone-rh-calc.