From Michell Instruments:
Humidity Calculator offers a comprehensive selection of humidity units. High and low humidity ranges are covered. It facilitates calculations with extremely dry gases containing only parts per billion moisture content. At the other end of the scale, it is capable of calculations involving very high concentrations of water vapor such as in paper production and certain metallurgical processes. Parameters included on the online calculator are relative humidity, absolute humidity, mixing ratio, moisture content in gases, moisture content in liquids, dew point, and vapor pressure.