Absolute humidity is the quantity of water in a particular volume of air. The most common units are grams per cubic meter, although any mass unit and any volume unit could be used. Pounds per cubic foot is common in the U.S., and occasionally even other units mixing the Imperial and metric systems are used.

If all the water in one cubic meter of air were condensed into a container, the container could be weighed to determine absolute humidity. The amount of vapor in that cube of air is