Propane can be measured in a few manners. The most common way is by gallons. It can also be measured by pounds (one gallon = 4.2lbs). When measured in gallons, a meter at the pump is used to determine the amount of propane that has been filled into a tank. When measured in pounds, the tare (empty starting) weight of the tank must be known first. After this weight is taken into account, the tank is then filled to capacity with the correct volume. (i.e. A BBQ tank is commonly referred to as a 20lb tank. This is because these tanks hold 20lbs or approximately five gallons of propane).