"Fire codes refer to liquefied petroleum [LP] gas capacities in a variety of ways: water capacity, gallons, or pounds. It is essential to understand the differences among the units, so code requirements can be applied correctly. 'Water capacity' (WC) is the amount of water a LP Gas vessel can hold at 60 °F (15.6 °C). For portable cylinders, like those in the foreground of this picture, water capacity is expressed in pounds. Cylinders may hold up to 1,000 pounds, or about 120 gallons WC."
Coffee Break Training - Fire Protection Series No. FP-2006-22
United States Fire Administration: http://www.usfa.fema.gov/