WC # is in pounds of water if the tank were totally filled with water. It is an indicative number used to identify the safe fill level for propane. Read the WC # on your tank, reduce it by 20%, multiply it by 4.24 ( the weight of a gallon of propane) to get the exact maximum propane gallon capacity your tanks are safely designed dor.
Propane is an expansive liquid at temperature changes, unlike water whose volume is constant between freezing and boiling points. Thus the safety level of filling propane tanks to a maximum of 80% to allow for that liquid expansion should warmer weather come along.