CFR /  Title 46  /  Part 151  /  Sec. 151.03-21 Filling density.

The ratio, expressed as a percentage, of the weight of cargo that may be loaded into a tank compared to the weight of water that the tank will hold at 60 [deg]F. The weight of a gallon of water at 60 [deg]F in air shall be 8.32828 pounds.