The legal measurement for a DOT spec tank seems a bit bizarre. DOT bottles are actually measured in their water capacity (WC) in lbs. That's because water is not compressible, whereas propane is, so they use water rather than water as the legal standard. You will see the WC weight and Tare Weight (TW) stamped into the metal on the bottle. TW is the weight of the empty bottle, while WC is the weight of water that would fill the tank. Convert the water weight to gallons by dividing by 8.35. If you want [liquid] propane weight, multiple the gallons by 4.23
LP gas can be sold by weight or liquid volume. When the propane is dispensed, it is in liquid form and so gallons can be measured as it passes through the pump. In the DOT bottle, it will be mostly liquid and some gas, but the quantity is the same.