Why is Gross Tonnage a volume?

If you look at the name, you would think that Gross Tonnage is a weight. This is not true, Gross Tonnage is a volume. To determine the size of the yacht the Register Ton is used. It is based on the volume where one register ton = 100cft, or 2.83 m3.

The Gross Tonnage (GT) is calculated using a formula that takes into account the yacht’s volume in cubic metres below the main deck and the enclosed spaces above the main deck.