The notional carrying capacity of a vessel used for port dues, taxation and so forth. National authorities calculate the figure by subtracting from the hull volume various allowances for crew’s quarters, navigation room, engine rooms, and so forth. Even small private yachts may have a register tonnage, which may be a bit of fun and may be written on forms by petty officials who ‘welcome’ you in this port or that. But for private craft the whole thing is a nonsense. The one thing you can say for sure about Register Tonnage, is that it is not a matter of weight.