Thailand employs the traditional measure GRT or Gross Registered Tonnage when referring to the size of a vessel. This size specification is often used and confused interchangeably with the terms displacement.
In fact the GRT of a vessel is not a measure of displacement or the length or weight of the vessel, it is the volume of space within the hull and enclosed space above the deck which are available for cargo, stores, fuel, passengers and crew. It is a measurement of the available volume, in cubic meters.
So how big is your Yacht in GRT, well the Harbour Department will calculate that for you and you can find plenty of calculators on line. To give you a feel for things, a regular sailing boat (keelboat) yacht of 45ft would come in at approx 8 GRT. Modern catamarans don’t fair well in this calculation, a 45ft Catamaran would be more like 23 GRT.
Note that this size classification is what most countries refer to, including Thailand, when they refer to a skipper license for a vessel “Tonnage” of 30 tonnes is generally specifying the vessels GRT.