Gross Tonnage (GT)

What is gross tonnage on a yacht?

Gross Tonnage is the headline regulatory number on every yacht specification sheet, and the figure most often misunderstood. It does not weigh the yacht. It measures the total volume of every enclosed space on board - accommodation, machinery, voids, tanks integral to the structure - and converts that volume into a dimensionless index using a fixed formula.

Under ITC 69, the formula is GT = K1 × V, where V is the total volume of all enclosed spaces in cubic metres and K1 = 0.2 + 0.02 × log10(V). For a yacht with around 2,500 m³ of enclosed volume, K1 lands at roughly 0.268, giving a GT of about 670. The measurement is performed by an IACS classification society - ABS, Lloyd's Register, Bureau Veritas, DNV or RINA - and approved by the flag state, which then issues the International Tonnage Certificate that accompanies registration.

GT sits alongside three other "tons" that recur on yacht specifications: displacement (the actual mass of water the hull displaces, in tonnes), Deadweight Tonnage (DWT) (carrying capacity in tonnes, largely irrelevant for yachts), and Gross Registered Tonnage (GRT) - the obsolete pre-1969 system expressed in 100-cubic-foot register tons. ITC 69 replaced GRT for newly measured ships from 18 July 1982; older yachts may still cite GRT in legacy paperwork, and it is not directly comparable to GT.

Why it matters for yacht owners

For an HNWI buyer, GT is the headline regulatory parameter after LOA - and arguably more consequential, because it dictates the operating regime your yacht will live under for its entire life. A 49.9m yacht at 499 GT and a 50.5m yacht at 520 GT look almost identical at the dock, but they sit in two different worlds. The second triggers SOLAS Chapter II-2 fire-protection upgrades, a higher-grade officer ticket (OOW 3000 GT minimum), additional safety equipment, MARPOL Annex applicability, and materially higher refit, insurance and crew costs.

This is why builders, designers and brokers obsess over the 500 GT line - and why so many recent 50-60m custom yachts deliberately come in at 499 GT, using exempt unenclosed spaces under ITC 69. The same logic applies again at 3,000 GT (entry to the Passenger Yacht Code) and at 10,000 GT (Master Unlimited captain). Resale also tracks GT: yachts that straddle a threshold - e.g. 510 GT - often trade at a discount to comparable sub-500 GT or comfortably-over-500 GT yachts, because they carry full upper-regime costs without the upper-regime size benefit. On a brokerage spec sheet, GT is the number to check first.

Key facts

• GT is a dimensionless volumetric index, not a weight; weights are quoted as displacement in tonnes.
• Formula under ITC 69: GT = K1 × V, where K1 = 0.2 + 0.02 × log10(V) and V is enclosed volume in m³.
• ITC 69 applies to all ships of 24m and over; below 24m, simplified national rules apply.
• Critical thresholds: 24m / ~100 GT (REG-YC Part A entry, formerly LY3), 500 GT, 3,000 GT (REG-YC Part A ceiling, formerly LY3), 10,000 GT.
• 500 GT is the inflection point: ISM Code, additional SOLAS fire protection, higher officer certification, manning escalation.
• The "499 GT design" is a legal ITC 69 exemption practice (open balconies and unenclosed deck areas) widely documented in industry coverage.
• Tonnage survey is carried out by ABS, Lloyd's Register, Bureau Veritas, DNV or RINA and approved by the flag state.
• Notable superyacht GT figures from public industry directories: Azzam ~13,136 GT, Eclipse ~13,564 GT, Dilbar ~15,917 GT.