Explainer: What is superyacht gross tonnage (GT)?

Superyacht gross tonnage (GT) is the unit of measurement for a yacht's internal volume. It takes into account internal volume across all decks – including cabins, dining rooms, saloons and engine rooms – but not the exterior spaces such as the decks or the hull itself.

So, why is superyacht gross tonnage important? And how is it used? BOAT explains...

How is gross tonnage calculated?

Gross tonnage is determined using a formula based on the yacht’s dimensions (length, breadth, and depth) and the volume of the enclosed spaces. It’s a cubic measurement, meaning it’s about the three-dimensional space the yacht occupies.

The unit of measurement was defined by the International Convention on Tonnage Measurement of Ships in 1969 and adopted by the International Maritime Organization (IMO) in 1969.

Why does gross tonnage matter?

Gross tonnage directly influences the extent to which a superyacht is subject to safety regulations, docking fees and survey and crew requirements. This is why you'll see a lot of shipyards building yachts in the sub-500GT category, as this is the most flexible (and least demanding) bracket for yacht ownership.

Yachts over 500GT are:

• Subject to more stringent safety and environmental rules under IMO. This includes standards for firefighting systems, pollution prevention, escape routes and lifeboats.
• Required to undergo more frequent surveys by classification societies (including Bureau Veritas or Lloyds Register), which inspect the yacht for compliance with structural and safety regulations. These surveys are often time-consuming and costly if the conclusion is that design and/or operational changes need to be made.
• Subject to more rigid crew size and certification requirements. Yachts with a higher gross tonnage will need more specialised crew (including engineers and deckhands) to operate legally. This naturally comes at a higher cost when factoring in salaries, accommodation and training.
• Subject to higher insurance premiums as a result of increased crew requirements, more complex operations and other potential risk factors associated with larger vessels.

What is the largest superyacht by gross tonnage?

The largest superyacht by gross tonnage is the 156-metre Dilbar, which was delivered by German shipyard Lürssen in 2016. She has a gross tonnage of 15,917GT and is the fifth-largest superyacht in the world. Designed by Espen Øino, Dilbar needs a crew of 80 to run smoothly but can accommodate a maximum of 96 crew members, meaning she can carry as many as 120 people at full capacity. 

Dilbar remains a highly secretive build but is known to have two helipads, a 25-metre-long swimming pool and her own garden, developed with a special variety of grass that tolerates salt air.