How the Thai Marine Department Identifies the Size of a Vessel
SUMMARY - Gross Registered Tonnage, or GRT, is the traditional measure Thailand uses to classify vessel size, based on enclosed volume rather than length, displacement, or weight. This article explains what GRT actually measures, why hull design like catamarans versus keelboats produces very different GRT figures at the same length, who calculates it, and why it matters for skipper license requirements. Island Spirit's brokers can help clarify how GRT applies to a specific yacht during the registration process.
by Ian Paskon
July 2026
What Is GRT and Why Does It Matter for Your Yacht in Thailand?
Thailand uses GRT, or Gross Registered Tonnage, as the traditional measure of a vessel's size, and it's a term that confuses a lot of new owners. GRT is often mistaken for a measure of displacement, length, or weight, but it isn't any of those things. It's actually a measurement of the available volume within the hull and enclosed space above deck, used for cargo, stores, fuel, passengers, and crew. That volume is measured in cubic meters, which is a very different way of thinking about size compared to length overall or weight.
Why Hull Design Changes a Yacht's GRT
Because GRT measures enclosed volume rather than length, two yachts of the same overall length can come out with very different GRT figures depending on their hull design. A regular sailing keelboat of around 45 feet typically comes in at approximately 8 GRT. A catamaran of the same 45 foot length, on the other hand, tends to measure closer to 23 GRT, since the wider beam and additional enclosed volume across two hulls adds up quickly. This is a detail that catches a lot of catamaran owners off guard, especially if they're used to thinking about size purely in terms of length. Understanding this difference matters whenever GRT comes into play for registration, licensing, or other regulatory purposes.
Who Calculates GRT and How
The Thai Marine Department is responsible for calculating the official GRT figure for a vessel, and this is the number that ends up on your registration documents. Plenty of online calculators exist as well, which can give owners a useful estimate ahead of the official process. Knowing your approximate GRT before you go through registration can help you anticipate what licensing or documentation requirements might apply to your vessel. It's worth checking this figure early, rather than being surprised by it later in the registration process.
Why GRT Affects Your Skipper License Requirements
This size classification is what most countries, including Thailand, refer to when discussing skipper license requirements for a vessel. A tonnage of 30 tonnes generally specifies the threshold referenced for a vessel's GRT in these classifications. That means a catamaran owner could find themselves crossing a licensing threshold at a shorter overall length than a sailing keelboat owner would, simply because of how GRT is calculated. Knowing where your yacht falls in this measurement helps you plan ahead for whatever skipper qualifications might be required.
Island Spirit: Helping You Understand the Numbers Behind Your Yacht
At Island Spirit Yacht Brokerage, we help buyers and owners understand the practical details behind a yacht's paperwork, and GRT is one of those details that often gets overlooked until it matters. Whether you're registering a new yacht or considering a purchase, our brokers can help you understand how GRT might affect your registration and licensing requirements. We work closely with the registration process in Thailand, so these kinds of technical details are part of our everyday conversations with clients. Reach out to our team if you'd like clarity on where your yacht stands.
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