Tonnage Measurement

If you’re considering registering your boat in a new country, you may need to think about a tonnage measurement. A Tonnage Certificate is a requirement for registration under most flag states, including the Red Ensign group (UK, Jersey, Gibraltar, Malta, Cayman Islands etc), Ireland, USA and many others. Tonnage determines whether your boat meets the size criteria for registration in another flag state. It also impacts registration fees and other requirements. This guide will explain tonnage in simple terms and walk you through the measurement and registration process step-by-step.

What is Tonnage Measurement?

Tonnage refers to your boat’s carrying capacity as determined by its enclosed volume. It serves as an indicator of the boat’s size. Tonnage is measured using a mathematical formula that accounts for the length, width, depth and enclosure of the spaces on board. Don’t worry – you don’t need advanced math skills. Maritime authorities handle the calculations during an official tonnage measurement conducted by a certified surveyor.

Why Tonnage Measurement Matters for Boat Registration

Countries set minimum and maximum tonnage limits for vessels that can be registered under their flags. Measurement ensures your boat falls within these size criteria. Tonnage also determines your boat’s registration fees – the higher the tonnage, the more you’ll pay annually to maintain registration. Registration may also depend on tonnage-based occupational limits and safety requirements. Having the right measurement is key to registering your boat under your preferred flag.

Types of Tonnage Relevant to Boat Owners

The most common types of tonnage are:

  • Gross Tonnage (GT) – total internal volume
  • Net Tonnage (NT) – GT minus some deductions
  • Displacement Tonnage – weight of water a boat displaces at the load waterline
    Different countries may use GT, NT or Displacement for registration, so you need to know your boat’s figures for each.

Tonnage Certificates: What They Are and Why You Need One

A Tonnage Certificate issued by the maritime authority provides your boat’s official tonnage figures. Also called a Tonnage Certificate of Documentation (COD), it’s required as proof of measurement for your registration. Registration application forms will ask for your official tonnage, so be sure to obtain your certificate before making a registration application.

Tonnage Certificate, Carving and Marking Note, Certificate of Documentation…

Each of the above terms, different names are used by different countries, may be used to denote the certificate issued by the flag state which sets out the certified tonnage of your vessel.

Which Flag States Require a Tonnage Certificate?

Most major flag states require a valid Tonnage Certificate as proof of measurement before completing boat registration. Some of the key ones that mandate a Tonnage Certificate include:

  • United States – US Coast Guard registration requires a Certificate of Documentation (COD) issued by the Coast Guard or a recognised classification society.
  • United Kingdom – UK Maritime and Coastguard Agency will need a newly issued Tonnage Certificate from an approved body before granting registry.
  • Germany – Official measurement by a classification society like Germanischer Lloyds is compulsory to register a vessel under the German flag.
  • Spain – Spanish Maritime Administration provides tonnage certification which must be presented to complete registration in Spain.
  • Portugal – A valid tonnage document endorsed by a recognised organisation is a prerequisite for Portuguese boat registration.
  • Italy – Italian registry demands a Tonnage Certificate issued by an authorised installer verifying the measurements.
  • Greece – For Greek registry, the Hellenic Register of Shipping issues tonnage certification after surveying the boat.

Choosing the Right Country for Boat Registration

Consider registration costs, taxation policies, crewing regulations, maritime laws, proximity to your preferred cruising grounds and tonnage criteria that match your boat. Some countries charge annual taxes on tonnage or have crew qualifications linked to tonnage. Research options to find the most suitable flag for you. You may find that your choice is limited according to your home residence, for example only US residents (including companies) can apply for an American flag registration.

Read more about Boat Registrations here.

Who Can Measure Tonnage?

Tonnage measurements must be conducted by certified marine surveyors or other officials appointed by the flag state in order to be accepted for registration purposes. The surveyors authorised to perform tonnage measurements vary depending on the country or classification society. Some common options include:

  • Classification Societies: Organisations like Lloyds Register, Det Norske Veritas, Royal Institute of Naval Architects that set technical standards and certify vessels, particularly for new designs. Most major nations recognise their tonnage figures.
  • Government Surveyors: Some maritime authorities employ their own surveyors who can measure domestic vessels. For example, US Coast Guard measures all US-flagged boats.
  • Approved Surveyors: Where the flag state does not have its own surveyors, it will appoint surveyors who have had the appropriate training to carry out measurements on their behalf, and who apply to them for permission to take measurements. In the UK, and for the Red Ensign group of flags, the International Institute of Marine Surveyors (IIMS) has been approved to manage registration applications on behalf of the MCA and other Red Ensign group countries.

Regardless of who does the measuring, be sure to obtain documentation that is recognised by your target flag state. Tonnage measurement by uncertified assessors may not be accepted for registration or have reduced validity periods. Always confirm credentials and eligibility before hiring a surveyor for an official tonnage assessment for your preferred flag.

How Tonnage Impacts Registration Fees

Fees are usually bracketed based on tonnage ranges. As an example, boats under 10GT may pay $500 annually while 10-25GT pay $750 and over 25GT pay $1000. Larger boats above a certain tonnage may face higher occupational or safety certification costs too. Understanding fee structures upfront allows budgeting registration expenses.

Steps to Get Your Boat’s Tonnage Measured

  1. Contact the country of your choice to determine the process for new boat registration applications. Check whether your vessel has already been measured and has a carving mark as this may be acceptable to your chosen flag state.
  2. Contact their approved classification society surveyor or maritime authority and book a tonnage measurement appointment.
  3. Prepare your boat by clearing all enclosed spaces.
  4. On the scheduled date, a certified surveyor will board your boat and take precise measurements.
  5. The authority will process the data and issue your Tonnage Certificate within 4-6 weeks.
  6. Keep the certificate in a safe place for future registrations or transactions requiring proof of tonnage.

FAQs on Tonnage Measurement for Boat Registration

Q. How long is a tonnage certificate valid?
A. The US COD is usually valid for 5 years but may need to be renewed if major modifications change the boat’s volume. Other certificates have no official expiry date.

Q. What if my boat is a home-built or unusual design?
A. Contact authorities early in the build process for guidance on measurement standards. Additional surveys may be needed for non-standard vessels.

Q. How do I prepare my boat for measurement?
A. Clear all lockers and spaces to allow full external and internal access. Have construction diagrams and access to bilges/double bottoms ready.


Understanding your boat’s tonnage is critical to navigating international registration requirements smoothly. With the proper paperwork and preparation, the measurement and registration process doesn’t need to be overwhelming. Follow the guidance here and you’ll be well on your way to sailing internationally under a new flag. Bon Voyage!

Contact me for a Red Ensign Group tonnage measurement in St Martin/Sint Maarten