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What is tonnage measurement?

Tonnage measurement is the work of identifying and measuring a boat.   The method is defined by law.  Tonnage measurement is required for part 1 registration on the UK flag, red ensign flags, and many others.  

Baseline Yachts are authorised tonnage measurers for vessels under 24m load line length operating on the UK, red ensign, and other flags. 

Here we provide simplified definitions of measurements taken for small vessels (under 24m load line length). 


We hope it helps you understand the numbers that appear on your UK part 1 registration.  It is not intended as survey guidance, or to depict any specific vessel.   

Profile view of motor yacht and sailing yacht

Length overall

Hull length from bow to stern, excluding (in most cases) deck fittings, swim platforms, bowsprits.

Plan view of motor yacht


Hull breadth at widest point, excluding rubrail or bumpers.

Profile view of motor yacht and sailing yacht


The depth of the hull structure, from keel to deck underside (or, for an open boat, top of gunwale).  
Not – the draft of the vessel underwater.  

3 dimensional view of motor yacht


An approximation of hull volume - not displacement.  

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