IMO Number

The IMO ship identification number is a permanent, unique fingerprint for a ship. Consisting of the letters IMO followed by seven digits, it is assigned to a vessel when it is built and stays with that hull for the whole of its life, regardless of any changes of name, flag, owner or operator. This permanence is precisely its value: a ship may be renamed and reflagged many times over its career, which can make it hard to track by name alone, but its IMO number allows authorities, insurers, class societies and databases to identify the same physical ship without ambiguity.

The scheme was introduced to enhance maritime safety and security and to help prevent fraud and pollution by making ships reliably identifiable. The number is calculated from a formula with a check digit that guards against transcription errors, and it must be permanently marked on the ship, both in a visible external position and in an accessible internal location, so that it cannot easily be removed or falsified. A parallel scheme assigns IMO numbers to registered shipping companies and managers, helping to link ships to those responsible for them.

The IMO number appears throughout a ship's documentation and on its certificates, and it is the key by which vessels are looked up in registries, classification records, Port State Control databases, casualty records and ship-tracking services. It works together with the other identifiers a ship carries: the MMSI, which identifies the vessel in digital radio and AIS transmissions and can change with flag, and the radio call sign. Whereas those identifiers may change, the IMO number is the constant, which is why investigators, regulators and commercial parties rely on it as the definitive reference for a particular ship, and why it underpins so much of the transparency and accountability in modern shipping.