IMO number

An IMO number is a unique seven-digit identifier assigned to a ship when it is built, which remains unchanged through every change of name, flag, or owner until the vessel is scrapped. Introduced through SOLAS to improve maritime safety and combat fraud, it is mandatory for most seagoing merchant ships of 100 GT and above, and must be permanently marked on the hull and internal structures.

The scheme is administered on behalf of the IMO by S&P Global (formerly IHS Markit / Lloyd’s Register–Fairplay), and the final digit is a check digit validating the preceding six.

Because names and flags change easily — sometimes precisely to obscure a ship’s history — the IMO number is the anchor of vessel identity in casualty records, port state inspections, sanctions enforcement, and commercial databases. IMO numbering schemes also exist for shipowning and management companies.