IMO Vessel Identification: A Guide for Shipping Operators

Published: 1 September 2025 · Last updated: 31 October 2026

The IMO number is the single most important vessel identifier in international shipping. It appears on clearance certificates, port state control records, sanctions databases, and every other document that follows a vessel through its working life. For operators applying for a Hormuz Transit Clearance Certificate, submitting the correct IMO number is essential — an incorrect or missing number delays processing and may require resubmission of the entire application.

What is the IMO number?

The IMO number is a permanent, unique seven-digit identifier assigned to a ship by the International Maritime Organization. It was introduced under IMO Resolution A.600(15) and became mandatory for most seagoing vessels of 100 GT and above under SOLAS amendments adopted in 1994.

The format is the prefix “IMO” followed by seven digits — for example, IMO 9999999. The number is allocated when the vessel is built and remains with the vessel for its entire operational life, regardless of change of flag, owner, operator, or name. Even if a vessel is renamed five times and changes flag three times, its IMO number does not change. It is the maritime equivalent of a national identity number that cannot be changed.

The check digit system

The seventh digit of the IMO number is a check digit, calculated from the first six digits. To verify a number, multiply each of the first six digits by 7, 6, 5, 4, 3, and 2 respectively, sum the products, and take the last digit of the result. This should equal the seventh digit.

For example, for IMO 9074729: (9×7) + (0×6) + (7×5) + (4×4) + (7×3) + (2×2) = 63 + 0 + 35 + 16 + 21 + 4 = 139. The last digit is 9, which matches the seventh digit. The HTCA portal validates the check digit automatically — if it fails, the application cannot proceed until the correct number is entered.

Where the IMO number is displayed on the vessel

SOLAS requires the IMO number to be permanently and prominently marked on the vessel. It must appear:

• On the vessel's hull, in a location visible from the water (typically on each side of the hull and, for larger vessels, on the stern)
• On the vessel's superstructure (typically on the front face of the bridge or superstructure front)
• In a location readable from the air on the horizontal surface of the vessel if practicable

The markings must be in permanent, contrasting lettering of at least 200mm in height. This is designed to allow identification of vessels by coastguard aircraft, satellite imagery, and port state control inspectors even when a vessel's name has been changed.

Where to find the IMO number in documents

The IMO number appears in all of the following documents and records:

• Certificate of Registry: The vessel's official registration document issued by the flag state. The IMO number is listed on the front page.
• Class certificate: The vessel's certificate of class issued by its classification society (e.g. Lloyd's Register, DNV, Bureau Veritas, ClassNK, ABS). The IMO number is on all class certificates.
• SOLAS safety certificates: The Safety Construction Certificate, Safety Equipment Certificate, and Safety Radio Certificate all carry the IMO number.
• MARPOL certificates: The International Oil Pollution Prevention (IOPP) certificate and International Energy Efficiency (IEE) certificate both carry the IMO number.
• Tonnage certificate: The International Tonnage Certificate (1969) carries the IMO number.
• Bills of lading and cargo documentation: Modern bills of lading and sea waybills typically include the IMO number of the carrying vessel.

IMO number vs. MMSI number

Operators sometimes confuse the IMO number with the MMSI number. They are different identifiers used for different purposes.

The MMSI (Maritime Mobile Service Identity) is a nine-digit number used for radio communications and AIS (Automatic Identification System) transmissions. It is assigned by the vessel's flag state telecommunications authority and is associated with the vessel's radio station licence. Unlike the IMO number, the MMSI can change — for example, when a vessel changes flag state it will typically be assigned a new MMSI.

The MMSI is the number visible on AIS tracking systems (such as MarineTraffic, VesselFinder, and AIS Hub). The IMO number is also typically included in AIS transmissions in the vessel's static data, but the primary radio identifier is the MMSI.

For HTCA transit clearance purposes, the IMO number is required. The MMSI is not a substitute. Submitting the MMSI in the IMO number field will cause the application to fail validation.

IMO number vs. official number

Some vessels, particularly older ones or those registered in certain jurisdictions, have an official number (also called the national registration number or port registry number). This is a different identifier assigned by the flag state registry. It may appear on the certificate of registry alongside the IMO number. Do not confuse the two. For the HTCA application, always use the seven-digit IMO number prefixed with “IMO”.

Vessels not required to carry an IMO number

Not all vessels are required to carry an IMO number. Exemptions under SOLAS include:

• Vessels without mechanical means of propulsion
• Pleasure yachts not engaged in trade
• Fishing vessels
• Vessels of less than 100 GT
• Ships not engaged on international voyages
• Warships and naval auxiliaries

If your vessel is not required to carry an IMO number, the HTCA application form includes a field for vessels without IMO numbers. You will be required to provide alternative identification (flag state registration number and certificate of registry) and may face additional processing requirements.

How to look up a vessel's IMO number

The authoritative source for IMO numbers is the IHS Markit (now LSEG) Sea-web database, which the IMO uses as its ship register. The IMO also maintains a publicly accessible vessel search via the Global Integrated Shipping Information System (GISIS). Additionally, AIS tracking platforms (MarineTraffic, VesselFinder) display IMO numbers in vessel profiles.

When in doubt, cross-reference the number against the vessel's class certificate and certificate of registry. These documents are the primary authority for the number.

Why accuracy matters for transit clearance

The HTCA uses the IMO number to cross-reference vessel details, validate the DWT declared for fee calculation purposes, and screen against applicable vessel-level sanctions lists. An incorrect IMO number may cause a mismatch with registry data, result in a different vessel's details being applied to the application, or trigger a false positive on a sanctions check. All of these scenarios delay processing. In cases where the mismatch is significant, the HTCA may be required to cancel the application and request resubmission.

Operators should verify the IMO number from the vessel's current certificate of registry before submitting the clearance application.