What is an IMO number?
The IMO ship identification number is a permanent 7-digit identifier assigned by the International Maritime Organization under SOLAS regulation XI-1/3. It is engraved on the ship and stays with the hull for its lifetime, regardless of changes in owner, flag or vessel name. Customs authorities, port state control, insurers and classification societies all rely on it as the single unambiguous vessel reference.
How the check-digit algorithm works
Multiply each of the first six digits by weights 7, 6, 5, 4, 3 and 2 (left to right). Sum the six products. Take the last digit of that sum — this must equal the seventh digit of the IMO number. If it does, the number is mathematically well-formed.
Worked example
Take IMO 9074729. Compute (9×7)+(0×6)+(7×5)+(4×4)+(7×3)+(2×2) = 139. The last digit of 139 is 9, which matches the seventh digit — so the number passes the check.
IMO vs MMSI vs Call Sign
The IMO number identifies the hull permanently. The MMSI is a 9-digit radio identifier that changes with the flag state. The Call Sign is a radio-station identifier issued by the flag. Only the IMO number survives re-flagging, sale or renaming.
When validation fails
A failed check most often means a typo — a transposed or dropped digit during manual data entry. It can also flag a fabricated number in a bill of lading or vessel listing. A valid result proves format only; always cross-reference against an official registry to confirm the vessel actually exists.
Frequently asked questions
What is an IMO number?
An IMO number is a unique 7-digit identifier assigned by the International Maritime Organization to ships of 100 gross tons and above. It stays with the vessel for its entire life — regardless of changes in owner, flag or name.
How does the IMO check-digit formula work?
Multiply each of the first six digits by weights 7, 6, 5, 4, 3, 2 (left to right). Sum the products. The last digit of that sum must equal the seventh digit of the IMO number.
Can you show a worked 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 of 139 is 9, which matches the check digit — so the number is valid.
How is an IMO number different from an MMSI or Call Sign?
The IMO number is permanent and identifies the hull. The MMSI is a 9-digit radio identifier tied to the vessel's current flag and can change on re-flag. The Call Sign is a radio-station identifier issued by the flag state.
What does an INVALID result usually mean?
The most common cause is a typo — a transposed or missing digit. It can also mean the number was fabricated (some fraudulent listings use random 7-digit numbers that fail the check).
Does a VALID result prove the vessel exists?
No. A valid check digit only proves the number is mathematically well-formed. To confirm the vessel exists and its current details, cross-reference the IMO number against an official registry such as Equasis or the flag state's registry.
Related guide
Read IMO Number Explained: Format, Check Digit and Common Uses for the full history, format rules, and how IMO differs from MMSI and call sign.