IMO numbers: why seven digits are the spine of every sanctions list

The one identifier that survives everything else

Ships change identity constantly and legally. A tanker sold in the morning can have a new name, a new flag, a new manager and a new radio call sign by the end of the week. Everything a sanctions list prints about a vessel is unstable except one field: the IMO ship identification number.

It is assigned once, when the hull is built, and stays with the hull until it is broken up. It is not reassigned to another ship afterwards. That single property is what makes it possible to say that the vessel the European Union listed under one name in 2024 and the vessel OFAC designated under a different name in 2025 are the same steel.

How the number is built

The number is seven digits. The first six are the identifier; the seventh is a check digit computed from them.

Take IMO 9187629. Multiply the first six digits by descending weights:

9 x 7 = 63
1 x 6 =  6
8 x 5 = 40
7 x 4 = 28
6 x 3 = 18
2 x 2 =  4
        --
        159  -> last digit 9 -> matches the seventh digit

Any single-digit typo, and most transpositions, break that equality. This site validates the check digit on every record it ingests. A number that fails is not silently dropped and it is not silently corrected: it is recorded as a warning against the source, and the record is held aside rather than attached to a hull it might not belong to.

Where each authority hides it

The three main lists print the number in three different places and formats.

OFAC puts it in the identification section as a document of type “Vessel Registration Identification”, with the value written as IMO 9187629. In the plain-text entries published in Recent Actions notices it appears mid-sentence, after the flag and before the program tag.

The UK prints it inside the ship details block, usually as IMO9826902 with no space, and occasionally as the bare seven digits. A small number of UK entries name the vessel itself as IMO 9058713 ("AVAX"), where the real name is the quoted string.

The EU gives it a column of its own in Annex XLII, as seven bare digits.

The formats matter because the gap between them is where mistakes live. A parser that only accepts IMO followed by a space will silently lose hundreds of UK entries.

The company-number trap

The IMO scheme also issues seven-digit numbers to registered owners and ISM managers. A designation entry for a shipping company can therefore carry a line reading “Identification Number IMO 6324125”, which looks exactly like a hull number and is not one.

Any process that scrapes designation text for seven digits following the letters IMO will invent ships that do not exist. The safeguard used here is context: a vessel entry is one that also marks itself as a vessel, and a number found in a company entry is stored as a company identifier or ignored.

Why this site refuses to merge without it

Names are not unique: several hulls in this database share a name with an unrelated ship. Flags change. Tonnages are printed inconsistently, sometimes as deadweight and sometimes as gross tonnage, occasionally unlabelled. Call signs follow the flag.

So the rule here is absolute: the IMO number is the only join key. Two records become one vessel only if they carry the same valid number. Two numbers are never merged, however similar the names.

A small number of list entries carry no IMO number at all. Rather than guess, they are held as candidates with a confidence score built from name, flag, tonnage and build year. A candidate is shown on a vessel page only when the evidence is strong, and it is labelled as unmerged when it appears. The methodology explains the scoring.

What the number does not tell you

An IMO number identifies a hull. It does not tell you who owns it, whether the flag it claims is genuine, or whether the vessel transmitting that number on AIS is the ship it says it is. A vessel with a fraudulent registration may broadcast a number belonging to another ship entirely. Verification means comparing the number against ship particulars — dimensions, build year, tonnage — through a registered source such as the IMO’s own database or Equasis, one vessel at a time.

For sanctions work, though, the number remains the anchor. Every list uses it, every list means the same thing by it, and it is the reason a permanent page per hull is possible at all.