What is an IMO number?

Every merchant ship of any size carries half a dozen identifiers — a name, a call sign, an MMSI, a national registration, a class number — and almost all of them can change. A vessel can be sold, renamed, reflagged and re-registered several times in a working life of thirty years. The IMO number is the one identifier designed to outlast all of that.

Introduced by the International Maritime Organization under the IMO Ship Identification Number Scheme (adopted in 1987 by resolution A.600(15) and later made mandatory under SOLAS chapter XI-1), the number is assigned to a ship's hull when it is built and stays with that hull until it is scrapped. Sell the ship, rename it, move it to a different flag — the IMO number does not move.

What the number looks like

An IMO number is the three letters IMO followed by a seven-digit number — for example, IMO 9811000. The numbers are issued in sequence by IHS Markit (now part of S&P Global) on the IMO's behalf, so a higher number generally means a more recently registered hull. There is no information about size, type or flag encoded in the digits themselves; the number is purely an index into the global record.

The same scheme also issues a separate IMO company number to registered owners and managers, so that the organisations behind a fleet can be tracked even when individual ships change hands. On Marifest, an owner's IMO company number and its GLEIF Legal Entity Identifier are both used to resolve who controls a vessel.

The check digit: how a typo is caught

The seventh and final digit is a check digit, calculated from the first six. The rule is simple: multiply the first six digits by 7, 6, 5, 4, 3 and 2 in turn, add the products, and take the last digit of the total. That last digit must equal the seventh digit of the IMO number.

Digit	Value	Weight	Product
1st	9	×7	63
2nd	0	×6	0
3rd	7	×5	35
4th	4	×4	16
5th	0	×3	0
6th	3	×2	6
Sum	—	—	120

The last digit of 120 is 0, so a valid number beginning 907403 ends in 0: IMO 9074030. This is why a search engine, a sanctions database or a registry can flag an invalid IMO number the instant it is entered — the checksum will not balance. It is a small piece of arithmetic that quietly prevents a great deal of mistaken identity.

Why it matters more than the name

Names are marketing; IMO numbers are facts. A vessel that has run up port-state detentions or appeared on a sanctions list has every incentive to take a fresh name and a new flag and trade on as if it were a different ship. Screening on the name alone would miss it. Screening on the IMO number does not — because the hull, and its history, follow that number.

A name can be repainted overnight. A flag can change in an afternoon. The IMO number is the one identifier that a vessel cannot shed without ceasing to exist. IMO Ship Identification Number Scheme, resolution A.1117(30)

This is exactly why every vessel file in the Marifest registry is keyed to the IMO number, and why the compliance screening matches that number — not the name — against the OFAC, EU, UN and UK lists. When a ship reappears under a new name, the IMO number ties the new entry straight back to the old record.

Which ships have one

The scheme is mandatory for most self-propelled seagoing merchant ships of 100 gross tons and above, including passenger ships, cargo ships, tankers and high-speed craft. It does not apply to vessels solely engaged in fishing in some jurisdictions, ships without mechanical propulsion, pleasure yachts, certain wooden ships, or naval and government vessels on non-commercial service — though many of these still appear in registries under other identifiers.

If you want to see how a given ship's identity has held steady while everything around it changed, the quickest route is to look it up. On Marifest you can search any of 97,000+ vessels by name or IMO number and read the full file — specs, ownership chain, flag and builder history, and the sanctions verdict — all anchored to that one fixed number.