IMO number vs MMSI number: how to identify a vessel

An IMO number is a permanent seven-digit number given to a ship’s hull, which stays the same for the ship’s whole life whatever its name, owner or flag. An MMSI number is a nine-digit radio identity that the ship’s flag state assigns, which the ship’s transponder broadcasts, and which changes when the ship changes flag. For checking which vessel is carrying your container, the IMO number is the one to trust.

That difference matters the moment you try to confirm a ship yourself. A reefer of grapes is six weeks from port, the booking names a vessel, and a search for that name returns three ships. This guide covers what each identifier is, where to find it, and how to use them together.

The four ways a ship is named #

What is an IMO number? #

The IMO number comes from the IMO Ship Identification Number Scheme, which the International Maritime Organization made mandatory through the SOLAS convention, regulation XI-1/3. It applies to cargo ships of 300 gross tonnage and upwards and passenger ships of 100 gross tonnage and upwards, so every container ship carrying your fruit has one.

The number is assigned once, free of charge, by S&P Global (formerly IHS Maritime and, before that, Lloyd’s Register-Fairplay). It stays with the ship through every sale, change of name and change of flag, and it is never given to another ship. It has to be marked permanently on the hull or superstructure, and it is printed on the ship’s certificates.

The last digit is a check digit. Multiply the first six digits by 7, 6, 5, 4, 3 and 2 in turn and add the results; the last digit of the total is the check digit. For IMO 9074729: 9×7 + 0×6 + 7×5 + 4×4 + 7×3 + 2×2 = 139, and the check digit is 9. A mistyped IMO number usually fails this test instead of quietly finding a different ship.

There is a separate IMO number for companies and registered owners. It also has seven digits, but it identifies the owner or manager, not the ship. Do not mix the two up when you copy a number from a registry page.

What is an MMSI number? #

MMSI stands for Maritime Mobile Service Identity. It is the number a ship’s radio equipment uses to identify itself, set out in ITU-R Recommendation M.585, and the number AIS transponders put in every message they send. That is why most tracking sites use it as their main key.

A ship’s MMSI has nine digits. The first three are the Maritime Identification Digits (MID), which the International Telecommunication Union allocates to each country, so they tell you the flag. 477 is Hong Kong, 636 and 637 are Liberia, 538 is the Marshall Islands, and Panama and Singapore each hold several blocks. The remaining six digits identify the ship within that country.

The flag state’s own authority assigns the MMSI, and the ITU publishes the registered ship stations in its free MARS database. When a ship changes flag it must take an MMSI from the new country. The old number can then be given to another vessel. So a history built on an MMSI is not guaranteed to be about one ship.

Call sign and vessel name #

The call sign is the ship’s radio call sign, assigned by the flag state from the letter blocks the ITU gives each country. It appears on documents and in AIS data and, like the MMSI, it changes when the ship changes flag.

The vessel name is the least reliable of the four. Owners change names on sale or charter, and popular names are reused, so several ships can carry the same name at the same time. A name search is how people end up confidently tracking the wrong ship.

How AIS uses them #

Every AIS message carries the MMSI. The IMO number, call sign and name travel in a separate static message that a ship sends every six minutes, together with its destination and ETA.

Those static fields are entered by the crew, not measured. The standard even has a value for “IMO number not available”. An MMSI with no IMO number, or with a name that does not match the IMO registry, is a sign that the static data is wrong or out of date. It is not a different ship.

Where to find the IMO and MMSI numbers #

On your documents. The bill of lading and the arrival notice normally name the vessel and the voyage number. Some also print the IMO number; few print the MMSI.

From the carrier. Carriers’ schedule and vessel pages usually list the IMO number next to the name.

From the registries. The IMO’s GISIS database has a Ship and Company Particulars module you can search by IMO number with a free public account. The ITU’s MARS database lists ship stations with their MMSI and call sign.

From public AIS sites. MarineTraffic, VesselFinder and similar sites show the IMO number, MMSI, call sign and flag together once you have found the ship. Start from the IMO number if you have it.

Check your vessel in thirty seconds #

Take the vessel name from your paperwork and search it on a public AIS site. If more than one ship comes back, open each candidate and compare the IMO number with your documents. It matches or it does not, and no amount of matching names, flags or likely-looking routes replaces that one check.

Then look at the flag. If it disagrees with your paperwork, either you have the wrong ship, or the right ship has changed flag. In the second case the MMSI and call sign on your documents are out of date and the IMO number is still correct.

Why the vessel is not what you should track #

The vessel on the booking is not always the vessel that sails. Ships are substituted, and containers are rolled to a later sailing. After a transshipment, your container is on a different ship altogether. The first vessel finishes its rotation on time while your reefer waits on a quay behind it.

So the IMO number is for confirming which ship a carrier’s milestone refers to. The thing to follow is the container or the bill of lading, because those stay with your cargo when it changes ships. Trackberry tracks the container and names the vessel it is on at each stage, so a change of ship after a transshipment shows up as a change on the shipment rather than as a map that stops making sense.

FAQ #

What is the difference between an IMO number and an MMSI? #

The IMO number is a permanent seven-digit number tied to the hull for the ship’s whole life. The MMSI is a nine-digit radio identity assigned by the flag state, which AIS broadcasts, and which changes when the ship changes flag. Use the IMO number to identify a ship, and expect the MMSI to be what tracking sites search by.

How do I find a ship’s MMSI number? #

Search the ship’s name or IMO number on a public AIS site, which shows the MMSI alongside it. The ITU’s MARS database also lists registered ship stations with their MMSI and call sign. The first three digits tell you the flag the MMSI was issued under.

Does the IMO number change when a ship is sold or renamed? #

No. It stays the same through every sale, renaming and change of flag, and it is never reassigned to another ship.

Can two ships have the same MMSI? #

Not at the same time if both are registered correctly, but an MMSI can be reassigned after a ship changes flag. Old data under an MMSI can therefore belong to a different vessel. Errors in how a transponder is set up also produce duplicates, which is another reason to confirm with the IMO number.

Is the IMO number the same as the container number? #

No. The IMO number identifies the ship. A container number, four letters and seven digits, identifies one box, which travels on many ships over its life. To follow your cargo, track the container or the bill of lading.

Sources #

• International Maritime Organization, IMO identification number schemes (SOLAS regulation XI-1/3, tonnage thresholds, marking, company numbers)
• IMO, GISIS: Ship and Company Particulars
• ITU, Table of Maritime Identification Digits
• ITU, Maritime mobile Access and Retrieval System (MARS)
• US Coast Guard Navigation Center, AIS static and voyage related data (message 5) (fields and six-minute interval)