What is an MMSI number?
· 4 min read
In short
An MMSI (Maritime Mobile Service Identity) is a nine-digit number that identifies a radio station at sea: a ship’s VHF radio with Digital Selective Calling (DSC), its AIS transponder, a coast station, a buoy or a rescue beacon. For a ship, the first three digits are the Maritime Identification Digits (MID), which name the country that issued the number. The MMSI identifies the radio licence, not the hull, so it usually changes when a ship changes flag; the ship’s permanent identifier is its IMO number.
What an MMSI is for
The MMSI is the address of a ship’s radio. Every AIS position report a ship sends carries its MMSI, which is how a receiver knows which ship a position belongs to. A DSC radio uses the same number to call one ship directly, and a DSC distress alert sends the MMSI with the ship’s position so rescuers know who is in trouble.
The numbering is set by the International Telecommunication Union in Recommendation ITU-R M.585. Each country’s radio administration issues MMSIs from its own block and reports ship stations to the ITU’s Maritime mobile Access and Retrieval System (MARS). In the United States the FCC assigns MMSIs with a ship station licence.
How to read the nine digits
The leading digits tell you what kind of station it is. M, I and D stand for the three Maritime Identification Digits; X is any digit.
| Format | Station |
|---|---|
| MIDXXXXXX | Ship station: the usual format, MID first |
| 0MIDXXXXX | Group of ships (one call reaches several) |
| 00MIDXXXX | Coast station, port station, pilot station, AIS base station |
| 111MIDXXX | Search and rescue aircraft |
| 8MIDXXXXX | Handheld VHF radio with DSC and GNSS |
| 98MIDXXXX | Craft associated with a parent ship, such as a tender |
| 99MIDXXXX | AIS aid to navigation: a buoy, light or virtual mark |
| 970XXYYYY | AIS-SART (search and rescue transmitter) |
| 972XXYYYY | Man-overboard (MOB) device |
| 974XXYYYY | EPIRB with AIS |
The MID is where the flag comes from. The ITU allocates MIDs to countries in Appendix 43 of the Radio Regulations: 353 and 636, for example, are allocated to Panama and Liberia. A country with a large fleet can hold several MIDs; the United States uses 338 and 366 to 369.
An MMSI has no check digit. Any nine digits look valid, so a single wrong key produces a different, plausible number. Receivers regularly pick up transmitters programmed with a placeholder MMSI or with a number that belongs to another ship.
Why the MMSI changes and the IMO number does not
An MMSI belongs to the radio licence, and the licence is issued by the ship’s flag state. When a ship moves to another flag, the new administration issues a new MMSI from its own MID block, and usually a new call sign too. The IMO number stays with the hull for its whole life.
That is why Voydar keys every vessel page on the IMO number and keeps each MMSI a ship has used as part of its identity history. The table below is live: every MMSI Voydar has received together with one ship’s IMO number.
One IMO number, 2 MMSIs
Live from VoydarWhat Voydar has received over AIS from EVER GIVEN (IMO 9811000). Each row is a radio identity the ship transmitted under; the country is decoded from the MMSI’s Maritime Identification Digits.
| MMSI | MID country | Call sign | First heard | Last heard |
|---|---|---|---|---|
| 636026627 | Liberia | A8AB9 | Sep 15, 2026, 11:36 UTC | Sep 29, 2026, 05:35 UTC |
| 353136000 | Panama | H3RC | Jun 19, 2026, 18:49 UTC | Sep 15, 2026, 09:45 UTC |
Voydar AIS reception. First and last heard are when Voydar’s feeds first and last received that MMSI with this IMO number, not official registration dates.
How a boat gets an MMSI
The rules are national. Ships on international voyages get their MMSI with a ship station licence from the flag state’s radio administration. In the United States, recreational boats that stay in U.S. waters can get an MMSI free or at low cost from organisations the FCC authorises (the U.S. Coast Guard lists BoatUS, the U.S. Power Squadrons and Shine Micro); a boat heading to a foreign country needs an FCC ship station licence instead.
The MMSI is then programmed into the DSC radio and the AIS transponder. On most equipment it can be entered only once or by a dealer, which is deliberate: a wrong MMSI in a distress alert points rescuers at the wrong boat.
Look up an MMSI
Type an MMSI into the box on this page to see which country issued it and what kind of station it is, then search Voydar for the ship. Every Voydar vessel page lists the MMSI it last transmitted and any earlier ones, next to its IMO number.
Questions and answers
- Is an MMSI the same as an IMO number?
- No. The IMO number identifies the ship itself and never changes. The MMSI identifies the ship’s radio licence and usually changes when the ship changes flag. Big ships have both; small boats often have only an MMSI.
- Can I find a ship by its MMSI?
- Yes. Search the MMSI on Voydar. If the ship has transmitted on AIS within Voydar’s coverage, its page shows its latest position and the IMO number it reported.
- What does MMSI stand for?
- Maritime Mobile Service Identity. The maritime mobile service is the radio service between ships, and between ships and coast stations.
- Can two ships have the same MMSI?
- They should not, but it happens: an MMSI has no check digit, and misprogrammed or copied numbers are regularly received. When two ships share an MMSI, the IMO number and the call sign in their AIS static data tell them apart.
Look up a ship
Search any vessel by name, IMO number or MMSI for its free Voydar profile: live position, identity history, ownership, inspections and sanctions.
Sources
Primary sources for the facts in this guide. Figures labelled as Voydar data come from Voydar’s own records, with their method stated next to them.
- ITU-R M.585: Assignment and use of identities in the maritime mobile service
- ITU: Maritime Identification Digits (Appendix 43 to the Radio Regulations)
- U.S. Coast Guard Navigation Center: MMSI format summary
- U.S. Coast Guard Navigation Center: MMSIs for recreational vessels
- FCC: Maritime Mobile Service Identities
Published Sep 28, 2026 · For information only. Not for navigation, and not a substitute for the official sources cited.