MMSI number decoder.

How an MMSI is built

A Maritime Mobile Service Identity is a nine-digit number used by marine VHF radios with digital selective calling and by AIS transponders. The first digits tell you what kind of station it is, and for most kinds three digits are a Maritime Identification Digits (MID) code that points to the administration that issued the number, such as 316 for Canada or 338, 366, 367, 368 and 369 for the United States. The International Telecommunication Union allocates MIDs and this tool uses its published table of 292 codes. There is no check digit in an MMSI, unlike an IMO number or a container number, so there is no arithmetic that proves a number is genuine.

The patterns decoded

These follow ITU-R M.585-9 as tabulated by the US Coast Guard Navigation Center. An individual ship station is the MID followed by six digits. A group ship call is a zero then the MID. A coast station starts 00 then the MID, and the sixth digit separates group, coast, port, pilot, AIS repeater and AIS base stations. Aircraft taking part in search and rescue start 111 then the MID. An AIS aid to navigation starts 99 then the MID, with a sixth digit of 1 for physical, 6 for virtual and 8 or 9 for mobile. Craft associated with a parent ship start 98 then the MID, and the Coast Guard notes this is not used in the USA. A handheld VHF with DSC starts with 8 then the MID. Freeform devices start 97: 970 is an AIS-SART, 972 a man-overboard device, 974 an EPIRB-AIS and 979 a Group B man-overboard device. For 970, 972 and 974 the next two digits are a manufacturer identity assigned by CIRM, not a country.

Worked examples

316001234 is an individual ship station with MID 316, Canada. 366999373 has MID 366, the United States. 003669999 would be a coast station with MID 366. 993161001 is a physical AIS aid to navigation under MID 316. 970123456 is an AIS-SART with manufacturer ID 12.

What the result does not mean

The MID shows who issued the number, not where the vessel is, which flag it sails under, or who owns it; vessels can change registry and keep or get a new MMSI. A well-formed number may be unassigned or reused. To identify a ship use its IMO number where it has one, which has a check digit, and the vessel IMO number validator covers that. Nothing you enter leaves your browser.

Sources (each opened 2026-10-19): US Coast Guard Navigation Center, MMSI Format (ship, coast, aircraft, AtoN, parent-ship craft, handheld and freeform device patterns, citing ITU-R M.585-9); ITU, Table of Maritime Identification Digits (MIDs). The ITU recommendation text itself was not retrieved; the structure comes from the Coast Guard table that cites it.

FAQ

What do the first three digits of an MMSI mean?

For an individual ship station they are the Maritime Identification Digits (MID), which identify the country or territory of the administration that issued the number. Other station types put a prefix such as 00, 99 or 111 before the MID.

Does an MMSI have a check digit?

No. Unlike an IMO number, an MMSI has no check digit, so a decoder can show whether the structure is valid but cannot prove the number is assigned to a real vessel.

How long is an MMSI?

Exactly nine digits. This tool rejects anything shorter or longer and ignores spaces and dashes.

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

An MMSI is a radio and AIS identity that can change, for example when a vessel changes flag. An IMO number is a seven-digit permanent ship identifier with a check digit.

What do MMSIs starting with 99 or 970 mean?

A 99 prefix followed by a MID is an AIS aid to navigation, such as a buoy. A 970 prefix is an AIS search and rescue transponder, 972 a man-overboard device and 974 an EPIRB-AIS, each followed by a manufacturer identity.

Does this tool look the MMSI up or track a vessel?

No. It only reads the digits locally and consults the ITU MID table built into the page. It cannot show a vessel name, position or owner.