Can a ship have an MMSI but no IMO number?

Yes. Most yachts, fishing boats under the scheme’s size limits and small craft have an MMSI for their radio but no IMO number.

Can a ship have an IMO number but no MMSI?

Yes: a ship that has been laid up, scrapped or lost keeps its IMO number in the records, while its radio licence and MMSI have lapsed. A ship under construction has an IMO number before its radio is licensed.

Why does the same ship appear under two MMSIs?

Usually because it changed flag and was issued a new MMSI. Sometimes a transponder is misprogrammed. The IMO number in the ship’s AIS static data shows which records belong together.

IMO number vs MMSI: what is the difference?

Updated Sep 28, 2026 · 3 min read

In short

The IMO number identifies the ship itself: seven digits, assigned once, never changed. The MMSI identifies the ship’s radio: nine digits, issued by the flag state, and usually replaced when the ship changes flag. Use the IMO number to identify a ship over time, and the MMSI to match a live AIS signal to it.

Side by side

IMO number and MMSI compared
	IMO number	MMSI
Identifies	The ship (hull), for life	A radio station: the ship’s DSC radio and AIS
Format	7 digits, the last a check digit	9 digits, no check digit; the first three are usually the country’s MID
Issued by	S&P Global on behalf of the IMO	The flag state’s radio administration, from ITU-allocated blocks
Changes	Never, and never reused	When the licence changes, usually on a change of flag
Who has one	Ships of 100 GT and above, with exceptions such as pleasure yachts	Any vessel with a DSC radio or AIS, including small boats
Where you see it	Certificates, marked on the ship, AIS static data every 6 minutes	Every AIS position report, DSC calls and distress alerts
Rules	SOLAS XI-1/3, IMO resolution A.1117(30)	ITU Radio Regulations, ITU-R M.585

A live example

The table below is not a mock-up: it lists every MMSI Voydar has received together with one IMO number, straight from its AIS records. When a ship changes flag, it starts transmitting a new MMSI and a new call sign from the new flag’s blocks, while the IMO number in its AIS data stays the same, so a reflagged ship shows up here as more than one row.

One IMO number, 2 MMSIs

Live from Voydar

What 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

Open the EVER GIVEN page

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.

Which one should you use?

To identify a ship over time (in a contract, a sanctions check, a news story or a spreadsheet), use the IMO number. Names are reused and changed, and MMSIs follow the flag.
To follow a ship live, the MMSI is what the AIS signal carries. A tracking site matches that MMSI to the IMO number the ship reports in its static data.
For a small boat without an IMO number, the MMSI is all there is. Voydar gives such boats a page keyed on the MMSI instead.

Voydar merges records only on identifiers that cannot drift: a checksum-valid IMO number, never an MMSI or a name. The MMSIs, names, flags and call signs a ship has used become its identity history, shown on its page.

And the call sign?

A ship has a third identifier, its radio call sign, such as H3RC. Like the MMSI, it comes from the flag state’s radio administration and usually changes with the flag. Full details are in the guides to the MMSI and the IMO number.

Questions and answers

Can a ship have an MMSI but no IMO number?: Yes. Most yachts, fishing boats under the scheme’s size limits and small craft have an MMSI for their radio but no IMO number.
Can a ship have an IMO number but no MMSI?: Yes: a ship that has been laid up, scrapped or lost keeps its IMO number in the records, while its radio licence and MMSI have lapsed. A ship under construction has an IMO number before its radio is licensed.
Why does the same ship appear under two MMSIs?: Usually because it changed flag and was issued a new MMSI. Sometimes a transponder is misprogrammed. The IMO number in the ship’s AIS static data shows which records belong together.

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.

Published Sep 28, 2026 · For information only. Not for navigation, and not a substitute for the official sources cited.