Built for the people who track, screen and trade ships: fleet managers, operators, analysts, compliance teams and anyone who has to be sure two records are the same ship. A vessel carries four different identifiers — the permanent IMO number vs the reassignable MMSI, plus its call sign and flag — they change at different times, and joining data on the wrong one silently breaks every dataset downstream. We assembled what each identifier is, which one is permanent, and how to follow a hull across a re-flag or a sale — into one reference.
This is the identity layer behind the live ports & vessel map and the technical companion to AIS decoded.
📖 See also: AIS decoded · Ship types & sizes · How to charter a ship
Contents
- At a glance
- The four identifiers
- The IMO number — the permanent key
- The IMO check-digit algorithm
- The MMSI — the radio identity
- Call sign and flag
- Which identifier changes, and when
- How to join data across a re-flag
- For developers
- FAQ
- References
At a glance
The short version, if you read nothing else:
- A ship carries four identifiers that answer different questions: the IMO number (the hull, forever), the MMSI (the radio/AIS station, for now), the call sign (radio identity), and the flag (state of registry).
- The IMO number is the only permanent one. Seven digits, physically marked on the hull, assigned once for the life of the ship — it does not change with owner, name, flag or class.
- The MMSI is not the ship — it's the radio. Nine digits whose first three (the MID) give the flag; it is reassigned when the ship changes flag, so it's a fine live key but a poor historical one.
- To follow a hull over time — across a sale, a rename, a re-flag — join on the IMO number. Joining on MMSI or name silently splits one ship into several records (or merges two).
- The IMO number has a built-in check digit, so you can validate it with arithmetic — a cheap guard against typos and fabricated numbers.
- Identity manipulation is a real signal: two ships broadcasting the same MMSI, an MMSI whose MID doesn't match the claimed flag, or a hull that changes name between port calls are all tells that AIS analysis surfaces.
The four identifiers
| Identifier | What it identifies | Format | Assigned by | Permanent? |
|---|---|---|---|---|
| IMO number | The hull (the physical ship) | IMO + 7 digits |
IMO scheme (administered by S&P Global / IHS Maritime) | Yes — for life |
| MMSI | The radio/AIS station | 9 digits (first 3 = MID) | The flag state's telecoms administration | No — changes on re-flag |
| Call sign | The radio identity | Letters/digits (e.g. 9HA1234) |
The flag state | No — changes on re-flag |
| Flag | The state of registry | Country (e.g. Panama, Liberia) | Chosen by the owner | No — changes on re-flag |
The mental model: the IMO number is the ship's fingerprint; the MMSI, call sign and flag are its current paperwork, which the owner can and does change.
The IMO number — the permanent key
An IMO number is a seven-digit, permanent identifier for a ship's hull — it never changes for the life of the ship. The IMO Ship Identification Number Scheme was introduced in 1987 (later mandatory under SOLAS — the Safety of Life at Sea convention — Ch. XI-1 Reg. 3) to give every ship exactly that permanent identity:
- Seven digits, prefixed "IMO" (e.g.
IMO 9074729). - Assigned once and marked on the hull (and in the engine room) — it never changes through the ship's life, regardless of sales, renames, re-flags or conversions.
- Assigned via the IMO scheme, administered by S&P Global (IHS Maritime). Mandatory marking under SOLAS XI-1/3 covers passenger ships ≥ 100 GT and cargo ships ≥ 300 GT (GT = gross tonnage); smaller craft, fishing vessels, wooden ships and pleasure craft are outside the mandate, though many can obtain a number voluntarily (the scheme now also assigns numbers to some fishing vessels).
- A parallel Company IMO number identifies the registered owner and ISM manager — useful for ownership screening.
Because it's permanent, the IMO number is the correct primary key for any dataset that tracks a ship over time: casualty history, port calls, ownership, sanctions status.
The IMO check-digit algorithm
The seventh digit is a check digit derived from the first six — so a valid IMO number can be verified arithmetically, and a typo or a fabricated number is often caught for free.
For an IMO number A B C D E F G:
check = ( A×7 + B×6 + C×5 + D×4 + E×3 + F×2 ) mod 10 → must equal G
Multiply the first six digits by weights 7, 6, 5, 4, 3, 2, sum them, and the units digit of the sum must equal the seventh digit.
Worked example — IMO 9074729:
9×7 + 0×6 + 7×5 + 4×4 + 7×3 + 2×2
= 63 + 0 + 35 + 16 + 21 + 4
= 139
units digit = 9 → equals G (9) ✓ valid
If the units digit doesn't match the last digit, the number is invalid — a fast, offline sanity check on any vessel record. (The literal "IMO" prefix is not part of the calculation — only the seven digits — a common implementer bug.)
The MMSI — the radio identity
The Maritime Mobile Service Identity is a 9-digit number identifying a radio/AIS station, not a hull (full field breakdown in AIS decoded):
- The first three digits are the MID (Maritime Identification Digits) — the flag/country. E.g.
232–235…UK,273…Russia,477…Hong Kong,538…Marshall Islands,636…Liberia,563–567…Singapore. - Special prefixes mark non-ship stations:
00MID…coast station,0MID…group of ships,111…SAR aircraft,99MID…AtoN (Aid to Navigation — e.g. a buoy or lighthouse transponder),98MID…craft associated with a parent ship, distress beacons970/972/974…. - It's assigned by the flag state's telecoms administration and programmed into the ship's transponder — so when the ship re-flags, its MMSI usually changes (and its MID no longer matches the old flag).
The MMSI is the right key for live tracking (it's what AIS broadcasts), but the wrong key for history, because it's tied to the current flag, not the hull.
Call sign and flag
- Call sign — a short radio identifier (letters and digits, e.g.
9HA1234) assigned by the flag state. Like the MMSI it's a radio identity and changes on re-flag. Its prefix also encodes the flag (ITU allocation), so a call-sign prefix that doesn't match the claimed flag is a red flag. - Flag — the state of registry whose law the ship flies under. Owners choose it; open registries (Panama, Liberia, Marshall Islands) dominate the merchant fleet for tax, cost and regulatory reasons. The flag drives which Port State Control MoU (the regional inspection Memorandum of Understanding — e.g. Paris, Tokyo) targeting applies to the ship, and can change with a sale or a commercial decision.
Neither is a durable key. Both are useful attributes to cross-check against each other and against the MMSI's MID.
Which identifier changes, and when
| Event | IMO number | MMSI | Call sign | Flag | Name |
|---|---|---|---|---|---|
| Ship renamed | — | — | — | — | changes |
| Sold (same flag) | — | — | — | — | often changes |
| Re-flagged | — | changes | changes | changes | often changes |
| Converted / rebuilt | — | maybe | maybe | maybe | maybe |
| Scrapped & replaced (new hull) | new hull = new IMO | new | new | new | new |
The pattern is stark: everything except the IMO number can change on a single re-flag, and they often change together — which is exactly why naïve tracking on name or MMSI falls apart.
How to join data across a re-flag
The practical rule for anyone building or reconciling vessel data:
- Store and join on the IMO number as the hull's primary key. It survives sales, renames and re-flags.
- Keep MMSI as the live-radio key — use it to attach the current AIS stream, but treat it as time-bounded (valid for the current flag period).
- Treat name, call sign and flag as attributes, not keys — snapshot them with a timestamp so you can see the history, not overwrite it.
- Validate the IMO with the check digit on ingest to catch typos and fabricated numbers.
- Flag mismatches as signals, not errors to silently fix: an MMSI MID that doesn't match the flag, two hulls sharing one MMSI, or a name that changes between consecutive port calls are all worth surfacing — they're the fingerprints of identity manipulation (see AIS decoded).
Do this and a ship that was "Vessel A / MMSI 636xxxxxx / Liberia" last year and "Vessel B / MMSI 352xxxxxx / Panama" today resolves to one hull on its IMO number — instead of two half-complete records.
For developers
- Identify on IMO, track on MMSI. Persist the IMO number as the vessel key; bind the live AIS stream by MMSI as a time-bounded association.
- Validate on ingest with the check-digit formula above — reject or quarantine numbers that fail.
- Cross-check identity fields: MMSI MID vs flag, call-sign prefix vs flag, name changes across port calls. Expose the mismatches; don't auto-correct them.
- The rynda ports & vessel map resolves live positions to hulls this way — MMSI for the live radio, IMO for the durable identity.
FAQ and common confusions
Q: What is an IMO number?
A: An IMO number is a seven-digit, permanent identifier for a ship's hull (prefixed "IMO", e.g. IMO 9074729), assigned once under the IMO scheme and marked on the hull. It never changes through the ship's life — regardless of sales, renames or re-flags — which makes it the right key for tracking a ship over time. The last digit is a check digit you can validate arithmetically.
Q: What is an MMSI?
A: A Maritime Mobile Service Identity is a nine-digit number identifying a ship's radio/AIS station (not the hull). Its first three digits (the MID) give the flag country, and it's assigned by the flag state — so it changes when a ship re-flags. Use it for live tracking (it's what AIS broadcasts), but not for a ship's history.
Q: What's the difference between an IMO number and an MMSI?
A: The IMO number identifies the hull permanently (never changes). The MMSI identifies the radio/AIS station and changes when the ship re-flags. Use IMO to identify a ship over its life, MMSI to attach its live AIS.
Q: Does the IMO number ever change?
A: No. It's assigned once and marked on the hull for the ship's entire life — through sales, renames, re-flags and conversions. Only a genuinely new hull gets a new IMO number.
Q: Can two ships have the same MMSI?
A: They shouldn't, but it happens — through error or deliberate manipulation. Two hulls broadcasting the same MMSI is a classic tell in AIS analysis. IMO numbers, by contrast, are unique per hull.
Q: How do I check if an IMO number is valid?
A: Use the check digit: (d1×7 + d2×6 + d3×5 + d4×4 + d5×3 + d6×2) mod 10 must equal the 7th digit. E.g. IMO 9074729 → 139 → units digit 9 → valid.
Q: Why does a ship's MMSI's first digits sometimes not match its flag?
A: The first three digits (MID) should match the flag. A mismatch usually means the ship re-flagged and didn't reprogram the transponder, or the identity is being manipulated — either way, worth flagging.
Q: Which identifier should I use as a database key?
A: The IMO number. Name, MMSI, call sign and flag all change; joining on them silently splits or merges ships. Keep them as timestamped attributes.
Q: What is a flag / flag of convenience?
A: The flag is the state of registry whose law the ship flies under. Open registries (Panama, Liberia, Marshall Islands) — sometimes called flags of convenience — host much of the merchant fleet for cost and regulatory reasons. The flag can change on a sale or commercial decision.
Q: Do all ships have an IMO number?
A: Mandatory for passenger ships ≥ 100 GT and cargo ships ≥ 300 GT under SOLAS. Smaller craft, fishing vessels, wooden ships, pleasure craft and warships are outside the mandate — though many obtain a number voluntarily, and the scheme now covers some fishing vessels. Small craft may carry only an MMSI.
References
- IMO — the Ship (and Company) Identification Number Scheme; SOLAS Ch. XI-1 Reg. 3.
- S&P Global / IHS Maritime — administers IMO number assignment on behalf of the IMO.
- ITU — MMSI structure and the Maritime Identification Digits (MID) table; call-sign allocations.
- ITU-R M.1371 — the AIS standard carrying MMSI (see AIS decoded).
- rynda ports & vessel map.
Document version
- 2026-07 — first public version. IMO number, MMSI, call sign and flag — what each identifies, which is permanent, the check-digit validation, and how to join a hull across a re-flag. Field definitions follow the IMO scheme and ITU; confirm edge cases against the current standards.