· 6 min read
IMO number explained: format, check digit and free lookup
An IMO number is a ship's permanent seven-digit ID. Learn how the check digit works, how it differs from MMSI and call sign, and where to look it up for free.
- IMO number
- Vessel identification
- MMSI
In short
An IMO number is a permanent ship identifier: the letters 'IMO' plus seven digits, assigned when the ship is built. It survives changes of flag, name or owner and is never reused. SOLAS regulation XI-1/3 makes it mandatory, and the last digit is a check digit. Look it up in IMO GISIS, Equasis or the ship's certificates.
An IMO number is the permanent seven-digit number that identifies a ship for its entire life. It is assigned when the ship is built and stays the same even if the ship changes its name, flag or owner. The last digit is a check digit, so most typing errors can be caught with simple arithmetic. You can look up IMO numbers for free in IMO GISIS and Equasis.
What is an IMO number?
An IMO ship identification number is the letters “IMO” followed by seven digits, for example IMO 9481427. The scheme started in 1987 with IMO resolution A.600(15) and became mandatory on January 1, 1996, under regulation XI-1/3 of the SOLAS Convention. The rule applies to passenger ships of 100 GT and above and cargo ships of 300 GT and above.
IHS Maritime assigns the numbers on IMO’s behalf, free of charge. As IMO explains, the number remains unchanged when a ship transfers to another flag and is never reassigned to another ship. SOLAS also requires the number to appear on the ship’s certificates and to be permanently marked both in a visible place on the hull or superstructure and in an accessible place inside the ship.
The scheme has grown over time: since 2013, fishing vessels of 100 GT and above can also apply for an IMO number on a voluntary basis. The number is also used as the ship’s identity in tracking systems such as AIS and LRIT.
Company and registered owner numbers
IMO runs a second numbering scheme for companies and registered owners. Adopted in 2004 with resolution MSC.160(78), it gives the managing company (the “Company” in the sense of the ISM Code) and the registered owner a permanent seven-digit number each. Under SOLAS regulation XI-1/3-1, these numbers appear on documents such as the Document of Compliance (DOC), the Safety Management Certificate (SMC) and the International Ship Security Certificate (ISSC) issued from January 1, 2009.
The IMO number you see next to each company in the Equasis management table comes from this scheme. Do not confuse it with the ship’s IMO number: one identifies the company, the other identifies the vessel.
How is the IMO check digit calculated?
The seventh digit is calculated from the first six:
- Multiply the first six digits, from left to right, by 7, 6, 5, 4, 3 and 2.
- Add up the results.
- The last digit of the total (the remainder after dividing by 10) must match the seventh digit of the number.
Example: for IMO 9481427 the calculation is (9×7) + (4×6) + (8×5) + (1×4) + (4×3) + (2×2) = 63 + 24 + 40 + 4 + 12 + 4 = 147. The last digit of the total is 7, the same as the last digit of the number, so the format is valid. The China P&I Club’s note on IMO numbers uses the same method and example.
Now imagine you typed the number as 9481327 by mistake: (9×7) + (4×6) + (8×5) + (1×4) + (3×3) + (2×2) = 144. The last digit is 4, which does not match the final 7, so something is wrong. The check digit catches most typing errors, but not all of them. A number with a valid format can also belong to a different ship, so it is good practice to check the IMO number together with the ship’s name and flag.
IMO number vs MMSI vs call sign: what is the difference?
A ship is identified by three complementary identities, not one:
| Identifier | Format | Assigned by | When does it change? | Where it is used |
|---|---|---|---|---|
| IMO number | “IMO” plus 7 digits | IHS Maritime on behalf of IMO | Never | Certificates, hull marking, ship databases, AIS static data |
| MMSI | 9 digits, the first three are the country code (MID) | The flag state’s administration | Usually when the flag changes | AIS and radio calls |
| Call sign | Letters and digits | The flag state’s administration, from the series ITU allocates to that country | When the flag changes | Radio communication, ship documents |
According to the ITU, the first three digits of an MMSI, the MID, denote the administration with jurisdiction over the ship station. That is why the MMSI of a Turkish-flagged ship starts with 271. Because the MMSI and call sign can change with the flag, the IMO number is the safer permanent reference for your records.
Where can you look up an IMO number?
- IMO GISIS: The “Ship and Company Particulars” module in IMO’s GISIS lets you search ships by IMO number and companies by IMO company number. You need a free public account.
- Equasis: After free registration, you can search by IMO number, ship name or call sign and see the ship’s management companies. Our Equasis guide walks through the steps.
- Ship documents: Certificates issued under SOLAS show the ship’s IMO number.
- The ship itself: The number is permanently marked on the hull or superstructure and inside the ship.
- AIS: Class A AIS units broadcast the IMO number as static data. It is entered manually when the unit is installed, so compare it with another source if something looks off.
Why does the IMO number matter for port service companies?
As a ship chandler, ship agent, technical service firm or surveyor, you work with ship names every day. But names change, and several ships can share the same name. The IMO number removes that confusion:
- Matching the right ship: Put the IMO number on quotes, orders and invoices, and you can match a renamed ship to your earlier records.
- Reaching the right company: Equasis and GISIS searches run on the IMO number, which is how you find the ship manager and registered owner.
- Sanctions screening: Sanctions lists such as the one published by the U.S. Treasury’s OFAC identify vessels by IMO number. Since the number stays the same when the name or flag changes, screening by IMO number is more reliable.
- Clean customer records: Using the IMO number as the key in your CRM or spreadsheets keeps the same ship from being entered twice.
- Continuous tracking: AIS position data arrives keyed to the MMSI, which can change with the flag. Track the ship by IMO number and your history stays attached to the same vessel after a flag change.
How does VesselCall use the IMO number?
In VesselCall, the IMO number is the common key for every step. VesselCall Outreach detects commercial vessels approaching the Turkish ports you choose using AIS data, then uses each vessel’s IMO number to look up its company roles in international ship databases. Your introduction e-mail goes from your own domain to the ship manager / commercial manager, or to the ISM manager if there is no such record, or to the registered owner if neither exists. The same vessel never receives a second e-mail within 7 days, even if it shows up at another port.
The Data package lets you search by IMO number, MMSI or ship name. One screen shows the vessel’s identity and technical particulars, manager and owner companies, class, P&I, PSC inspection summary and its Turkish port calls since monitoring started.
