What is an IMO number?
· 4 min read
In short
An IMO number is a ship’s permanent identifier: the letters IMO followed by seven digits, the last of which is a check digit. It is assigned once, stays with the ship from build to scrapping, and does not change when the ship is renamed, reflagged or sold. Ships of 100 gross tonnage and above need one, with some exceptions such as pleasure yachts. It is the most reliable way to identify a ship, which is why every Voydar vessel URL starts with it.
A number for the hull, for life
The International Maritime Organization calls it the IMO Ship Identification Number. It started as the permanent numbers Lloyd’s Register gave the ships in its register from 1963, made seven digits long in 1969 and adopted by the IMO as the basis for its scheme in 1987. The scheme is now set out in IMO resolution A.1117(30) of December 2017.
The IMO number “remains unchanged during the entire life of the ship”, whatever happens to its name, flag, owner or type, and a number is never reused for another ship. S&P Global (formerly IHS Markit and Lloyd’s Register-Fairplay) assigns the numbers on the IMO’s behalf, free of charge.
Which ships have one
SOLAS regulation XI-1/3 made the number mandatory from 1 January 1996 for passenger ships of 100 gross tonnage and upwards and cargo ships of 300 gross tonnage and upwards. SOLAS also requires the number to be permanently marked on the ship. Resolution A.1117(30) extends the scheme to:
- ships of 100 gross tonnage and above, including fishing vessels;
- passenger ships under 100 gross tonnage, high-speed passenger craft and mobile offshore drilling units covered by SOLAS regulation V/19-1;
- motorised inboard fishing vessels under 100 gross tonnage down to 12 metres long, when authorised to fish outside their flag state’s waters.
It does not apply to ships without mechanical propulsion, pleasure yachts, ships on special service (such as lightships and rescue vessels), hopper barges, floating docks, warships and troop ships, or wooden ships other than fishing vessels. That is why most yachts and small boats have an MMSI but no IMO number.
A separate IMO number scheme, adopted in 2004 by resolution MSC.160(78), gives a permanent seven-digit number to the company and the registered owner of each ship. Those are company numbers, not ship numbers.
How the check digit works
The seventh digit protects the number against typing errors. Multiply the first six digits by 7, 6, 5, 4, 3 and 2, add the results, and take the last digit of the sum. If it equals the seventh digit, the number is well formed.
| Digit | 9 | 8 | 1 | 1 | 0 | 0 | Sum | Check |
|---|---|---|---|---|---|---|---|---|
| Weight | 7 | 6 | 5 | 4 | 3 | 2 | ||
| Product | 63 | 48 | 5 | 4 | 0 | 0 | 120 | 0 |
The last digit of 120 is 0, and the seventh digit of 9811000 is 0, so the number passes. Change any single digit and the check fails. A passing number is well formed, not proof that a ship holds it: to confirm that, look the ship up. The IMO’s own format illustration, IMO8712345, does not pass (its weighted sum is 128), which is a reminder that the letters and digits alone prove nothing.
The check also catches the placeholder values that turn up in AIS data, such as 1234567 or 9999999, which is why Voydar tests every IMO number before using it to identify a ship.
IMO numbers in Voydar
Live from Voydar113,860 ships in Voydar’s catalog carry an IMO number that passes the check digit. 2,337 further records carry a number in the IMO field that fails it, usually a placeholder or typo sent over AIS; Voydar never uses those to identify a ship.
Voydar count of distinct numbers in the IMO field of its vessel records, tested with the A.1117(30) check digit.
Will IMO numbers run out?
Not soon. According to the IMO’s FAQ on the scheme, the check-digit rule allows about 900,000 seven-digit numbers between 1000000 and 9999999; on 30 September 2022, 453,524 of them (50.4%) had been assigned. Because fishing vessels are now included, S&P Global has proposed extending the number to eight or ten characters, and ran an industry impact study in 2022–2023. No change has been adopted.
Where to find a ship’s IMO number
On the ship: marked on the hull or superstructure and printed on its certificates. Online: in the ship’s AIS static data, which Class A transponders broadcast every six minutes, and in public registers such as the IMO’s GISIS database. On Voydar: the IMO number is at the top of every vessel page and in its URL, so seadar.io/vessel/9811000 finds Ever Given whatever the ship is called today.
For how the IMO number relates to the other identifiers a ship carries, see IMO number vs MMSI.
Questions and answers
- Does an IMO number ever change?
- No. It stays with the ship for life, through renaming, reflagging and changes of owner, and it is never reused after the ship is scrapped or lost.
- Do yachts have IMO numbers?
- Pleasure yachts are exempt from the scheme, so most do not. Large yachts used commercially, and some others, do carry one.
- How do I check if an IMO number is valid?
- Multiply the first six digits by 7, 6, 5, 4, 3 and 2 and add them up. The last digit of the sum must equal the seventh digit. The lookup box on this page does it for you and then searches for the ship.
- Who issues IMO numbers?
- S&P Global, on behalf of the International Maritime Organization. Applications are processed free of charge.
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.
- IMO: IMO Identification Number Schemes
- IMO resolution A.1117(30): IMO Ship Identification Number Scheme
- IMO: Ship numbering FAQ (S&P Global)
- IMO: Global Integrated Shipping Information System (GISIS)
Published Sep 28, 2026 · For information only. Not for navigation, and not a substitute for the official sources cited.