An IMO prefix, spaces, hyphens and a spreadsheet’s trailing .0 are all stripped before checking.
Try , the worked example from our IMO number guide.
One per line, or separated by commas. Straight out of a spreadsheet is fine.
How the IMO check digit works
take the first six digits weight them 7, 6, 5, 4, 3, 2 add the six products the last digit of that sum is the seventh digit 9 0 7 4 7 2 | 9 x7 x6 x5 x4 x3 x2 | 63 0 35 16 21 4 | sum 139, last digit 9
Why the leading digit is part of the rule
An IMO ship number opens 1, or 5 to 9. That is specification rather than a habit, and it earns its keep: it is what rejects a seven-digit fragment of an MMSI that happens to satisfy the check digit by chance. Drop the rule and a checker starts calling pieces of a different identifier valid, which is worse than telling you nothing. The 1 is recent. Numbers were issued from 5000000 upward for decades, and once the 9 series ran out the next ones began at 1000000, so a hull delivered today can carry a number that older checkers refuse. A leading 2, 3 or 4 has never been issued and is still wrong.
What a bad IMO number actually costs
This is not tidiness. Our own production database holds about 5,900 numbers derived from AIS that look like IMO numbers and are not. Left untested they get queried against every data source, which spends a metered allowance on nothing, and one of them can match a real record and staple one ship’s ownership onto another hull. The check digit is the cheapest defence against that there is, and most people have never been shown it.
What this page does not do
It does not look a ship up. It is arithmetic on a number: no vendor is contacted, no particulars are returned, and nothing you type is sent anywhere or stored. A valid result means the seven digits are self-consistent, not that the hull exists, and the two are genuinely different claims. Looking a vessel up against a licensed register is what the Vessel Register does behind a sign-in.
Questions
- What is an IMO number?
- A seven-digit number assigned to a ship's hull for life. It survives a change of name, owner, flag and class, which is exactly why it is the key the whole trade sorts on: a name can be reused by another ship the week after, and often is.
- How do I check an IMO number by hand?
- Weight the first six digits by 7, 6, 5, 4, 3 and 2, add the products, and the last digit of the sum must equal the seventh digit. On 9074729 the products are 63, 0, 35, 16, 21 and 4, which sum to 139, and the last digit of 139 is 9. It checks out.
- Why did a number that looks fine come back invalid?
- Two usual causes. Either one of the first six digits is mistyped, in which case the arithmetic above no longer lands on the seventh, or it is not an IMO number at all: a nine-digit MMSI truncated to seven is the common one, and an IMO number opening with a digit below 5 is the tell.
- Does a valid check digit mean the ship exists?
- No, and the distinction matters. It means the seven digits are internally consistent. Roughly one seven-digit number in ten passes by chance, so a valid result rules out typos rather than confirming a hull. Only a register can do the second thing.
- Can I check a whole list at once?
- Yes. Paste or search a column of IMO numbers in the second box, newline or comma separated. It counts how many are valid and, where only the seventh digit is wrong, shows the number that was probably meant.
- Is this checker free?
- Yes, with no account and no limit. It runs entirely in your browser, so nothing you paste is sent to Neptune Atlas or stored anywhere.
Read the full IMO number guide
Also worth reading: ship names and renaming on why the number outlives the name, and who owns a ship on the four roles a single hull carries. There are also free laytime and TCE calculators.
Neptune Atlas checks this on the way in, on every hull in a register of the world merchant fleet, and keeps the four ownership roles apart. See what it costs.