What is an IMO number?

An IMO number is a unique 7-digit identifier assigned to a merchant ship's hull by the International Maritime Organization. It stays the same for the ship's entire life, even if it is sold, re-flagged, or renamed.

How is the IMO check digit calculated?

Multiply the first six digits by the weights 7, 6, 5, 4, 3, 2 respectively, then sum the products. The check digit is the units digit (the rightmost digit) of that sum.

Does the "IMO" prefix count as part of the number?

No. The prefix is just a label. Only the 7 digits matter, and the validator strips the prefix automatically before checking.

Do all ships have an IMO number?

Most seagoing merchant ships of 100 gross tonnage and above carry one. Some vessels such as fishing boats below a size threshold, warships, and pleasure yachts are exempt, though many obtain one voluntarily.

Is an MMSI number the same as an IMO number?

No. An MMSI is a 9-digit radio identifier that can change if the ship changes flag, while the IMO number is a permanent 7-digit hull identifier that never changes.

IMO Number Format Reference

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What an IMO number is

The IMO number is a permanent identity for a ship’s hull, assigned by the International Maritime Organization. Unlike the flag state, name, or call sign, it never changes — which is precisely why insurers, port authorities, and casualty databases rely on it to track a vessel across its entire lifetime.

How it works

An IMO number is seven digits long. The first six digits are the sequential serial number; the seventh is a check digit that guards against typos. To compute the check digit, multiply each of the first six digits by a descending weight and take the units digit of the total:

digits   d1 d2 d3 d4 d5 d6
weights   7  6  5  4  3  2

sum = 7*d1 + 6*d2 + 5*d3 + 4*d4 + 3*d5 + 2*d6
check digit = sum mod 10   (the last digit of sum)

For example, IMO 9074729: the first six digits are 9 0 7 4 7 2.

7*9 + 6*0 + 5*7 + 4*4 + 3*7 + 2*2
= 63 + 0 + 35 + 16 + 21 + 4
= 139
139 mod 10 = 9  →  check digit is 9, which matches.

Tips and notes

Because the algorithm only validates the check digit, a structurally valid number is not proof that a ship actually exists — it merely means the number is internally consistent. To confirm a real registration, cross-reference the IMO number against an official registry such as Equasis or the IHS Markit database. Company and registered-owner IMO numbers share the same checksum scheme, so this validator works for those too once any leading letters are removed.