IMO Number Format Reference

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.

Who assigns IMO numbers?

IMO numbers are administered by Lloyd’s Register on behalf of the International Maritime Organization. When a merchant vessel is built, the shipyard or owner applies for a number through an approved flag-state or classification society. The number is then permanent — it survives re-flagging, sale, and even renaming. Because of this permanence, casualty databases, detention records, and port state control inspections all key on the IMO number rather than the ship’s name.

The scheme was introduced under IMO Resolution A.600(15) in 1987 and progressively made mandatory for most seagoing vessels above 100 GT. Some categories are exempt: fishing vessels, vessels engaged in domestic voyages only, government vessels, vessels without mechanical propulsion, and pleasure yachts — although many operators obtain an IMO number voluntarily for insurance and chartering purposes.

Checking a company or manager number

The same 7-digit + check-digit scheme also applies to company and registered-owner IMO numbers, introduced under the ISM Code. These identify the company responsible for ship management rather than the hull itself. If you are working with a company number (printed with a C prefix), strip the prefix before entering the digits and the validator will work identically.

Where the number appears

You will encounter the IMO number in:

• Ship classification records — classed by a member society of IACS
• Port state control inspection reports — Tokyo, Paris, and US Coast Guard MOU databases
• Insurance certificates — hull and P&I club policies typically reference it
• Lloyd’s Casualty Reports — the permanent identifier in any casualty write-up
• AIS data streams — the static AIS message field IMO carries it alongside the MMSI

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.

A single-digit error almost always produces a different check digit and fails immediately. A transposition of two adjacent digits may or may not fail depending on the positions involved — that is why visual verification against a second source remains good practice for critical documents.

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