Vessel Identification Guide
IMO Numbers Guide
The permanent, unchanging identifier for vessels worldwide — understand how IMO numbers work and why they matter for maritime data.
Example IMO Number
What is an IMO Number?
An IMO number is a unique, permanent 7-digit reference number assigned to seagoing ships and registered ship owners and management companies by the International Maritime Organization (IMO). It remains unchanged throughout the life of the vessel, regardless of changes in ownership, flag, or name.
The IMO Ship Identification Number Scheme was introduced in 1987 under SOLAS regulations to enhance maritime safety and security by providing a permanent way to identify ships. This makes it invaluable for tracking vessel history, ownership changes, and compliance records.
Unlike the MMSI number which is assigned to radio equipment and can change, the IMO number is physically marked on the ship's hull and permanently linked to the vessel's structure.
IMO Number Structure
Every IMO number follows a specific format with a built-in error check.
Format: IMO XXXXXXC
- "IMO" prefix (always present)
- 6 digits = Unique vessel identifier
- 1 check digit = Error detection
Check Digit Calculation
The check digit is calculated by multiplying each of the first 6 digits by a weight (7 to 2), summing the products, and taking the last digit.
Example: 9074634
(9×7)+(0×6)+(7×5)+(4×4)+(6×3)+(3×2) = 63+0+35+16+18+6 = 138
138 mod 10 = 8... wait, that's wrong! This example needs recalculation.
Validate IMO Numbers
The check digit allows you to validate IMO numbers programmatically. If a number fails the check digit validation, it's either incorrectly entered or invalid.
Which Vessels Have IMO Numbers?
Not all vessels are assigned IMO numbers. Here's who gets them.
Assigned IMO Numbers
- • Seagoing passenger ships (any size)
- • Cargo ships of 300 GT and above
- • Mobile offshore drilling units
- • Motor yachts of 100 GT and above (voluntary)
- • Registered ship owners (company numbers)
- • Registered ship managers (company numbers)
Not Assigned IMO Numbers
- • Fishing vessels
- • Non-propelled vessels (barges)
- • Government ships (naval, coast guard)
- • Pleasure yachts (unless voluntary)
- • Ships under 300 GT
- • Inland waterway vessels
IMO Number vs MMSI
Both identifiers are essential, but they serve different purposes.
| Aspect | IMO Number | MMSI |
|---|---|---|
| Format | 7 digits (IMO + 6 + check) | 9 digits |
| Assigned to | Ship's hull | Radio equipment |
| Permanence | Never changes | Can change (reflagging) |
| Issued by | IHS Markit (on behalf of IMO) | Flag state authorities |
| Indicates country | No | Yes (via MID) |
| Best for | Historical tracking, ownership | Real-time positions |
When to Use Each
Use IMO numbers when tracking vessel history, verifying ownership, or analyzing long-term patterns. Use MMSI for real-time position queries and current voyage tracking. Our API supports both identifiers for maximum flexibility.
Where to Find an IMO Number
IMO numbers are displayed and recorded in multiple locations.
On the Hull
Permanently marked or engraved on the ship's hull, visible at the stern and on the main deck.
Ship's Certificates
Recorded in safety certificates, classification documents, and official ship registration papers.
AIS Transmissions
Broadcast in AIS Message Type 5 (static data). Available through maritime data APIs.
Maritime Databases
Searchable in IHS Markit Sea-web, Lloyd's List Intelligence, and other vessel registries.
GISIS Database
The official IMO Global Integrated Shipping Information System maintains the authoritative record.
Via API
Query our API with MMSI to retrieve the associated IMO number, or vice versa.
Query Vessels by IMO Number
Use our API to look up vessel positions, history, and particulars using IMO or MMSI numbers.