Operational Guide · Registers & Logbooks

# Tonnage Certificate and IMO Number

Two numbers that follow the ship for its entire life: tonnage (which determines obligations and fees) and the IMO number (which identifies it permanently, even through a change of name or flag).

Tonnage Certificate IMO Number tonnage ship identification

## Operational Explanation

The Tonnage Certificate, issued under the 1969 International Convention on Tonnage Measurement of Ships, certifies the ship's gross tonnage (GT) and net tonnage (NT), calculated according to a universal method that replaced the previously fragmented national systems. Tonnage determines numerous regulatory obligations (applicability thresholds for certificates and requirements) and commercial ones (port dues, Suez/Panama tolls).

The IMO Number, introduced by Resolution A.600(15) of 1987 and made mandatory by SOLAS Regulation XI-1/3 from 1996, assigns a permanent identifier to the ship that never changes, regardless of changes of name, flag or ownership: a tool specifically designed to combat maritime fraud.

## Regulatory Reference

International Convention on Tonnage Measurement of Ships, 1969 for the calculation of GT/NT; Resolution A.600(15) (1987) and SOLAS Regulation XI-1/3 (mandatory from 1996) for the IMO Number, applicable to passenger ships ≥100 GT and cargo ships ≥300 GT.

## Scope of Application

Every ship subject to the Tonnage Convention (for the Tonnage Certificate) and every passenger ship ≥100 GT or cargo ship ≥300 GT (for the IMO Number requirement).

## Procedure / How to Complete It

1. Verify that the Tonnage Certificate is consistent with the ship's current configuration, especially after structural modifications that could alter GT/NT.
2. Verify that the IMO Number is correctly recorded on all of the ship's certificates and permanently marked on the hull per the applicable requirements.
3. In the event of a change of name, flag or ownership, confirm that the IMO Number remains unchanged on all updated documentation.
4. Verify consistency between the declared IMO Number and the one recorded in international databases (e.g. PSC systems, class registers).
5. Recalculate and recertify tonnage in the event of significant structural modifications (e.g. hull lengthening, superstructure modifications).

## Practical Example

Verification example: after a change of flag and renaming of the ship, the Tonnage Certificate is reissued by the new Administration with GT/NT unchanged, while the IMO Number remains identical to the one originally assigned, ensuring the ship's historical traceability.

## Real Cases

The IMO Number system was introduced specifically in response to cases of maritime fraud in which ships with a serious history (accidents, involvement in illicit trafficking) repeatedly changed name and flag to evade controls: the permanent identifier now makes it possible to reconstruct a ship's full history regardless of such changes.

## Common Mistakes Mistake Library

Mistake | Consequence | How to avoid it
--- | --- | ---
Tonnage Certificate not updated after a structural modification that alters GT/NT | Certificate not representative of the actual configuration, possible implications for applicable regulatory thresholds | Always recertify tonnage after significant structural modifications
IMO Number not permanently marked or not legible on the hull | PSC deficiency for non-compliance with marking requirements | Periodically verify the legibility and permanence of the IMO Number marking
Inconsistency between the IMO Number on documentation and on international registers after a change of flag/name | Traceability difficulties and possible suspicion during inspection | Systematically verify IMO Number consistency across all documentation after every change of flag/name/ownership

## PSC Observations

PSCOs check consistency between the IMO Number shown on the certificates, the one marked on the hull, and the one recorded in PSC information systems; discrepancies can raise suspicion of fraud or unclear ship identity.

## Operational Tips

- Always recertify tonnage after every significant structural modification; do not wait for the next routine survey.
- Periodically verify that the permanent IMO Number marking on the hull remains legible.
- After every change of flag, name or ownership, systematically verify that the IMO Number remains consistent across all updated documentation.

## Checklist

- Tonnage Certificate consistent with the ship's current configuration
- IMO Number correctly recorded on all certificates
- Permanent IMO Number marking on the hull legible
- IMO Number consistency verified after changes of flag/name/ownership
- Tonnage recalculated and recertified after significant structural modifications

## FAQ

Does the IMO Number change if the ship changes flag or name?
No: it is designed to remain permanently unchanged for the ship's entire life, regardless of changes of flag, name or ownership, precisely to facilitate traceability and prevent maritime fraud.
What is the difference between gross tonnage (GT) and net tonnage (NT)?
Gross tonnage measures the total volume of the ship's enclosed spaces; net tonnage represents a measure of the spaces usable for commercial transport, both calculated according to specific formulas under the 1969 Convention.
Which ships are required to carry an IMO Number?
Passenger ships of 100 GT and upwards and cargo ships of 300 GT and upwards, per SOLAS Regulation XI-1/3, mandatory from 1 January 1996.
🎬 Additional photos, videos and interactive diagrams for this topic will be available in a future version of the platform.

## Related Topics

International Load Line Certificate Harmonized System of Survey and Certification (HSSC)

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