What the ITC 1969 changed
Before the convention, national tonnage rules diverged and were gamed. Open shelter deck ships and other measurement artifacts existed purely to shrink registered tonnage. The ITC 1969 was adopted on 23 June 1969 and entered into force on 18 July 1982. It replaced the old gross and net register tons with two universal, formula based parameters. Gross tonnage is a function of the total volume of all enclosed spaces: GT equals K1 multiplied by V, where V is that volume in cubic metres and K1 is a logarithmic coefficient. Net tonnage is derived from cargo space volume with adjustments for moulded depth, draught and, for passenger ships, passenger numbers. Both are dimensionless functions of volume, not weights. Net tonnage is floored: it cannot be taken as less than 0.30 of gross tonnage. Every ship in scope carries an International Tonnage Certificate (1969) stating both values, issued by the flag administration or its recognised organisation. Re measurement is not automatic. A new certificate is required when the ship's characteristics change in a way that increases net tonnage. Where an alteration reduces net tonnage, a new certificate is generally withheld for twelve months, except on transfer of flag or after a major conversion.
GT: the threshold number of world shipping
Gross tonnage is the convention world's favourite threshold. SOLAS applies its cargo ship construction requirements from 500 GT. The Maritime Labour Convention certificate requirement starts at 500 GT for ships on international voyages. SOLAS Chapter IV radio requirements apply to cargo ships from 300 GT. CII and the EU carbon regimes apply from 5,000 GT. The Hong Kong Convention applies from 500 GT. Manning scales, survey fees, registration fees and many insurance and P&I calculations run on gross tonnage as well. That is why measurement decisions taken at the design stage echo for a ship's whole life. A vessel engineered to sit just below a threshold lives under a different rulebook than a near sister a few tons above it.
NT, dues and the canal exceptions
Net tonnage, as the earning capacity proxy, is the traditional base for port and harbour dues, pilotage and light dues in many jurisdictions. The famous exceptions are the canals. Both the Suez and Panama authorities apply their own measurement systems and issue their own tonnage certificates. That is why a ship's file can legitimately contain multiple tonnage documents showing different numbers, each valid in its own regime. It is not an error, and it should not be corrected.
Tonnage versus deadweight, and why precision matters
Deadweight is the mass a ship can carry at a given draught, comprising cargo, fuel and stores. It is a commercial measure of capacity and it has nothing to do with gross or net tonnage. Confusing the two is the most common tonnage error in contracts and in casual usage. A ship is described by gross tonnage for regulatory purposes and by deadweight for carrying capacity, and the two rank ships differently. For operators, tonnage hygiene is simple but real. Keep the tonnage certificate consistent with the ship as built and as modified. Re measure after conversions that change enclosed volume. Check that charterparties, insurance documents and registrations quote the right figure for the right purpose. The number is quiet, but almost every fee and threshold in the ship's life listens to it.