Tonnage Measurement: Gross & Net Tonnage

1. The Tonnage Convention 1969

International Convention on Tonnage Measurement of Ships, 1969.

• An International Conference on Tonnage Measurement was convened by IMO in 1969 with the intention of producing a universally acceptable system of tonnage measurement.
• The International Convention on Tonnage Measurement of Ships 1969 was prepared at this conference, and it came into force on 8 July 1982.
• All ships constructed on or after that date were measured for tonnage in accordance with the 1969 Convention.
• Ships built prior to that date were, if the owner so desired, permitted to retain their existing tonnages for a period of 12 years from that date — i.e. all ships were required to be measured in accordance with the 1969 Convention by 18 July 1994.

Key dates. Adopted 1969 → in force 8 July 1982 → all ships compliant by 18 July 1994 (12-year transition for existing ships).

2. Gross Tonnage (GT)

Gross Tonnage is found by adding to the under-deck tonnage:

• The tonnage of all enclosed spaces between the upper deck and the second deck.
• The tonnage of all enclosed spaces above the upper deck.
• Any portion of hatchways exceeding 4% of the gross tonnage.

The Gross Tonnage Formula:

GT = K₁ × V
where K₁ = 0.2 + 0.02 log₁₀ V
and V = total volume of all enclosed spaces in cubic metres.

Gross tonnage is therefore a dimensionless measure of the total enclosed volume of the ship — it is not a weight. It is used as the basis for manning, safety equipment scales, registration and many port charges.

3. Net Tonnage (NT)

Net Tonnage (Register Tonnage) is obtained by deducting from the Gross Tonnage the tonnage of spaces which are required for the safe working of the ship:

• (a) Master's accommodation.
• (b) Crew accommodation and an allowance for provision stores.
• (c) Wheelhouse, chartroom, radio room and navigation aids room.
• (d) Chain locker, steering gear space, anchor gear and capstan space.
• (e) Space for safety equipment and batteries.
• (f) Workshops and storerooms for pumpmen, electricians, carpenter, boatswain and the lamp room.
• (g) Donkey engine and donkey boiler space if outside the engine room.
• (h) Pump room if outside the engine room.
• (i) In sailing ships, the storage space required for the sails, with an upper limit of 24% of the gross tonnage.
• (j) Water ballast spaces if used only for that purpose; the total deduction for water ballast, including double bottom spaces, may not exceed 19% of the gross tonnage.

Net Tonnage Formula:

For other ships: NT = K₂ Vc (4d / 3D)²

For passenger ships (13 passengers or more): NT = K₂ Vc (4d / 3D)² + K₃ (N₁ + N₂ / 10)

Where each symbol means:

Limiting Values:

• The factor (4d / 3D)² is not taken to be greater than unity.
• The term K₂ Vc (4d / 3D)² is not to be taken as less than 0.25 GT.
• NT is not to be taken as less than 0.30 GT.

The d/D effect. Vessels with high freeboards (low d/D ratio) will have low net tonnages. Because squaring this ratio can result in excessively low net tonnages, the limiting value of 0.30 GT is applied.

4. Why Tonnage Matters

Tonnage is a fundamental ship measurement used throughout maritime regulation and commerce:

• Manning — the Minimum Safe Manning Document and crew requirements are scaled to the ship's tonnage.
• Safety regulations — many SOLAS and MARPOL requirements apply by tonnage (e.g. 300, 400, 500, 1600 GT thresholds).
• Certification — which certificates apply (IOPP for ≥150 GT tankers / ≥400 GT others; Cargo Ship Safety Radio ≥300 GT; etc.).
• Registration — the International Tonnage Certificate records GT and NT; registration fees and statutory limits use them.
• Port and canal charges — port dues, pilotage, light dues, towage and canal tolls are commonly based on GT/NT.
• Load line & freeboard — tonnage interacts with freeboard and the d/D ratio that affects net tonnage.

Surveyor asks: "What is GT and NT, and give the formulas?" Answer with GT = K₁ V (K₁ = 0.2 + 0.02 log₁₀ V) as the total enclosed volume, and NT = K₂ Vc (4d/3D)² + K₃ (N₁ + N₂/10) for passenger ships, remembering the 0.25 GT and 0.30 GT limits and the key dates (1969 → 8 July 1982 → 18 July 1994).