What are gross tonnage (GT), net tonnage (NT) and deadweight (DWT), and how do they differ?
Short answer
| GT | NT | DWT | |
|---|---|---|---|
| Measures | Total enclosed volume | Useful (earning) volume | Weight the ship can carry |
| Unit | Dimensionless | Dimensionless, always less than GT | Tonnes |
| Basis | Tonnage Convention 1969, regulation 3 | Tonnage Convention 1969, regulation 4 | Not from the Tonnage Convention |
| Formula | GT = K1 · V | NT = K2 · Vc · (4d/3D)² + K3 · (N1 + N2/10) | Loaded displacement − lightship displacement |
| Used for | Which rules apply, manning, registration | Port and canal dues | Commercial size, chartering |
- GT, NT and DWT are three different kinds of number. The trap is to treat them as the same.
- Is GT in tonnes? No. GT and NT are dimensionless indices of volume. Only DWT (and displacement) are weights in tonnes.
- GT decides which SOLAS, MARPOL and manning requirements apply, at thresholds like 500 GT, 3,000 GT and 5,000 GT.
- NT limits: the (4d/3D)² factor is not taken greater than 1, the K2Vc term is not less than 0.25 GT, and NT is not less than 0.30 GT.
- DWT is the cargo plus bunkers, fresh water, stores, lube oil, crew and effects, and ballast. Cargo deadweight is what is left for cargo.
- Suez and Panama Canals use their own tonnage measurements for dues, separate from the 1969 NT.
- GT and NT are on the International Tonnage Certificate (1969), issued by DGMA or its RO under the MS Act 2025.
- As Chief Engineer: every tonne carried as fuel or stores is a tonne not available for cargo.
Full answer
The full answer explains each point step by step, with the regulations and figures behind it and the discussion from other candidates. It is in the MEO Class I section of the Marine One forum, open to members enrolled for the MEO Class I exam.