Gross tonnage (GT)

Gross tonnage (GT) is a dimensionless volumetric measure of a ship’s total enclosed space under the 1969 International Convention on Tonnage Measurement: $GT = K_1 \cdot V$, with $V$ the total enclosed volume in cubic meters and $K_1 = 0.2 + 0.02 \log_{10} V$. It is not a weight. GT drives manning rules, survey regimes, port dues, and the size thresholds in SOLAS, MARPOL, and the IGF Code (for example the 500 GT IGF floor).