Tonnage (GT/NT) Calculator

Gross Tonnage depends only on Total Enclosed Volume in this calculator -- cargo volume, draft, depth, and passenger count all affect Net Tonnage instead, not GT. Because the ITC 69 coefficient itself rises with volume, GT grows somewhat faster than Total Enclosed Volume does, not in a straight line.

The IMO's International Convention on Tonnage Measurement of Ships, 1969 (ITC 69) that this calculator follows sets a floor: Net Tonnage must be at least 30% of Gross Tonnage regardless of how small the computed cargo/draft/passenger contribution comes out to. This calculator enforces that floor directly, so a very small Cargo Space Volume or shallow Molded Draft won't push Net Tonnage below that minimum. The ITC 69 convention sets two floors, and this calculator enforces both. The cargo term itself -- K2 x Vc x (4d/3D)^2 -- is never taken as less than 25% of Gross Tonnage, and the final Net Tonnage is never taken as less than 30% of Gross Tonnage.

Increasing Molded Draft never decreases Net Tonnage, but its effect levels off once the draft-to-depth ratio hits this formula's cap -- beyond that point, further increases in draft alone stop adding more Net Tonnage, since the draft/depth factor in the ITC 69 formula is capped rather than unbounded.

No -- treat this as an educational, simplified implementation of the ITC 69 convention for planning purposes only. Official Gross and Net Tonnage figures used for port fees, canal dues, safety equipment requirements, and crew manning rules must come from a certified tonnage survey, not a simplified online calculator.