Ship Tonnage Calculator
Enter a hull’s dimensions and block coefficient for displacement and deadweight, and an enclosed volume for gross tonnage — the tonnage basics behind a ship’s size and what she can lift.
Weight, volume, and capacity
Tonnage is one word for several ideas. Displacement is Archimedes at work — the ship weighs exactly the water her hull pushes aside. Take the empty ship out of that and you have deadweight, the earning capacity. Gross and net tonnage step away from weight entirely to measure enclosed space, which is what registries and ports charge against. Keeping the four straight is a rite of passage for every deck and engineer cadet.
Weight is only half of container capacity; the slot side is measured in TEU, over on the Cargo TEU Calculator.
Frequently Asked Questions
Why does a ship have so many different 'tonnages'?
Because they measure different things. Displacement and deadweight are weights (in tonnes): displacement is the weight of the ship, and deadweight is what she can carry. Gross and net tonnage are volumes, not weights — measures of enclosed space used for registration, port dues, and regulations. A single ship can be, say, 50,000 t displacement, 40,000 DWT, and 30,000 GT, and none of those numbers contradict each other.
How is displacement estimated here?
From the block-coefficient volume of the underwater hull times the water density: displacement = length × beam × draft × block coefficient × 1.025 for seawater. The block coefficient (Cb) is how boxy the hull is — around 0.6 for a fine, fast hull and up to about 0.85 for a full-bodied bulk carrier or tanker. It is an estimate; a real figure comes from the ship's hydrostatic tables.
What is deadweight, and how is it found?
Deadweight tonnage (DWT) is everything a ship can carry when loaded to her marks — cargo, fuel, fresh water, stores, and crew. It equals the loaded displacement minus the lightship weight (the empty ship). Enter a lightship figure and the calculator subtracts it from displacement to give DWT. Cargo capacity is a little less than DWT, because fuel and stores use up some of it.
How is gross tonnage worked out?
Modern gross tonnage uses the Moorsom formula GT = K1 × V, where V is the total enclosed volume in cubic metres and K1 = 0.2 + 0.02 × log₁₀(V). It is a pure volume measure with no units. Enter an enclosed volume and the tool applies the formula. The official GT on a ship's Tonnage Certificate comes from a full measurement under the IMO Tonnage Convention, so treat this as an approximation.
Teaching approximations from simplified formulas — not certificated tonnage or a stability assessment. Always work from the ship’s approved hydrostatics and Tonnage Certificate.