Ship Dimensions and Tonnage Explained: LOA, GT, DWT and the Rest

Why These Numbers Aren't Interchangeable

Every ship particulars sheet lists a run of figures — length, breadth, draught, then a second cluster of tonnages that don't map directly onto weight or size the way you'd expect. Confusing gross tonnage with deadweight, or LOA with LBP, is an easy mistake with real consequences: port dues, canal tolls, manning scales, and safety equipment requirements are all pegged to specific figures in this list, not to "how big the ship is" in general.

Length

• LOA (Length Overall): the absolute maximum length of the ship, end to end, including any overhanging structure. This is the figure used for berth planning, dry dock sizing, and canal/lock transit limits.
• LBP (Length Between Perpendiculars): measured from the forward perpendicular (through the point where the design waterline meets the stem) to the after perpendicular (typically through the rudder stock centreline). LBP is shorter than LOA and is the figure used in most stability, hydrostatic, and classification calculations — it represents the length that actually matters for how the hull behaves in the water, not the physical extremities.

Breadth and Draught

• Breadth (Beam): the maximum width of the hull, usually quoted "moulded" (to the inside of the shell plating) or "extreme" (outer edge to outer edge, including any rubbing strakes or sponsons).
• Draught: the vertical distance from the waterline to the bottom of the keel. It changes constantly with loading condition — a ship's particulars sheet usually quotes the maximum summer draught, but the actual draught at any given moment (forward, aft, and mean) is what a bridge team works from for under-keel clearance and squat calculations.

The Tonnages — None of Them Are Weight

This is the part that trips people up: despite the name, gross and net tonnage are not units of weight. They're measures of internal volume, defined under the International Convention on Tonnage Measurement of Ships, 1969.

• Gross Tonnage (GT): a dimensionless figure derived from the ship's total enclosed volume. GT is the figure that determines which regulations apply to a vessel — manning scales, SOLAS/MARPOL applicability thresholds, and STCW certificate-of-competency limits are almost all written in terms of GT, not length or deadweight.
• Net Tonnage (NT): also volume-based, representing the ship's revenue-earning space (broadly, cargo and passenger capacity) after deducting non-revenue spaces like machinery and crew accommodation. NT is the figure most commonly used for port dues and canal tolls — the Suez and Panama Canals each use their own modified tonnage calculations derived from NT.

The Weights — Where DWT and Lightship Actually Matter

• Deadweight (DWT): the actual weight-carrying capacity of the ship — cargo, fuel, fresh water, stores, and crew combined — measured in tonnes, at the summer load line. This is the figure charterers and cargo planners care about, and it's a genuine weight, unlike GT/NT.
• Lightship (Light Displacement): the weight of the ship itself, empty — hull, machinery, fittings, permanently installed equipment, with no cargo, fuel, stores, or crew aboard.
• Displacement: lightship plus everything currently aboard (cargo, fuel, stores, crew) — the actual total weight of the ship at any given moment, equal to the weight of water it displaces.

A quick way to hold this together: Displacement = Lightship + Deadweight (at any given loading condition, up to the deadweight's maximum at the load line). GT and NT sit in a completely separate volume-based system and don't convert directly to either figure — a common source of confusion when someone asks "how much does that ship weigh" and gets handed a GT figure that measures something else entirely.

A Worked Example

SEAHAKE, the tanker at the centre of this week's rescue boat davit incident review, gives a real set of particulars to anchor these definitions against: LOA 177.72m, breadth (extreme) 28.09m, maximum draught 11.00m, Gross Tonnage 21,329, Deadweight 32,463 tonnes. Note the GT and DWT numbers aren't close to each other, and shouldn't be — one measures enclosed volume, the other measures carrying capacity by weight. Treating them as roughly the same figure is exactly the mistake this reference exists to prevent.

Test Your Knowledge

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Related Reading

• SEAHAKE — the incident these particulars are drawn from