A bulk carrier is measured in deadweight tonnes. A containership is measured in TEUs. A ro-ro vessel is measured in lane metres. The diversity of measurement systems in shipping reflects the diversity of what ships carry — and understanding each one is essential for anyone working across maritime operations, chartering, or cargo management.
Displacement: The total mass of the ship plus everything it carries, equal to the weight of water displaced — divided into lightweight (the ship itself) and deadweight (cargo, fuel, crew, and consumables).
Deadweight Tonnage (DWT): The maximum weight a ship can carry in cargo and consumables. The primary measure for bulk carriers and tankers.
Gross Tonnage (GT): Based on total internal volume of the vessel — used for port dues, safety regulations, and crew requirements.
Net Tonnage (NT): Based on revenue-earning spaces only (cargo holds, passenger cabins) — used for canal dues and commercial assessments.
TEU: Twenty-foot Equivalent Unit — the standard measure for containership capacity, counting the number of 20-foot containers a vessel can carry.
Lane metres: The primary measure for ro-ro and ro-pax vessels, where 1 lane metre equals 2 square metres of usable vehicle deck space.
How Ship Weight and Capacity Are Measured
When we talk about the weight of a ship, we are usually referring to displacement — the mass of the vessel plus everything aboard it, equal to the weight of water displaced to keep it afloat. This follows Archimedes’ principle directly, and it applies to every floating vessel regardless of type or size. Displacement divides into two components: the lightweight, which is the ship itself including its structure, machinery, and permanent fittings, and the deadweight, which is the carrying capacity — cargo, fuel, ballast, water, provisions, crew, passengers, and consumables. A vessel in lightship condition carries no deadweight items at all.
In practice, different vessel types are measured differently depending on their commercial purpose. Three tonnage concepts underpin most maritime measurement: Deadweight Tonnage (DWT), which measures how much weight a ship can carry; Gross Tonnage (GT), which is based on total internal volume and governs port dues, safety regulations, and manning requirements; and Net Tonnage (NT), which measures only revenue-earning spaces and is used for canal dues and commercial assessments. None of these figures is interchangeable — knowing which one applies requires knowing what type of vessel is being discussed and what the measurement is being used for.
Deadweight tonnage, TEUs, and lane metres are not competing systems — they are purpose-built measures for fundamentally different types of cargo operation. The measure must match the vessel type, and confusing one for another produces figures that are commercially meaningless.
Bulk Carriers
Bulk carriers transport dry commodities — coal, iron ore, grain, bauxite, and similar materials — and are defined almost entirely by their deadweight tonnage. The range is enormous: from small handy-size vessels of around 500 DWT serving regional trades to the largest ore carriers exceeding 300,000 DWT, with some of the world’s biggest bulk carriers approaching 400,000 tonnes. Subtypes within the category — ore carriers, coal carriers, and timber carriers — all use DWT as their primary capacity measure, reflecting the fundamental importance of weight-carrying ability to the economics of dry bulk shipping.
Tankers
Tankers carry liquid cargo — crude oil, refined petroleum products, chemicals, and liquefied gases — and like bulk carriers, their capacity is defined primarily by deadweight tonnage, with reference to GT and NT where regulatory or commercial requirements call for them. The range spans from small product tankers of around 500 DWT serving coastal or inland trades to Very Large Crude Carriers (VLCCs) and Ultra Large Crude Carriers (ULCCs) in the 300,000 to 400,000 DWT range and above. The size classification of tankers — from general purpose through Aframax and Suezmax to VLCC and ULCC — is essentially a DWT-based hierarchy that determines which ports, terminals, and trade routes each vessel can economically serve.
Containerships
Container ships depart from the weight-based measurement convention entirely. Their capacity is expressed in TEUs — Twenty-foot Equivalent Units — the number of standard 20-foot containers the vessel can carry. A ship rated at 10,000 TEU can carry 10,000 standard 20-foot containers, or 5,000 40-foot containers (each counting as two TEUs), or any combination of container sizes up to that total. TEU capacity defines operational scale in container shipping more usefully than tonnage, because the commercial unit being traded is the container slot, not a weight figure. The world’s largest containerships now exceed 24,000 TEU.
Ro-Ro and Ro-Pax Vessels
Roll-on/Roll-off vessels carry wheeled cargo — cars, trucks, trailers, and heavy machinery — that drives or is towed on and off the vessel via ramps. Their capacity is measured in lane metres, where one lane metre equals two square metres of usable vehicle deck space. Total capacity depends on available lane metres and the mix of vehicle sizes loaded for each voyage. Ro-Pax vessels combine ro-ro cargo capacity with passenger accommodation, and their capacity is expressed as a combination of lane metres and maximum passenger numbers — reflecting the dual commercial purpose that defines this vessel type.
Vessel Types and Their Capacity Measures
| Vessel Type | Primary Capacity Measure | Typical Range | Cargo Carried |
|---|---|---|---|
| Bulk Carrier | Deadweight Tonnage (DWT) | 500 – 400,000 DWT | Coal, ore, grain, bauxite, timber |
| Tanker | Deadweight Tonnage (DWT) | 500 – 400,000+ DWT | Crude oil, refined products, chemicals, LNG/LPG |
| Containership | TEU (Twenty-foot Equivalent Units) | 500 – 24,000+ TEU | Containerised general cargo |
| Ro-Ro Vessel | Lane Metres | Varies by deck layout | Cars, trucks, trailers, heavy machinery |
| Ro-Pax Vessel | Lane Metres + Passenger Count | Varies by design | Wheeled cargo and passengers |
While displacement gives the overall weight of a vessel at any given moment, the industry measures that matter commercially — DWT, TEU, lane metres — are purpose-built for each cargo type. This diversity in measurement is not complexity for its own sake; it is the practical expression of how fundamentally different the business of moving ore, oil, containers, and vehicles actually is.
Quick Reference: Cargo Vessel Types
Frequently Asked Questions
What is the difference between DWT, GT, and NT?
Deadweight Tonnage (DWT) measures how much weight a vessel can carry — cargo, fuel, and consumables. Gross Tonnage (GT) is based on total internal volume and governs port dues and safety requirements. Net Tonnage (NT) covers revenue-earning spaces only and is used for canal dues and commercial assessments. None of the three is interchangeable.
Why are containerships measured in TEUs rather than tonnes?
The commercial unit in container shipping is the container slot, not a weight figure. TEU capacity defines how many standard 20-foot containers a vessel can carry, which is the operationally and commercially meaningful measure for scheduling, pricing, and cargo planning.
What is a lane metre?
A lane metre is the standard measure of capacity for ro-ro and ro-pax vessels. One lane metre equals two square metres of usable vehicle deck space. Total ro-ro capacity is expressed as the total lane metres available across all vehicle decks, with actual capacity depending on the mix of vehicle sizes carried.
What determines the size class of a tanker?
Tanker size classes — general purpose, Aframax, Suezmax, VLCC, ULCC — are defined by DWT ranges and, in some cases, by the physical dimensions that determine which waterways, ports, and terminals a vessel can access. The size class is a commercial and operational classification as much as a measurement.
Sources: IMO Convention on Tonnage Measurement of Ships 1969 (London Convention) · UNCTAD Review of Maritime Transport · Lloyd’s Register vessel classification and tonnage guidelines · BIMCO shipping terminology reference
