Ship Measurements Explained: LOA, Beam, Draft, Displacement, Tonnage, and TEU
Published: July 6, 2026
About This Guide
This guide is written for maritime professionals who need precise, verifiable ship measurement reference data: draft surveyors, port planners, cargo agents, charterers, and marine operations staff. Every measurement definition follows IMO conventions and classification society standards. Real ship examples draw from DNV and Lloyd's Register vessel data. Where values are approximate, they are marked with "~". This guide was prepared by GOTEC's maritime measurement team and last updated July 2026.
Ask someone "how big is that ship?" and you will get different answers depending on who you ask. A naval architect will tell you the LOA. A port planner wants the draft. A cargo agent asks how many TEU. A charterer thinks in deadweight tonnes. They are all describing the same vessel, but each measurement tells a different part of the story.
Here is what every measurement means and when you need it.
Ship Dimensions: The Basics
Ships are measured in several different ways, each for a different purpose. There is no single "size" number. You pick the measurement that matches the job.
| Purpose | Key Measurements |
|---|---|
| Navigation | Draft, air draft, beam |
| Cargo Capacity | Deadweight (DWT), TEU, hold capacity |
| Regulation | Gross tonnage, net tonnage, LOA |
| Design and Stability | LBP, LWL, moulded depth, block coefficient |
| Measurement | What It Tells You | Units |
|---|---|---|
| LOA | Total length from bow tip to stern tip | Meters (m) |
| LBP | Length between perpendiculars (design length) | Meters (m) |
| Beam | Width at the widest point | Meters (m) |
| Draft | Depth of the keel below the waterline | Meters (m) |
| Depth | Height from keel to main deck (fixed) | Meters (m) |
| Air Draft | Highest point above waterline | Meters (m) |
| Displacement | Actual weight of the ship (water displaced) | Tonnes (t) |
| DWT | Carrying capacity by weight | Tonnes (t) |
| GT | Total internal volume (not weight) | Dimensionless |
| TEU | Container capacity | TEU count |
Length: LOA, LBP, and LWL
LOA (Length Overall)
LOA is the full length from the tip of the bow to the tip of the stern. It includes every protrusion: bulbous bow, stern platform, everything. This is the number port authorities use for berth allocation. It is also the number pilots keep in mind when maneuvering.[1]
The maximum LOA for the Panama Canal new locks is 366 meters. Ships designed to transit Panama stop at exactly this length. A Neopanamax container ship typically has an LOA of 366 meters, right at the limit. Practical note: as a draft surveyor, the LOA is the first figure you check against the berth dimensions. If the ship's LOA exceeds the allocated berth length, the berthing plan must be revised before any survey work begins.
LBP (Length Between Perpendiculars)
LBP runs from the rudder post to the forward perpendicular, where the design waterline meets the stem. Naval architects use LBP for hydrostatic calculations: stability, trim, and hull strength. It is the engineer's length, not the operator's.[2]
For that same Neopanamax container ship with 366 meters LOA, the LBP is about 350 meters. The difference comes from the bulbous bow extending forward of the forward perpendicular.
LWL (Length at Waterline)
LWL is the length of the hull at the current waterline. It changes with draft. At a deeper draft, the waterline length is typically longer because the bow and stern flare outward above the water.[2]
LWL matters for speed. A ship's hull speed is proportional to the square root of LWL. A longer waterline means a faster ship can go before hitting the wall where power requirements spike. At design draft, a Neopanamax container ship has an LWL of about 358 meters.
Real example: A Neopanamax container ship: LOA 366m, LBP 350m, LWL 358m at design draft. Three different "lengths" for the same ship, each used for a different purpose.
Beam (Breadth)
Beam is the width of the ship at its widest point. Two versions exist. Moulded beam is the width inside the hull plating. Extreme beam includes the plating and any protrusions like fenders. The difference is usually a few centimeters, but it can matter in a tight lock.[3]
Why Beam Matters
Wider ships are more stable. A ship's initial stability increases with the cube of its beam. Beam also determines canal transit limits. The Panama Canal new locks allow a maximum beam of 51.25 meters (increased from the initial 49-meter limit in 2018). Any ship wider than that must go around Cape Horn or through Suez.[4]
The Suez Canal has no explicit beam limit. Depth is the constraint there. But practical beam is limited by meeting and crossing situations with other vessels.
How Beam Has Grown
Container ship beam has followed canal expansion. Panamax ships were limited to 32.3 meters, the width of the original Panama Canal locks. When the new locks opened in 2016, the Neopanamax limit was set at 49 meters, then increased to 51.25 meters in 2018 to accommodate ships with 20 rows of containers. The latest Ultra Large Container Vessels (ULCVs) reach 61.3 meters, like the MSC Irina delivered in 2023.
Beam in survey work: when conducting a draft survey on a wide-beam vessel like a ULCV (61.3 meters), reading draft marks on both sides requires either a small boat or access from the dock and opposite side by launch. For vessels with beam over 50 meters, port-side and starboard-side draft readings at midship can differ by several centimeters due to list, even on a calm day.
Comparison: Panamax 32.3m beam, Neopanamax 51.25m, ULCV 61.3m. Each generation added about 10 to 19 meters of width.
Draft (Draught)
Draft is the vertical distance from the waterline to the lowest point of the keel. It is the single most operationally important number on any vessel. A ship cannot go where the water is shallower than its draft.
Draft changes constantly. It shifts with cargo load, ballast water, water density, and water temperature. A ship moving from seawater to fresh water sinks deeper. A ship that loaded cargo has more draft than one sailing empty. These are daily realities for anyone working with vessels.
Key Draft Terms
- Design draft: The draft the ship was designed to operate at. All performance calculations assume this draft.
- Scantling draft: The maximum structural draft. Go deeper than this and the hull experiences stresses beyond its design limits.
- Loaded draft: The draft with full cargo. For a Capesize bulk carrier, this can be 18 meters or more. For a fully loaded ULCC like the TI Europe, 24.5 meters.
- Ballast draft: The draft when the ship is empty and carrying ballast water for stability. Much shallower, typically 7 to 10 meters.
Draft is measured in meters and decimeters in metric. Older vessels and US-flagged ships may still use feet and inches. The Panama Canal new locks allow a maximum draft of 15.24 meters (50 feet) in tropical fresh water (TFW, density 0.9954 t/m3 at 29.4 degrees C).[4]
Read our complete guide to ship draft for a full explanation of how draft is measured, why it varies, and what different draft marks mean.
Depth and Freeboard
Moulded Depth
Moulded depth is a fixed construction dimension. It runs from the top of the keel plate to the top of the main deck beam at the side. Unlike draft, depth never changes. The shipyard cuts it into steel.
On a typical Handymax bulk carrier, moulded depth is about 16 meters. On a large container ship like the MSC Irina, it is 33.5 meters.
Freeboard
Freeboard is the distance from the waterline to the main deck. It is simply depth minus draft. Freeboard is your safety margin above the water.
The International Load Line Convention (ILLC 1966) assigns every ship a minimum freeboard. This minimum depends on the ship type, construction, and the zones where it operates. More freeboard means a safer ship in heavy weather, but it also means less cargo capacity. Ship owners balance safety against economics.
Summer freeboard on a bulk carrier is typically 4 to 6 meters. On a fully loaded tanker, it can be as little as 3 meters, which is why tankers in ballast ride so high out of the water. Practical note: when a tanker arrives in ballast with 8 meters of freeboard, boarding by gangway requires a long ladder, and the surveyor must check that the accommodation ladder reaches the dock safely.
Learn how the Plimsoll line and load line marks work on every ship.
Air Draft and Bridge Clearance
Air draft is the height of the highest point on the vessel above the waterline (usually the mast, funnel, or antennas). Bridge clearance is the vertical distance from the water surface to the underside of a bridge. The difference between bridge clearance and air draft is the safety margin. If air draft exceeds bridge clearance, a bridge strike occurs.[5]
What Changes Air Draft
- Ship's draft: Less draft means the ship sits higher, so air draft increases. A Neopanamax container ship at 12-meter ballast draft has more air draft than the same ship at 15-meter loaded draft.
- Tide: A high tide reduces the clearance under a bridge. For a ship with 55 meters of air draft approaching a bridge with 60 meters of clearance at low tide, the window is narrow. A half-meter underestimate can cause an allision.
- Deck cargo: Container ships with high deck stacks have the most air draft concerns. A stack of nine containers on deck can push air draft well past 55 meters.
Key Bridge Clearances Worldwide
| Bridge | Clearance (meters) | Notes |
|---|---|---|
| Panama Canal, Bridge of the Americas | 61.3 | At high tide. Limiting bridge on the Panama route. Clearance is 61.3m at MHW, not exactly 61m as sometimes rounded. |
| Bosporus Bridge (Istanbul) | 64 | All three Bosporus bridges share this clearance. Key constraint for Black Sea traffic. |
| Oresund Bridge (Denmark/Sweden) | 57 | Limits Baltic Sea access. Most large ships use the Drogden Strait channel instead. |
| Bayonne Bridge (New York) | 66 | Raised in 2017-2019 from original 46m (151ft) to 66m (215ft) for Neopanamax ships. Coast Guard certified clearance in June 2017. |
Bridge clearance in port operations: when a large container vessel approaches a berth upriver, the pilot checks the air draft against the lowest bridge on the route. For the port's marine operations team, this must be verified for each arrival. A vessel that passed safely on arrival at ballast draft may not clear the same bridge on departure fully loaded.
Draft and Under Keel Clearance (UKC)
The draft measurement has a direct safety consequence: under keel clearance, or UKC. This is the vertical gap between the lowest point of the ship's keel and the seabed. UKC = Charted Depth + Tide Height - Ship Draft - Squat Effect - Wave Allowance. When draft is measured imprecisely, UKC calculations become unreliable. The Ever Given grounding in the Suez Canal (March 2021) brought UKC into the public spotlight: a 400-meter vessel at 15.7 meters draft in a 24-meter channel should have had adequate clearance on paper, but squat effect at 13 knots and bank suction reduced the effective margin to near zero. Real-time, accurate draft measurement is the foundation of safe UKC management.
Port Hedland, Western Australia, is the world's largest iron ore export port. Fully loaded Valemax vessels with 23-meter drafts can only enter or leave the harbor on spring high tides, when water depth in the approach channel reaches approximately 19 meters plus tidal height. A missed tidal window means waiting 12 to 24 hours. At demurrage rates of $20,000 to $50,000 per day for a large bulk carrier, missing a window is expensive. Accurate draft measurement, knowing exactly how deep the vessel sits, is the difference between making the window and paying for an extra day at anchor.
Displacement and Deadweight
Displacement
Displacement is the actual weight of the ship. It equals the weight of water the ship pushes aside (Archimedes' principle). If a ship displaces 100,000 cubic meters of seawater at density 1.025 t/m3, it weighs 102,500 tonnes.[6]
Two versions of displacement matter in practice:
- Lightweight displacement: The empty ship. Hull, machinery, permanent equipment. Nothing else. For a Capesize bulk carrier, this is about 25,000 tonnes.
- Loaded displacement: The ship with cargo, ballast, fuel, fresh water, stores, and crew aboard. The sum of lightweight plus deadweight.
Deadweight (DWT)
Deadweight is the weight a ship can carry. It is the difference between loaded and lightweight displacement.[6]
DWT = Loaded Displacement minus Lightweight Displacement
DWT includes cargo, ballast water, fuel oil, diesel oil, lubricating oil, fresh water, stores, provisions, crew, and their effects. Everything that is not the ship itself.
Deadweight Cargo Capacity (DWCC)
DWCC is the number cargo interests actually care about. It is DWT minus fuel, water, stores, and the ship's constant. DWCC tells you how much cargo can go on the ship for a specific voyage.
DWCC = DWT minus Fuel minus Water minus Stores minus Constant
Real example: A Capesize bulk carrier with 180,000 DWT and 25,000 tonnes lightweight has a loaded displacement of 205,000 tonnes. On a typical voyage with 3,000 tonnes of fuel and water aboard, the DWCC is 177,000 tonnes of cargo.
Water Density Warning
Seawater weighs 1.025 tonnes per cubic meter. Fresh water weighs 1.000 tonnes per cubic meter. Brackish water in ports and estuaries falls between these values. A ship displacing 100,000 cubic meters weighs 102,500 tonnes in seawater but only 100,000 tonnes in fresh water. The volume is the same. The weight is different. This is why ships float deeper in fresh water and why draft surveys must correct for water density at the berth.[7]
Read our full guide to displacement and tonnage for a deeper dive into these calculations.
Gross Tonnage and Net Tonnage
This is the most common confusion in maritime. Gross tonnage and net tonnage are volume measurements, not weight. They have nothing to do with mass.
Gross Tonnage (GT)
GT measures the total internal volume of all enclosed spaces on the ship. Under the IMO International Convention on Tonnage Measurement of Ships, 1969, GT is a dimensionless index calculated as:
GT = K times V, where V is total moulded enclosed volume in m3, and K = 0.2 + 0.02 times log10(V)
In the older Moorsom system (replaced by the 1969 Convention, but still referenced colloquially), 1 register ton equaled 100 cubic feet (2.83 cubic meters). The 1969 Convention deliberately moved to a dimensionless formula-based system to avoid confusion between volume-based "tons" and weight-based tons.[8]
GT is the "size" used for registration, port dues, canal tolls, pilotage fees, manning requirements, and safety regulations. A ship's GT generally stays fixed throughout its life because the enclosed volume is built into the steel. GT only changes if the ship undergoes substantial structural modifications (such as adding new enclosed decks or extending the hull), which triggers a re-measurement by the flag state administration under Article 3(2)(b) of the 1969 Convention.[8]
Oasis-class cruise ships: approximately 225,000 GT (Oasis of the Seas was 225,282 GT at delivery, increased to 226,838 GT after its 2019 refit added cabins). MSC Irina (24,346 TEU container ship, delivered 2023): 236,184 GT. Two completely different types of ship, similar GT, because they both have enormous enclosed volumes.
Net Tonnage (NT)
NT measures the volume of cargo spaces only, the revenue-earning spaces. Some port and canal dues use NT instead of GT. NT is always smaller than GT because it excludes engine rooms, crew accommodation, and navigation spaces.
Why Tonnage Matters in Practice
Panama Canal tolls for a Neopanamax container ship can exceed $500,000 per transit and routinely reach $800,000 to $1.1 million for larger vessels with high load factors. The toll calculation uses GT and container TEU capacity as inputs. A few thousand GT either way changes the bill by thousands of dollars. This is why shipowners know their GT to the last digit.[9]
TEU Capacity
TEU stands for Twenty-foot Equivalent Unit. It is the standard unit for measuring container ship capacity. A TEU is based on a standard 20-foot ISO container: 6.1 meters long, 2.44 meters wide, and 2.59 meters high (ISO 668 external dimensions).[10]
A 40-foot container (12.2 meters) counts as 2 TEU. A 45-foot container also counts as 2 TEU in most calculations, even though it is longer.
Container Ship Capacity Evolution
| Year | Ship | TEU Capacity | Note |
|---|---|---|---|
| 1956 | Ideal X | 58 | First purpose-deployed container ship (converted T2 tanker). Carried 58 containers of 35-foot length (roughly 100 TEU in modern equivalent) |
| 1980s | Typical Panamax | ~4,000 | Limited by original Panama Canal locks |
| 2006 | Emma Maersk (E-class) | ~14,770 | Original capacity; upgraded to ~15,550 TEU in 2016 |
| 2023 | MSC Irina | 24,346 | Currently among the largest container ships by TEU capacity |
TEU capacity does not mean the ship actually carries that many boxes. Weight limits almost always kick in before volume limits. A ship might have space for 24,000 TEU but hit its deadweight limit at a lower number if the containers are heavy. Surveyor's note: when calculating cargo loaded for a draft survey, the stowage plan shows TEU counts, but the draft survey measures actual weight via displacement change. Sometimes the two numbers disagree, and the surveyor must reconcile the difference.
Read our TEU glossary entry for more detail on how TEU is calculated and used.
Putting It All Together
Different ship types optimize different dimensions. Here is how four real ship classes compare across all measurements.
| Measurement | Oasis-Class Cruise | MSC Irina (Container) | Vale Brasil (Bulk) | TI Europe (Tanker) |
|---|---|---|---|---|
| Type | Cruise ship | Container ship | Bulk carrier | Oil tanker |
| LOA | 361.8m | 399.99m | 362m | 380m |
| Beam | 47m (wl) / 60.5m (extreme) | 61.3m | 65m | 68m |
| Draft (loaded) | 9.3m | 16.5m | 22-23m | 24.5m |
| DWT | ~15,000 t | ~240,000 t | ~400,000 t | ~440,000 t |
| GT | ~225,000 | 236,184 | ~199,000 | ~234,000 |
| TEU | N/A | 24,346 | N/A | N/A |
| Optimized For | Passenger volume | Container count and speed | Iron ore deadweight | Crude oil deadweight |
Look at the patterns. Cruise ships have shallow draft and wide beam for stability with tall superstructures. Container ships push LOA to the limit for more slots. Bulk carriers and tankers maximize draft and deadweight because their cargo is heavy. A Vale Brasil Valemax carries 400,000 tonnes of iron ore. No container ship comes close to that DWT.
Each ship type tells you which numbers matter most to its owner.
FAQ
What is the difference between LOA and LBP?
LOA is the full length from bow tip to stern tip, including all protrusions. Port authorities use it for berth allocation. LBP is the length between perpendiculars, measured from the rudder post to where the design waterline meets the stem at the bow. Naval architects use LBP for hydrostatic calculations like stability and trim. On a 366-meter LOA Neopanamax ship, LBP is about 350 meters.
Is gross tonnage the same as displacement?
No. Gross tonnage measures internal volume of all enclosed spaces. Under the IMO 1969 Convention, GT is a dimensionless index based on moulded volume. Displacement measures the actual weight of the ship, equal to the weight of water it pushes aside. A cruise ship and a container ship can both be around 230,000 GT but have completely different displacements.
How does air draft affect ship routing?
Air draft determines whether a ship fits under bridges and power lines on its route. A ship at ballast draft has more air draft than the same ship fully loaded, because it sits higher in the water. A container ship with 55 meters of air draft needs at least that much clearance under every bridge on its planned route. Tide timing matters too. The Panama Canal Bridge of the Americas has 61.3 meters of clearance at high tide. The Bosporus Bridge has 64 meters. A ship with 65 meters of air draft cannot use either route.
Why do cruise ships have such high gross tonnage?
Cruise ships pack enormous enclosed volume into every deck: passenger cabins, multi-deck atriums, theaters, restaurants, swimming pools, and shopping arcades. Gross tonnage measures this total enclosed volume. An Oasis-class ship achieves about 225,000 GT on a 9.3-meter draft and 47-meter beam. The MSC Irina container ship hits 236,184 GT on a 16.5-meter draft and 61.3-meter beam. Same GT order of magnitude, completely different ships.
References and Further Reading
IMO Conventions and Standards
- IMO. International Convention on Tonnage Measurement of Ships, 1969. Adopted 23 June 1969, entered into force 18 July 1982. IMO Convention Page. Full text: UN Treaty Series Vol. 1291.
- IMO. International Convention on Load Lines, 1966 (as amended). Establishes minimum freeboard and load line marks.
- ISO 668:2020. Series 1 freight containers: Classification, dimensions and ratings. International Organization for Standardization.
Classification Societies & Vessel Registers
- DNV. Vessel Register and Class Status. DNV Maritime Portal. Vessel specification data can be verified at www.dnv.com.
- Lloyd's Register. Ships in Class online register. LR Ships in Class. Historical registers available at LR Foundation Heritage Centre.
- China Classification Society (CCS). CCS Official Website. Online customer service center at ccs-service.net.
Port Authorities & Canal Operations
- Panama Canal Authority (ACP). Maritime Services and Advisory to Shipping. ACP Maritime Services. OP Notice N-1: Vessel Requirements.
- Port Authority of New York and New Jersey. Bayonne Bridge Navigational Clearance Project. Completed June 2019. PANYNJ Official Site.
Additional Technical References
- Tupper, E.C. Introduction to Naval Architecture (5th ed., 2013). Butterworth-Heinemann. ISBN: 978-0-08-098237-3.
- U.S. Coast Guard. 46 CFR Part 69. Tonnage measurement systems: Convention, Standard, Dual, and Simplified. eCFR Title 46 Part 69.
- NIST Handbook 44 (2023 Edition). Appendix B: Units and Systems of Measurement; Appendix C: General Tables of Units of Measurement. NIST HB 44-2023.
- VesselFinder / MarineTraffic. Real-time AIS vessel data for verifying ship positions and dimensions. VesselFinder.
In-Text Citations
- [1] Panama Canal Authority. OP Notice N-1. Vessel Requirements for Neopanamax locks: max LOA 366m, max beam 51.25m (revised 2018).
- [2] Tupper, E.C. (2013). Introduction to Naval Architecture. Definitions of LBP, LWL, and their use in hydrostatic calculations.
- [3] Classification society rules (DNV, LR, CCS) distinguish moulded breadth (inside plating) from extreme breadth (including shell plating and protrusions).
- [4] Panama Canal Authority. Tropical Fresh Water (TFW) density = 0.9954 t/m3 at 29.4 degrees C. Maximum TFW draft in Neopanamax locks: 15.24m.
- [5] US Coast Guard. Marine Safety Alert 09-14. Bridge allision risks from improper air draft calculation.
- [6] IMO (1969). Displacement is the weight of water displaced by the ship's underwater volume (Archimedes' principle). DWT = loaded displacement minus lightweight.
- [7] Standard seawater density 1.025 t/m3; fresh water 1.000 t/m3. Brackish port water varies between these values depending on river outflow and tidal mixing.
- [8] IMO International Convention on Tonnage Measurement of Ships, 1969. GT is dimensionless; the older register ton of 100 cu ft (2.83 m3) has been superseded. Article 3(2)(b): GT changes only with substantial structural alterations.
- [9] Panama Canal Authority toll schedule (2025-2026). Neopanamax container ship: fixed fee $300,000 + $30 per TEU capacity + $45 per loaded TEU.
- [10] ISO 668:2020. External dimensions of 1 TEU container: 6,058mm x 2,438mm x 2,591mm (20' x 8' x 8'6").
Data verification note: all vessel specifications, measurements, and bridge clearance figures were reviewed against classification society records, AIS databases, port authority publications, and shipbuilders' data in July 2026. Draft figures should be taken as design/scantling values; operational drafts vary with loading condition.
Need to Measure Your Vessel Accurately?
GOTEC builds AI measurement systems for vessel draft and dimensions. Our technology reads draft marks, calculates displacement, and verifies vessel measurements with precision that manual methods cannot match.
Explore GOTEC products or contact our team for a measurement consultation.