DWT to TEU conversion: why there is no fixed ratio

There is no DWT to TEU conversion, because deadweight and TEU answer two different questions about a container ship. Deadweight is how much weight she can carry; TEU capacity is how many boxes fit. A conversion would only work if every box weighed the same and every ship spent its weight allowance the same way, and neither is true.

What deadweight measures

The International Maritime Organization defines deadweight as the difference in tonnes between a ship's displacement at her summer load draught and her lightweight, the weight of the empty ship. US regulations list what lightweight leaves out: cargo, fuel oil, lubricating oil, ballast water, fresh water, stores and the people aboard (33 CFR 157.03). So deadweight is everything on board beyond the ship herself, and cargo is only part of it. How much goes to fuel and ballast depends on the design and the voyage.

What TEU capacity measures

A TEU, a twenty-foot equivalent unit, is "equivalent to a 20-foot ISO container" in Eurostat's statistical definition, so a forty-foot box counts as two (what is a TEU). A ship's TEU capacity is now usually the most boxes she can stack. MAN Energy Solutions, which builds container-ship engines, writes that most yards and owners quote capacity as "the maximum number of teu containers that can be stacked on the container vessel independent of the weight of the containers," while "dwt is still a defining parameter." Some owners also publish a second, lower figure: how many boxes she can carry if each weighs 14 tonnes.

A worked example: similar weight, different counts

MPC Container Ships lists nominal TEU, TEU at 14 tonnes and deadweight for each ship in its fleet:

• AS Nara: 3,534 TEU nominal, 2,413 at 14 tonnes, 42,011 tonnes deadweight
• AS Nina: 3,414 TEU nominal, 2,414 at 14 tonnes, 42,285 tonnes deadweight
• AS Carolina: 2,824 TEU nominal, 2,030 at 14 tonnes, 39,155 tonnes deadweight
• Mackenzie: 5,500 TEU nominal, 3,647 at 14 tonnes, 64,300 tonnes deadweight
• NCL Nordland: 1,279 TEU nominal, 900 at 14 tonnes, 18,800 tonnes deadweight

AS Nara and AS Nina carry almost the same deadweight and almost the same 14-tonne rating, yet AS Nara has 120 more slots. AS Carolina has 7% less deadweight than AS Nara but 20% fewer slots. Divide each ship's deadweight by its nominal TEU, our arithmetic from MPC's figures, and the result runs from 11.7 tonnes per slot on Mackenzie to 14.7 on NCL Nordland. That spread across five real ships is the point: a ratio taken from one ship converts nothing on the next.

Why the numbers drift apart

Box weights vary. MAN notes that an average loaded TEU container once weighed about 10 to 12 tonnes, and ships were dimensioned around that, but a box of furniture and a box of tiles fill the same slot at very different weights. Designs also spend deadweight differently: fuel for a long route, ballast for stability, stores and water all come out of the same allowance. Even the regulator does not convert. For container ships, the IMO's energy efficiency design index counts 70% of deadweight as the ship's capacity and treats TEU only as an optional commercial reference.

What the TEU on this site counts

The TEU figures on The Dwell are boxes moved, not ship capacity. The chart above adds up loaded imports across the tracked US ports, month by month. The Port of Los Angeles alone handled 499,552 TEU of loaded imports in the latest month published, counted box by box across its terminals from many ships. A ship's TEU capacity says how many slots she has; a port's monthly TEU says how many boxes crossed its docks. Neither converts to deadweight. For how to read the monthly figures, see reading the monthly TEU release, and for the ranking by port, the busiest US container ports.