TEU: the unit that measures the container age

A TEU, twenty-foot equivalent unit, is the volume of one standard twenty-foot container. A forty-foot box is two TEU. When MSC Irina is rated at 24,346 TEU, that is the nominal count of twenty-foot slots aboard, stacked up to 26 tiers high.

Nominal is the operative word. The rating assumes every slot filled with a standard, lightly loaded box; real payloads run lower once weight limits, refrigerated containers and the mix of forty-footers intervene. That gap is why laden-load records, like CMA CGM Jacques Saadé departing Singapore with 20,723 full boxes, are reported separately from capacity ratings, and why Maersk once rated Emma Mærsk at 11,000 TEU while the industry called her 15,500: Maersk counted only laden 14-tonne boxes.

The unit's history is the industry's history. Ideal X carried 58 boxes (35-footers, pre-standard) in 1956; the ISO container was standardised in the 1960s; and capacity has doubled roughly every fifteen years since, until now, with the growth curve flattening hard against the 400-metre, 61.5-metre envelope the canals and ports allow.