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.