ISO 668

[2] Introduced in 1968, ISO 668 currently regulates both external and internal dimensions of containers, as well as the minimum door opening sizes, where applicable.

[6] ISO 668 also specifies the respective associated gross weight ratings, and includes requirements for load transfer areas in the base structures of containers, since Amendment 1 of 2005.

[7] The maximum gross mass (MGM) rating of laden 20- and 30-foot length units was notably increased to 30,480 kg (67,200 lb) by Amendment 1 of 2005.

Production statistics show that the vast majority of containers have, for many years, been built with a stacking strength at or above the 213,000 kg[clarification needed] figure stipulated in ISO 1496‑1.

ISO standards require 45‑foot containers to include a second set of four strong vertical columns (like corner posts), manufactured in them, symmetrically at the 40‑foot length position (meaning 2+1⁄2 ft (76 cm) inwards from their actual outside corners), to support being stacked interchangeably with 40‑foot containers.

Load bearing of stacked containers shown is at the 40 ft coupling. The two 20‑foot containers at the bottom are rigidly joined with four twistlocks between them, so that they could also be placed higher in the stack. Note: 48-foot and 53-foot units can only be stacked in a 40-foot compliant stack if they are just 8 feet wide, or have special structural provisions to handle their usual 6 inch (15.2 cm) extra width. This is usually not the case. Not shown is the rare, but also possible combination of a 30-foot container coupled to a 10-foot box, in a 40(+) foot long stack.