ISO 668

ISO 668 – Series 1 freight containers – Classification, dimensions and ratings is an ISO international standard which nominally classifies intermodal freight shipping containers, and standardizes their sizes, measurements and weight specifications.

The current version of the standard is the Seventh edition (2020), which integrates version E. The standard was prepared by Technical Committee ISO/TC 104: Freight containers, Subcommittee SC 1: General purpose containers.

Introduced in 1968, ISO 668 currently regulates both external and internal dimensions of containers, as well as the minimum door opening sizes, where applicable. Minimum internal dimensions were earlier defined by ISO standard 1894: 'General purpose series 1 freight containers – Minimum internal dimensions'. Its second edition appeared in 1979, but was withdrawn, once revised by ISO 1496‑1 of 1990. The current version of ISO 1496‑1 is 2013, including Amendment 1 of 2016, last reviewed and reconfirmed in 2019.

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. Amendment 2 of 2005 then also standardized 45 ft (13.7 m) length containers.

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. Until then, the MGM for 20- and 30-foot boxes was 24,000 kg (52,910 lb), and 25,400 kg (56,000 lb) respectively.

However, since Amendment 2 of 2016, the maximum gross mass for ISO-standard (Series 1) containers of all sizes, (except 10‑foot units), has until now been further increased to a maximum of 36,000 kg (79,370 lb). Draft Amendment 1 of ISO 668: 2020 – for the eighth edition – maintains this. This is remarkably heavy, given that U.S. trucks, including the weight of the truck itself, are federally limited to a max gross mass of 80,000 lb (36,290 kg) – without special transport permits and regimes.

A separate standard is set for the required stacking strength, or 'maximum superimposed mass' (MSM) for standard containers. The ISO standard for Series 1 containers, ISO 1496‑1, established this, among other characteristics, for many years set at 192,000 kg (423,300 lb). However, in order to keep pace with the increase of container maximum gross weight, the continuing growth in container ship size, and the related height of container stacks on board the ships, the required stacking strength was increased to a superimposed weight of 213,360 kg (470,400 lb) in 2005 per Amendment 3. This value was since maintained in the latest revision of Standard 1496‑1 (2013). 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 figure stipulated in ISO 1496‑1. In fact, most major container operators and lessors now cause containers to be built above the required figure, with the most common superimposed strength being 216,000 kg (476,200 lb).