Ever wondered why a 20ft shipping container isn’t actually 20 feet long? The answer lies in ISO 668: a global standard for how containers are built and measured. In this article, we’ll look at what ISO 668 is and how it helped shape intermodal transport as we know it.
What is ISO 668?
ISO 668 is an international standard that defines the external dimensions, weight limits, and structural requirements for freight containers used in intermodal transport. First published in 1968 by the International Organization for Standardization (ISO), the regulation established standardized container sizes that made intermodal trade possible and today allows the liner shipping industry to move an estimated $4 trillion in goods each year.
Freight Transport Prior to ISO 668
The story of container transport begins in the 18th century, when standardized wooden boxes were used to move coal within the mining regions of England and Wales. By the early 1900s, this idea had evolved into closed containers—made with wood or iron—that could be transported by rail or road, laying the groundwork for modern intermodal shipping.
The first international container standard appeared in 1933, created by the Bureau International des Containers et du Transport Intermodal (BIC) in a bid to standardize containers used for trade between European countries. This early framework had major limitations by modern standards: containers were not stackable, nor were they designed for intercontinental or intermodal use.
In the 1950s, Malcolm McLean’s Sea-Land containers and advances in telecommunications fueled interest in creating a “shared language” for containerized transport. Container sizes, corner fittings, and designs still varied widely by region, and cargo often had to be unloaded from one container and reloaded into another, adding time, cost, and safety risks to the transport process.
In 1968, ISO Technical Committee 104 (ISO/TC 104) established ISO 668, introducing the uniform container dimensions, fittings, and tolerances that make intermodal transport possible today.
ISO 668 Specifications
ISO 668 sets size and weight requirements for Series 1 containers used in intercontinental traffic, including:
- External dimensions and tolerances
- Minimum door opening sizes (where applicable)
- Maximum gross mass (gross weight) ratings
- Load transfer areas (as of 2005)
These specifications ensure containers are compatible with ships, railcars, chassis, and handling equipment worldwide.
ISO 668 works closely with ISO 1496, which defines internal dimensions, structural strength, and testing criteria. Together, they ensure containers are not only the right size but also strong and safe for stacking, lifting, and transport.
External Shipping Container Dimensions
ISO 668 details the external lengths, widths, and heights of Series 1 freight containers. Like other ISO standards, ISO 668 predominantly details external shipping container dimensions using the metric system, though imperial measurements are now also included. We’ve detailed the most common shipping container dimensions here as a guideline—but please refer to the most recent ISO 668 edition for up-to-date measurements and tolerances.
As defined in ISO 668, all Series 1 containers share a uniform width of 2.438 metres (8 ft). Their lengths, however, are slightly shorter than their nominal names. For instance, the length of a 20ft container is actually 19 ft 10½ in, with a dimensional tolerance of just ¼ inch (more on that below). This allows for twist-lock couplers to connect shorter containers to form exact multiples of 40ft, ensuring that the four outer corner castings align perfectly with containers above or below when stacked.
Dimensional Tolerances
ISO 668 also defines dimensional tolerances for ISO shipping containers. According to the standard, the maximum permitted dimensional tolerances are:
- +0 / -0.25 inches for a 20ft shipping container
- +0 / -0.375 inches for a 40ft shipping container.
These tight tolerances ensure that shipping containers can fit securely on ships and chassis while allowing for minor production differences between manufacturers.
ISO 668:2020 and Related Standards
The latest edition of the standard, ISO 668:2020 (7th Edition), was published in 2020, with a two-page amendment added in 2022. ISO standards are typically reviewed every five years. As a result, ISO 668 is currently under review to ensure it continues to reflect modern shipping practices.
Other relevant ISO container standards include:
- ISO 830: Freight Containers — Vocabulary
- ISO 1161: Series 1 Freight Containers — Corner Fittings — Specification
- ISO 6346: Freight Containers — Coding, Identification and Marking
- ISO 17712: Freight Containers — Mechanical Seals
