Source: adapted from Container Alliance.
The standard 40-foot container is the most commonly used for shipping goods in ocean freight, along with the 20-foot container. The dimensions are usually measured using the imperial system (feet), and both the exterior and interior (usable) dimensions are specified. The standardization of cargo containers ensures containers can be stacked most efficiently, literally one on top of another. Although freight containers are manufactured worldwide, they are built to specific ISO specifications, allowing for interoperability across intermodal transportation systems.
Many shippers prefer to use the largest load unit possible (e.g. 40-foot high cube containers) because it conveys economies of scale. However, weight restrictions can make the 20-foot a desirable option, particularly for transporting commodities. For instance, the weight restrictions on both the regular and the high-cube forty-footers are at around 30 short tons (or 28 metric tons), virtually the same as a 20-footer, so there are no gains in using a high-cube container for heavy goods. Payload weight decreases even further with 45- and 48-foot containers, underscoring their unique role in transporting bulky yet comparatively lightweight goods. Most containers are made from weathering steel (also known as Corten steel), which helps prevent rusting. They are water-resistant, protecting their contents from water intrusions.
Standardization enabled the expansion of the containerized seagoing transportation of goods. The usage of standardized containers resulted in a vast reduction in shipping costs. It allowed many countries that were previously isolated from global trade to put their products on the world market and consume imported goods.