Why are Ocean Freight Containers or Shipping Containers 20 feet and 40 feet long?

Ever wondered why shipping containers are 20 feet and 40 feet long, exactly? Why not 18, 36, or 50? The solution does not reside in port design or international control — but in one potent innovation that shook the world’s supply chains.

In the 1950s, American trucker Malcolm McLean had a problem: cargo loading was slow, inefficient and costly. Merchandise was being processed piece by piece. His groundbreaking idea? A standard metal box that could be loaded once, moved from truck to train to ship without unloading. And so the intermodal shipping container was born — along with the modern global supply chain.

But why was the original standard set at 20 feet?

It was a conscious, pragmatic choice. McLean wanted a size that would:

• Easily fit on a truck chassis, for easy highway transport

• Have no height, length or weight above accordingly through conventional rail cars

• Integrate into the cranes and forklifts of the 1950s, particularly at ports where infrastructure was light

• Find the right balance between volume and weight — neither so large as to be unwieldy, nor so small as to be inefficient

That 20-foot length was a happy medium for efficiency, safety, and versatility. Eventually the 40-foot container was brought on board to maximize volume per crane lift—it was twice the length of a 20-foot container, so scaling up was simple. Both lengths were officially standardized in 1968 by the International Organization for Standardization (ISO). Ports and infrastructure around the world adapted to this new system rather than defining it.

The result? So resilient that today, more than 90% of international trade is carried in containers—typically either 20 or 40 feet long. These containers are the foundation for just-in-time inventory, cross-border e-commerce, multi-modal logistics and every modern supply chain you can think of.

So the next time you encounter a shipping container, keep in mind — it’s more than just a steel box. This is the DNA of global trade, cast in a daring, pragmatic vision.