A single twenty-foot equivalent unit is the capacity of a standardized twenty-foot long shipping container
In common use, the twenty-foot equivalent unit (or TEU) is a measure of shipping capacity based upon the volume of a standard twenty-foot long by eight-foot wide by about 8.5-foot tall metal container. The unit is useful measuring shipping capacity at all levels of the shipping/cargo industry including a single ship, a train, an entire fleet of ships, a shipping company, or entire port of commerce.
It should be noted that the number's derivation relative to shipping volume is inexact for two primary reasons:
Further, shipping containers come in myriad sizes and lengths to facilitate large and odd-sized cargo. For example, a ship may need to utilize a container 45 feet long to transport a particularly large piece of cargo. This container would be classified as 2 or 2.25 TEUs (depending on the exactitude of the measurement).
The metric is used primarily in the shipping industry to measure and compare total shipping capacity between companies. Round numbers giving approximations of scale (rather than exact numbers of minutia) are typically preferred as, again, the metric is non-exact.