ISO 1496-1 originally set the shipping container stacking weight limit at 192,000 kg (423,288 lbs) across the four corner posts. A 2005 revision increased the rated capacity to 213,360 kg (470,400 lbs) for new-build containers. Both figures assume a 1.8G dynamic acceleration factor, meaning the container is rated to hold that load even under the rolling, pitching, and heaving forces of ocean transit. On stable ground, the effective safety margin is even larger. //

A two-high stack of empty containers presents roughly 320 square feet of windward surface area on the long side. At 90 mph wind speed, that surface generates approximately 4,100 lbs of lateral force. Without twist-locks anchoring the upper container, friction alone between the flat steel surfaces will not hold.