Even with proper cargo-loading and secure stowage of containers aboard ship, factors such as severe weather and rough seas, or more catastrophic and rare events like ship-grounding or collision can result in containers being lost overboard while at sea. The question is: How many? A figure of around 10,000 containers a year is often reported, but according to the National Cargo Bureau, ?there have been no comprehensive statistics kept, as to the number of containers lost overboard.?

The WSC conducted a survey of its members, who represent over 90 percent of global containership capacity. Members were asked to provide the actual number of containers lost overboard for three previous consecutive years. Based on the survey results, the council estimated that on average there are approximately 350 containers lost at sea each year, not counting catastrophic events. Catastrophic losses, wherein 50 or more containers are lost overboard in a single incident, are rare. When catastrophic losses were included , an average total loss per year of approximately 675 containers was observed.

Some carriers reported that they lost no containers during the period, while others noted a catastrophic loss. The number of containers lost in a catastrophic event can vary greatly - from 50 to several hundred.

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