The Douglas Sea Scale was devised by the English Admiral H. P. Douglas in 1921. Its purpose is to estimate the sea's roughness for navigation. The Douglas Scale consists of two codes, one for estimating the state of the sea (fresh waves attributable to local wind conditions), the other for describing sea swell (large rolling waves attributable to previous or distant winds).

It was difficult to relate the Beaufort wind scale to a ship's features, especially as sails were replaced with the rigid structures of powered ships. The Douglas Sea Scale standardized the many variations being used by ship captains from many nations.