The Moorsom System is a method created in Great Britain of calculating the tonnage or cargo capacity of sailing ships as a basis for assessing harbour and other vessel fees. It was put into use starting in 1849 and became British law in 1854.
Previous methods of calculating tonnage, as Builder's Old Measurement, were not being consistently applied and, because they were designed for sailing ships, could not be applied appropriately or fairly for steamships. Substantial portions of a steamship were required for boilers, machinery and coal, thus limiting the proportion of the ship's space available for cargo.
In 1849 Great Britain appointed a Commission with Admiral George Moorsom as secretary to resolve these problems. The Commission determined that fees should be proportional to the earning capacity of the ship, whether for cargo or passengers.
The result was called The Moorsom System, which set forth the rules for the measurement of the internal volume of entire ship.
- The total internal volume was then divided by 100 cubic feet (2.8 m3) to produce the gross tonnage.
- Net tonnage was the volume remaining after subtracting the volume of the space used for machinery and other non-revenue producing functions.
The Commission sought to avoid a significant change in the fees charged to an existing vessel when the new system was implemented.
George Moorsom ordered the entire fleet of British merchant ships to be measured according to the new System and then divided the total gross tonnage by the total registered tonnage.
While the rules for measuring ships changed over the years, the standard of 100 cu ft (2.8 m3) per ton remained in effect until a new system was established by The International Convention on the Tonnage Measurement of Ships, effective for new ships in July 1982.