# Builder's Old Measurement

## Measurement of the internal volume of a sailing vessel (c. 1650–1849) / From Wikipedia, the free encyclopedia

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**Builder's Old Measurement** (**BOM**, **bm**, **OM**, and **o.m.**) is the method used in England from approximately 1650 to 1849 for calculating the cargo capacity of a ship. It is a volumetric measurement of cubic capacity. It estimated the tonnage of a ship based on length and maximum beam. It is expressed in "tons **burden**" (Early Modern English: **burthen**, Middle English: * byrthen*), and abbreviated "tons bm".

The formula is:

- ${\text{Tonnage}}={\frac {({\text{Length}}-({\text{Beam}}\times {\frac {3}{5}}))\times {\text{Beam}}\times {\frac {\text{Beam}}{2}}}{94}}$

where:

*Length*is the length, in feet, from the stem to the sternpost;*Beam*is the maximum beam, in feet.[1]

The Builder's Old Measurement formula remained in effect until the advent of steam propulsion. Steamships required a different method of estimating tonnage, because the ratio of length to beam was larger and a significant volume of internal space was used for boilers and machinery. In 1849, the Moorsom System was created in the United Kingdom. The Moorsom system calculates the cargo-carrying capacity in cubic feet, another method of volumetric measurement. The capacity in cubic feet is then divided by 100 cubic feet of capacity per gross ton, resulting in a tonnage expressed in tons.

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