Builder's Old Measurement

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:

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

where:

• Length is the length, in feet, from the stem to the sternpost;
• Beam is the maximum beam, in feet.

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 (2.8 m³) of capacity per gross ton, resulting in a tonnage expressed in tons.