## ::Builder's Old Measurement

### ::concepts

Times::length    Tonnage::first    Tonnage::cubic    British::formula    Burthen::capacity    Depth::title

Builder's Old Measurement (BOM or bm) 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{{#invoke:Category handler|main}}, Middle English: byrthen {{#invoke:Category handler|main}}), 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.<ref name=Kemp>{{#invoke:citation/CS1|citation

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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 Great Britain. 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.

Builder's Old Measurement sections