Builder's Old Measurement
Builder's Old Measurement (BOM or bm) is the method of calculating the size or cargo
Cargo is goods or produce transported, generally for commercial gain, by ship, aircraft, train, van or truck. In modern times, containers are used in most intermodal long-haul cargo transport.-Marine:...

 capacity of a ship
Since the end of the age of sail a ship has been any large buoyant marine vessel. Ships are generally distinguished from boats based on size and cargo or passenger capacity. Ships are used on lakes, seas, and rivers for a variety of activities, such as the transport of people or goods, fishing,...

 used in England from approximately 1720 to 1849. It estimated the tonnage
Tonnage is a measure of the size or cargo carrying capacity of a ship. The term derives from the taxation paid on tuns or casks of wine, and was later used in reference to the weight of a ship's cargo; however, in modern maritime usage, "tonnage" specifically refers to a calculation of the volume...

 of a ship based on length and maximum beam
Beam (nautical)
The beam of a ship is its width at the widest point. Generally speaking, the wider the beam of a ship , the more initial stability it has, at expense of reserve stability in the event of a capsize, where more energy is required to right the vessel from its inverted position...

. The formula is:

  • Length is the length, in feet, from the stem
    Stem (ship)
    The stem is the very most forward part of a boat or ship's bow and is an extension of the keel itself and curves up to the wale of the boat. The stem is more often found on wooden boats or ships, but not exclusively...

     to the sternpost
    A sternpost is the upright structural member or post at the stern of a ship or a boat, to which are attached the transoms and the rearmost left corner part of the stern...

  • Beam is the maximum beam, in feet.

Thus, Builder's Old Measurement estimates the cargo-carrying capacity of a ship in tons, a weight that is also termed deadweight tonnage
Deadweight tonnage
Deadweight tonnage is a measure of how much weight a ship is carrying or can safely carry. It is the sum of the weights of cargo, fuel, fresh water, ballast water, provisions, passengers, and crew...


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
Moorsom System
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...

 was created in Great Britain
Great Britain
Great Britain or Britain is an island situated to the northwest of Continental Europe. It is the ninth largest island in the world, and the largest European island, as well as the largest of the British Isles...

. Instead of calculating deadweight, the Moorsom system calculates the cargo-carrying capacity in cubic feet, a volumetric measurement rather than a weight 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.

History and derivation

The first tax on the hire of ships in England was levied by King Edward I
Edward I of England
Edward I , also known as Edward Longshanks and the Hammer of the Scots, was King of England from 1272 to 1307. The first son of Henry III, Edward was involved early in the political intrigues of his father's reign, which included an outright rebellion by the English barons...

 in 1303 based on tons of burthen. Later, King Edward III levied a tax of 3 shillings on each "tun" of imported wine, equal to £ today (using the last year of Edward III's reign, 1377, as the base year). At that time a "tun" was a wine container of 252 gallons weighing about 2240 lb (1,016 kg). In order to estimate the capacity of a ship in terms of 'tun' for tax purposes, an early formula used in England was

  • Length is the length (undefined), in feet
  • Beam is the beam
    Beam (nautical)
    The beam of a ship is its width at the widest point. Generally speaking, the wider the beam of a ship , the more initial stability it has, at expense of reserve stability in the event of a capsize, where more energy is required to right the vessel from its inverted position...

    , in feet.
  • Depth is the depth of the hold, in feet below the main deck.

The numerator yields the ship's volume expressed in cubic feet.

If a "tun" is deemed to be equivalent to 100 cubic feet, then the tonnage is simply the number of such 100 cubic feet 'tun' units of volume
  • 100 the divisor is unitless, so tonnage would be expressed in 'ft³ of tun'.

In 1678 Thames shipbuilders used a deadweight method assuming that a ship's burden would be 3/5 of its displacement. Since displacement is calculated by multiplying length x beam x draft x block coefficient, all divided by 35 ft³ per ton of seawater, the resulting formula for deadweight would be:

  • Draft is estimated to be half of the beam.
  • Deadweight of cargo is assumed to be 3/5 of the displacement.
  • Block coefficient is based on an assumed average of 0.62.
  • 35 ft³ is the volume of one ton of sea water.

Or by solving :

In 1694 a new British law required that tonnage for tax purposes be calculated according to a similar formula:

This formula remained in effect in until the Builder's Old Measurement rule was put into use in 1720, and then by Parliamentary law in 1773.


  • Depth to deck
The height from the underside of the hull, excluding the keel itself, at the ship's midpoint, to the top of the uppermost full length deck.
  • Depth in hold
Interior space; The height from the lowest part of the hull inside the ship, at its midpoint, to the ceiling that is made up of the uppermost full length deck. For old warships it is to the ceiling that is made up of the lowermost full length deck.
  • Main deck
Main deck, that is used in context of depth measurement, is usually defined as the uppermost full length deck. For the 16th century ship Mary Rose
Mary Rose
The Mary Rose was a carrack-type warship of the English Tudor navy of King Henry VIII. After serving for 33 years in several wars against France, Scotland, and Brittany and after being substantially rebuilt in 1536, she saw her last action on 1545. While leading the attack on the galleys of a...

, main deck is the second uppermost full length deck. In a calculation of the tonnage of Mary Rose the draft
Draft (hull)
The draft of a ship's hull is the vertical distance between the waterline and the bottom of the hull , with the thickness of the hull included; in the case of not being included the draft outline would be obtained...

was used instead of the depth.

External links

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