Old Measurement - om this is sometimes seen written as GRT and applies to the measurements of ships built, registered or surveyed prior to 1 January 1836, and in particular to merchant ships of the British Empire.

In om terms a ship was measured for overall length, from fore side of the stem to aftside of the sternpost, and for the beam - outer edge to outer edge across the widest part of the ship. The depth was an inside measurement of the depth of the hold for a single decked vessel, and the total of the space between decks for a multi decked vessel. In old measurement, the dimensions are given in feet and inches - eg. 108' 6" x 22' 3" x 16' 8"

New Measurement - nm this is sometimes seen written as NRT, and came into effect on 1 January 1836 when the method of recording the measurements of tonnage and other dimensions of British merchant ships was changed, giving way to the term "New Measurement".

In nm terms overall length, beam and depth of ships were all measured from the inner edges of the hull ie. inner edge to inner edge from stem to stern for length, inner edge to inner edge at the widest part of the ship for beam, and an overall depth from the top of the gunwales to base of the hold. Dimensions in new measurement terms are given in decimals of a foot, but this is often not quite correct eg. 108.2' x 23.0' x 16.4'.

Tonnage - in some records, ships are recorded giving their overall weight when empty and in others they have the words "burthen" or "burden" or "laden" appearing with the weights. In this case, it means how much the ship weighed when fully loaded.