Ounce (British tower and London systems)

This page describes an obsolete unit.

The unit described on this page was in use prior to modern methods of precision measurement. It was based on a standard which is no longer available, and which is not capable of being calibrated against modern measurement units. Therefore, the equivalence to modern SI units or to current United States customary units can only be considered approximate.

The British traditional system of units of weight or mass (see Mass versus weight for the distinction) included several different types of pound, two of which were the Tower pound and the London pound. These pounds were divided into ounces, the same ounce being used for both, except that the London pound contained 16 ounces and the Tower pound 12.

One Tower or London ounce contained 450 troy grains.

Relationship between London and tower weights

1 London pound	=	1⅓ tower pounds	=	7,200 troy grains
¾ London pound	=	1 tower pound	=	5,400 troy grains
1 London ounce	=	1 tower ounce	=	450 troy grains
1 London pennyweight	=	1 tower pennyweight	=	22½ troy grains