The troy ounce (troy oz) is the unit of weight traditionally used for precious metals such as platinum. It differs from the more common ounce (oz) used in the UK and US, being slightly heavier.
1 troy ounce = 1.097 oz = 0.0311 kilograms
32.1507 troy oz = 1 kilogram
The troy system of weight is named after the city of Troyes in France, and was widely used in Europe during the Middle Ages. It stopped being commonly used when other systems began to be preferred, continuing only in the specialised fields of precious metals, gems and medicines, up to the nineteenth century. Today it is only used for the trading of precious metals and gemstones.