Britannia silver coins from 1997 to 2011 are .958 Britannia silver & contain 1 troy ounce of silver.
Total weight is 32.45 grams, coin diameter: 40mm, contains one troy ounce or 31.10 grams of silver

2013 to current day, coin weight is 31.1g, Britannia silver coins updated to using .999 silver, Diameter 38.61 mm, whilst still containing 31.1 grams or 1 troy ounce of silver.