To convert a 'regular' ounce to a troy ounce you can multiply by .91146. A troy ounce weighs less than an avoirdupois ounce. This is what most American consider an ounce (16 oz in a pound). To convert a troy ounce into a regular ounce multiply by 1.091.

A troy ounce is slightly heavier than a regular ounce. 1 troy ounce is equal to 1.09714286 ounces. For example, if a piece of silver weighs 5 troy ounces, then you multiply by 1.09714286 to get 5.4857143 ounces.