The troy ounce (oz t) is a unit of imperial measure, now commonly used to measure the mass of precious metals.One troy ounce is defined as exactly 31.10348 g.
The international yard and pound agreement did not define any troy weights. Rather, it defined the avoirdupois pound in metric terms, from which we can derive an exact value for the troy ounce. Specifically, it defined one pound as 0.45359237 kg.
We can derive an exact value for the troy ounce as follows:
1 lb = 0.45359237 kg = 453.59237 grams
An avoirdupois pound also equals 7,000 grains (the international yard and pound agreement did not change the grain values of non-metric units).
1 lb = 7000 grains
7000 grains = 453.59237 grams
1 grain = 453.59237 grams/7000 grains = 0.06479891 grams
1 troy ounce = 480 grains×0.06479891 g = 31.1034768 grams