The troy ounce is the unit in which gold, silver and platinum are quoted. NIST's conversion factors list it as 31.10348 g, against 28.34952 g for the avoirdupois ounce on a kitchen scale.
Conversion
g = troy oz × 31.10348
troy oz = g ÷ 31.10348
Example
A 100 g bar is 100 ÷ 31.10348 = 3.215 troy ounces, but 100 ÷ 28.34952 = 3.527 ordinary ounces. One troy ounce is 31.10348 ÷ 28.34952 = 1.097 ordinary ounces.
The grams in an ounce guide compares them, and how many ounces in a pound covers the common pound. Use oz to grams for the food ounce only.
Troy ounce vs avoirdupois ounce
The two ounces share a name but not a size. 31.10348 − 28.34952 = 2.75 g, so 10 troy ounces weigh 311.03 g while 10 ordinary ounces weigh 283.50 g. Because the troy ounce is the heavier unit, converting a metal price or weight with the food ounce understates the mass by about 8.9% (28.34952 ÷ 31.10348 = 0.911).
Common mistakes
Using the 28.35 g factor for gold or silver, and mixing up the troy ounce with the fluid ounce, which measures volume (29.57 mL) rather than mass.