If you are buying, selling or weighing gold or silver: The type of ounce used is the troy ounce, not the avoirdupois ounce. An "ordinary" (Avoirdupois) Ounce is exactly 28.349523125 grams - or 437.5 grains.

A Troy Ounce is exactly 31.1034768 grams - or 480 grains.

Despite being an ancient system, the troy ounce is still in use. Part of the reason for this is that most of the worlds gold bullion bars are still in 400 ozt 'good delivery bars' and it would be a mega-hassle to melt them all down and convert to grams. More and more gold bars are seen in gram sizes nowadays however, such as the 1 gram bullion 'chip gold' bars.