Ounces to grains conversion factor is 437.5 (for troy weight it is 480). To find out how many grains in an ounce, multiply by the conversion factor or use the converter below. 1 Ounce = 437.5 Grains. Ounce is an imperial and U.S. customary mass unit. Troy ounce is used for the measurement of precious metals, gold, silver etc.