1 ounce (oz) = 437.5 grains (gr) = 0.0283495231 kilograms (kg) = 28.3495231 grams (g) = 28 349.5231 milligrams (mg) = 0.0625 pounds (lb). In 1958 the US and countries of the Commonwealth (Canada, Australia and New Zealand) defined the mass of the international avoirdupois ounce is defined to be equal to 28.349 523 grams.