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. There are 16 avoirdupois ounces in an avoirdupois pound. One ounce is equal to 437.5 grains. The ounce is commonly used as a unit of mass in the United States, Canada and sometimes in Australia and New Zealand. 1 ounce (oz) = 0.0283495231 kilograms (kg) = 28.3495231 grams (g) = 28 349.5231 milligrams (mg) = 0.0625 pounds (lb).