An ounce (oz) is a unit of weight/mass in the imperial and U.S. customary systems of measurement. It is not to be confused with the fluid ounce, a unit of volume, which also uses the same abbreviation "oz." These are separate, unrelated units that cannot be used interchangeably. For example, 1 fluid ounce of a liquid, in most cases, does not weigh 1 ounce. This is because we need to take density into account.
The ounce has had many different definitions over the course of history. The name "ounce" is derived from the Latin uncia, which was of a Roman pound. Today, the ounce is mostly used in the United States, as well as some other countries with past British influence.
In the United Kingdom, the ounce is no longer a legal unit of measure, but it is still used on an informal basis. The definition of the ounce used in both systems is the avoirdupois ounce, which is based on the avoirdupois pound (lb). Use of "ounce" and "pound" on this page refer to the avoirdupois measures unless otherwise specified.
Most other countries more commonly use grams (g) and kilograms (kg). Kilograms are the base unit of weight/mass in the International System of Units (SI).
Below are some equivalences between ounces and other common measurements of weight:
16 oz = 1 lb
1 oz = 28.349523125 g = 0.028349523125 kg
Did you know?
A hen's egg, such as one you might eat for breakfast, weighs about 2 ounces. A hummingbird's egg, however, is very tiny. It weighs about of an ounce.