Ounce (oz)

Definition

One sixteenth of an avoirdupois pound, exactly 28.349523125 grams.

History

The ounce comes from the Roman uncia, a twelfth of the libra; the modern avoirdupois ounce is instead one sixteenth of the pound. Through the 1959 agreement it works out to exactly 28.349523125 grams.

Where it is used

Ounces are used in the United States for food portions, packaged goods and letter mail, and worldwide for boxing weights and some recipes.

Typical uses include recipe portions, letter mail, packaged food.

Conversion pages

These pages convert ounce to or from another unit in weight.

• Gram to ounce
• Ounce to kilogram

Other Weight units

• Kilogram (kg)
• Gram (g)
• Pound (lb)
• Stone (st)
• Milligram (mg)
• Microgram (mcg)
• Tonne (t)

Sources

• NIST HB 44 NIST Handbook 44, Appendix C §5 (Tables of Equivalents) · Dataset reviewed Nov 11, 2026