Ounces to Grams

The ounce and the gram

The ounce comes from the Latin uncia, meaning "one-twelfth" — a Roman unit that was 1/12 of a Roman pound. The modern avoirdupois ounce is 1/16 of an avoirdupois pound, not 1/12, but the name stuck across the change in fraction. The 1959 International Yard and Pound Agreement fixed the avoirdupois ounce at exactly 28.349523125 grams.

The gram is one thousandth of a kilogram, the SI base unit of mass. Originally defined in 1795 as the mass of one cubic centimeter of water at 4°C (close to but not exactly 1 gram in modern measurements), the gram is now defined as 1/1000 of the kilogram, which since 2019 has been defined by the Planck constant. The chain is: Planck constant → kilogram → gram.

The exact conversion

grams = ounces × 28.3495

Exact to many decimal places (28.349523125 g per oz, by 1959 definition). For everyday purposes 28.35 is fine; for engineering or scientific work, use the full value.

Watch out: avoirdupois vs troy ounces

There are two main ounce definitions still in use. The avoirdupois ounce (28.35 g) is what you'll find on food packaging, postal scales, and almost every everyday measurement. The troy ounce (31.10 g) is used only for precious metals — gold, silver, platinum, palladium. A "1 oz" gold coin is a troy ounce; a "1 oz" chocolate bar is an avoirdupois ounce. The difference is about 10%, which is meaningful for both eating chocolate and trading gold but in different ways.

When you'd use this conversion

Cooking is the most common case. American recipes use ounces for most ingredient weights; international recipes use grams. A British baking recipe that calls for "250 g of butter" maps to about 8.8 oz. Spices and yeast are often given in grams worldwide because the small quantities are awkward in ounces.

Postal and shipping rates are another routine context. US postal rates for letters and small parcels are in ounces; international postal rates and customs forms use grams. A package marked "8 oz" needs to become 227 g for customs declarations.

Pharmaceuticals and supplements use grams almost universally. Even in the US, drug labels report doses in mg (milligrams). A "200 mg" tablet is 0.2 g, or about 0.007 oz — far too small a fraction to express usefully in ounces, which is why pharmacy is metric.

Common mistakes

Confusing ounce (weight) with fluid ounce (volume) is the most frequent error in cooking. A fluid ounce of water happens to weigh almost exactly one ounce, which is the source of the confusion — but a fluid ounce of honey weighs about 1.4 oz, and a fluid ounce of olive oil weighs about 0.9 oz. The units measure different things even when they share a name. If a recipe says "8 oz flour" without specifying, it usually means weight in baking and volume elsewhere; check the recipe's other measurements to figure out which.

Using troy ounces where avoirdupois is meant (or vice versa) is the precious-metals trap. A troy ounce of silver is 31.10 g; an avoirdupois ounce of anything else is 28.35 g. In any coin-dealing or metals-trading context, the ounce is troy unless explicitly stated.