Grams to Ounces

The gram and the ounce

The gram is one thousandth of a kilogram, the SI base unit of mass. The kilogram has been defined since 2019 by the Planck constant — a fundamental physical constant — replacing a physical platinum-iridium artifact that had served as the definition for the previous 130 years. The gram, being a derived unit (1/1000 of a kilogram), follows whatever the kilogram is defined as.

The ounce traces to the Roman uncia, a unit equal to one-twelfth of a Roman pound. The modern avoirdupois ounce is one-sixteenth of an avoirdupois pound, and since the 1959 International Yard and Pound Agreement has been defined as exactly 28.349523125 grams. There's also a troy ounce (31.10 g) used for precious metals, but unless you're trading gold, "ounce" means the avoirdupois ounce.

The conversion factor

ounces = grams ÷ 28.3495

Or equivalently, ounces = grams × 0.035274. Both give the same answer to several decimal places. The first is more directly tied to the underlying definition.

A mental shortcut

Divide by 28 instead of 28.35 for a fast approximation. 100 g ÷ 28 = 3.57 oz (actual: 3.53). 500 g ÷ 28 = 17.9 oz (actual: 17.6). The error is well under 2%, fine for kitchen work or casual conversion. For precision, use the full divisor.

When you'd actually use this

Reading metric food labels in an American context is the most common case. A 250 g bar of chocolate is 8.8 oz. A 500 g bag of pasta is 17.6 oz. A 100 g serving size for nutrition information needs to translate to 3.5 oz for US-familiar portions.

International recipes are the close second. A British or European recipe calling for "150 g of flour" or "75 g of sugar" needs translation for US cooks who measure in ounces or cups. Baking is where this matters most — small differences in ingredient ratios can change the final product significantly.

Postal weight on imports and customs declarations is a third area. A package shipped from a metric country lists weight in grams (or kilograms for larger items). US postal infrastructure uses ounces and pounds, so the conversion happens at the customs declaration step or in shipping receipts.

Common mistakes

Mixing up grams and milligrams when scaling down a recipe or supplement. A "100 mg" dose is 0.1 g, not 100 g. The factor of 1,000 between mg and g is easy to lose track of when you're deep in a calculation, especially because both abbreviations look similar.

Misidentifying troy ounces as avoirdupois ounces in precious-metals contexts. A "1 oz gold coin" is a troy ounce — 31.10 g, not 28.35 g. The difference is about 10%, which on gold at several thousand dollars per ounce is hundreds of dollars per coin.