How Many Grams in an Ounce? | Precise Kitchen Math

One standard avoirdupois ounce equals exactly 28.35 grams when rounded for practical use, and 28.349523125 grams by precise legal definition.

That half-cup of flour or the 4-ounce chicken breast on your scale — the number behind it comes from a 1959 international agreement that standardized weights across the US and UK. Whether you’re halving a British baking recipe, calculating protein macros, or weighing shipping packages, the ounce-to-gram conversion is one of the most useful kitchen and household math facts you can memorize. Here’s exactly how it works, why that odd number exists, and the one common mistake that ruins recipes.

Why 28.35? The Ounce Defined

The number 28.349523125 comes from the definition of the avoirdupois pound: 0.45359237 kilograms exactly. Since one pound contains 16 ounces, dividing that kilogram value by 16 produces 0.028349523125 kilograms — which is 28.349523125 grams. That’s the legally precise figure used by the US National Institute of Standards and Technology (NIST) for official measurements.

For everyday cooking and nutrition labels, 28.35 grams is the accepted standard. The small rounding from 28.349523125 to 28.35 introduces an error of roughly 0.002%, far below anything a kitchen scale could detect. Many recipes and food labels round further to 28 grams, which saves mental math but introduces a 1.2% error — enough to matter when scaling a recipe by weight rather than volume.

The One Critical Exception: Troy Ounces

Not all ounces are the same. Precious metals — gold, silver, platinum, and palladium — use the troy ounce, which equals exactly 31.1034768 grams. That’s nearly 2.8 grams heavier than the standard ounce you use in the kitchen. Confusing the two is a costly error. A 1-ounce gold bar described in troy ounces weighs 31.1 grams, not 28.35. Food, body weight, and everyday retail goods always use the avoirdupois standard; jewelry and bullion always use troy.

Ounces to Grams: The Simple Math

Converting is straightforward multiplication or division. To convert ounces to grams, multiply the number of ounces by 28.35:

grams = ounces × 28.35

To convert grams back to ounces, divide the gram value by 28.35 or multiply by 0.035274:

ounces = grams ÷ 28.35 (or grams × 0.035274)

A kitchen scale that offers both units does this automatically — practical for most home cooks. For precision work like supplement dosing or commercial baking, the exact factor 28.349523125 is worth using.

What This Conversion Does NOT Cover

Fluid ounces are a volume measure, not a weight. One US fluid ounce of water weighs roughly 1.043 ounces (about 29.57 mL in volume), but the conversion changes with every ingredient’s density. A fluid ounce of olive oil weighs less than a fluid ounce of honey. Using the 28.35-gram conversion on a fluid ounce will always produce a wrong result. Always check whether your recipe measures by weight (ounces or grams on a scale) or by volume (fluid ounces in a measuring cup).

Common Ounce-to-Gram Reference Table

The table above uses the 28.35 factor. For most kitchen purposes, these numbers are accurate enough; a quality digital scale reads within a fraction of a gram anyway.