Food label serving sizes: why grams and ounces both appear

Pick up almost any packaged food sold in the United States and the Nutrition Facts label states weight twice: once in grams, once in ounces, usually as something like “Serv. Size 1 bar (40g)” or “NET WT 12 OZ (340g)”on the front of the package. This isn’t a design choice — it’s a federal labeling requirement, and the two numbers are derived from each other using the exact avoirdupois factor of 28.349523125 grams per ounce.

Why both units are required

The FDA's food labeling regulations require packaged food sold in the US to declare net quantity of contents in both US customary units (ounces, pounds) and metric units (grams, kilograms). The dual requirement exists because the US never completed a full switch to metric for consumer goods, while FDA regulation itself is written with metric as the primary reference for consistency with international food science and nutrition research. The full rules are laid out in the FDA's Food Labeling Guide.

How the two figures are usually rounded

Manufacturers typically set a serving size as a clean metric weight first — 28 g, 30 g, 100 g, 240 g — because that's how the product is formulated and packaged internally. The ounce equivalent shown alongside it is then rounded for readability, which can create small mismatches if you convert it back yourself:

Notice that 28 g rounds up to a clean “1 oz,” even though the exact conversion is 0.988 oz — a small, FDA-permitted rounding for label readability. This is the reason a grams-to-ounces conversion done by hand sometimes disagrees slightly with the number printed on the package.

Net weight vs serving size — two different gram figures

A package shows two distinct weight figures that are easy to conflate: the net weight (total contents of the package, usually printed on the front) and the serving size (one portion, printed inside the Nutrition Facts panel). Both are dual-labeled in grams and ounces, but they answer different questions — net weight tells you how much product you bought in total, serving size tells you how the nutrition numbers on the label are scaled. A 340 g (12 oz) bag of chips might list a 28 g (1 oz) serving size, meaning the bag contains roughly 12 servings; confusing the two figures is a common source of nutrition-tracking error.

When precision actually matters

For everyday shopping, the printed ounce figure is close enough. It matters more if you're tracking nutrition intake precisely (use the gram figure, since it's usually the less-rounded number), scaling a bulk recipe from a packaged ingredient's stated weight, or comparing unit prices across products that print different rounding. In any of those cases, convert from the gram figure directly rather than trusting the printed ounce equivalent — a grams-to-ounces converter applies the exact 28.349523125 g factor instead of a marketing-friendly rounding.