Insights
How nutrition labels actually work
Explanations of how US nutrition labels work: Daily Values, FDA rounding rules, RACC serving sizes, the gap between a rounded and unrounded %DV, and the label mechanics that tie them together. Every number is traced to the exact provision of 21 CFR it comes from, and dated to the regulation text it was read from — so you can check it, cite it, and know when it last changed.
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Do the calories in USDA's branded food data add up?
5,953 branded foods in USDA FoodData Central (release 2026-04-30) state calories that the 4/4/9 rule cannot explain, even after allowing for label rounding, fiber and sugar alcohols. The rule is 4 calories per gram of protein and carbohydrate, 9 per gram of fat. We checked 450,408 foods. 20,463 miss their stated calories by more than 10 calories and more than 20% per 100 g, and about half of those are label rounding. These are USDA's copies of the labels, not the packages themselves, so a gap can also start in USDA's data.
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Why is the Daily Value for vitamin K so different for infants?
FDA sets a separate Daily Value for each of four population groups. For vitamin K the gap between the highest and lowest is 48× — the widest spread of any nutrient on the panel.