For the same uniform food, half the portion gives half the underlying nutrient amount. The displayed percentage may not halve perfectly because the original label is rounded. Use this as a practical estimate rather than demanding exact agreement between printed integers.
Start with the amount in the panel
Imagine a label listing 460 mg sodium per serving. Half gives 230 mg. Against the FDA sodium Daily Value of 2,300 mg, that is 230 ÷ 2,300 × 100 = 10%. If the full serving was displayed as 20% DV, the two views line up neatly in this example.
A less tidy label
Now imagine 500 mg sodium per serving. Half is 250 mg, or about 10.9% of that reference. A full-serving panel may show a rounded integer percentage. Halving that printed integer can introduce another small rounding difference. The food did not change; the representations did.
Keep the denominator fixed
Eating half a portion changes the numerator, not the reference amount. Do not halve both the sodium and the Daily Value: that would leave the percentage unchanged. The same reasoning applies to other nutrient references, each with its own denominator.
Use the stated mass portion when you can. “Half” of a layered snack that separates unevenly may not contain half of every component. For a mixed product, portioning assumptions belong beside the calculation. The FDA Daily Value source supports the reference denominator; these invented amounts demonstrate the portion arithmetic, not a recommended sodium allocation.
Sources and example boundaries
- FDA: Daily Values
Percent Daily Value is a reference comparison, not a personalised target.
- FDA: sodium
Sodium units and the distinction between sodium and salt.
All food figures and prices in the worked example are invented to explain the method. They are not restaurant nutrition, tested recipes, shopping prices or personal dietary advice. Use the current label for the product you actually have.