Why equal fiber concentration can give different portion totals
Separate dietary fiber per 100 g from the amount in a bowl, and check how changing the portion affects the result.
Two foods can have the same calculated dietary fiber concentration while the amounts in their usual portions differ. Keep concentration and portion totals in separate columns so that a larger serving is not mistaken for a more concentrated product.
The serving-based examples in the FDA nutrition label guide provide the basis for scaling printed nutrient amounts. The numbers below are invented for arithmetic practice, without an individual fiber target.
Compare two fictional cereal records
Cereal A lists 3 g dietary fiber per 30 g serving. Cereal B lists 5 g per 50 g serving. Dividing each nutrient amount by its serving weight and multiplying by 100 gives 10 g dietary fiber per 100 g for both cereals.
At the printed serving sizes, B supplies 2 g more fiber. At equal food weight, the calculated concentration is identical. A comparison chart should show which of these questions it is answering rather than selecting whichever number makes one product look better.
Follow a change in your bowl
Suppose you weigh 45 g of A. The portion estimate is 3 × 45 ÷ 30 = 4.5 g fiber. A 45 g portion of B gives 5 × 45 ÷ 50 = 4.5 g as well.
If you reduce the portion of A to 36 g, its estimate becomes 3.6 g. The 9 g reduction in cereal weight changes the calculated fiber amount by 0.9 g. The product itself has not changed, and its per-100-g value remains 10 g.
Check category names before combining records
Use the dietary fiber row, not total carbohydrate, and retain the label's market and terminology. Do not add fiber to the total carbohydrate figure as if they were unrelated totals on a US panel. A quantity worksheet should preserve the source categories instead of inventing a new nutrient definition.
If one label only says “less than” an amount, keep that inequality visible. An exact-looking concentration cannot be recovered from a bound. Likewise, an absent row should not be filled with zero merely to complete a comparison.
The most useful saved result is explicit: “4.5 g estimated dietary fiber in a weighed 45 g portion, calculated from this package.” It communicates both the amount and how it was obtained, while leaving personal dietary decisions outside the arithmetic.