Use the package’s cup weight, not a generic cup-to-gram conversion
Distinguish a household measure from measured mass and calculate a portion using the package-specific relationship.
A cup measures volume, while grams measure mass. There is no single gram weight that works for a cup of every food. When a package pairs a household measure with grams, keep that product-specific relationship attached to the calculation.
The FDA serving-size guide describes household measures accompanied by metric amounts. That pairing provides a usable label basis without claiming that every scoop or slice has an identical measured weight.
Follow the package-specific example
Imagine a fictional cereal lists three quarters of a cup as 30 g, with 150 mg sodium. If you measure half a cup using the same assumed packing, its share of the stated volume is 0.5 ÷ 0.75, or two thirds.
The corresponding label-based estimate is 20 g cereal and 100 mg sodium. The gram amount is inferred from this particular package's pairing, not from a universal rule for cereal cups.
Use a weighed portion when that is your evidence
Suppose your half-cup scoop actually weighs 24 g. A calculation based on that measurement uses 24 ÷ 30 = 0.8 servings, yielding 120 mg sodium. That differs from the volume-based estimate because the measured quantity differs.
Neither route should be hidden. Label one result “estimated from household measure” and the other “calculated from weighed portion.” You can then decide which evidence better matches the task without mixing parts of the two methods.
Check how the measure was filled
Level and heaped measures, settling, broken pieces, and different shapes can change the mass within a volume. Zero the container before weighing, and record whether the food was dry or prepared. A scoop of powder and a cup of finished drink are not the same state.
Do not use a cup weight found for another brand or ingredient merely because the name is similar. If no suitable gram pairing is supplied, weighing can provide the portion mass needed for a mass-based calculation.
When sharing a recipe note, include both the household description and the actual mass if available. “Half a cup, weighed at 24 g for this product” is more reproducible than “half a cup” alone. The resulting nutrient estimate still depends on the rounded package data and cannot measure how nutrients vary among individual pieces.