Cornstarch

One US cup of cornstarch weighs between 112 and 128 grams, depending on how the cup is filled and which source measured it. That range is the answer; a single number would not be. Every figure behind it is below, with the method each source used.

Also called corn starch, cornflour, corn flour (British), maize starch.

Two sources 13.3% apart, over the 10% tolerance for the milled category, so this entry publishes a range rather than a figure. USDA FDC 169698 gives a cup of cornstarch at 128 g (0.5410 g/ml). King Arthur's Ingredient Weight Chart gives 28 g per quarter cup, implying 112 g per cup or 0.4734 g/ml.

Neither source names a method on the page cited. King Arthur's Recipe Success Guide separately states their measuring method for dry ingredients as spoon-and-sweep, which is likely the source of the difference: pure cornstarch is the most cake-prone ingredient in the milled category because it is 100% starch with no diluent, and the same reason it is added to powdered sugar as an anti-caking agent explains why cornstarch on its own compacts hard in the box and needs breaking up. A dip-and-sweep cup pushed into a settled box captures more material than a spooned cup does, and 12.5% is within the range that measurement technique can plausibly account for.

This mirrors the flour case, where dip-and-sweep and spoon-and-sweep produce different weights of the same substance, but with only one source per method the entry cannot be split into states under rule 6 and is held as a range instead. The 128 g figure appears everywhere the USDA data has been quoted, including Wikipedia's cornstarch article which lists a density of 0.54 g/ml matching USDA exactly. That does not add a source; Wikipedia's density is USDA's cup portion divided by the customary cup volume, so citing it would be citing USDA twice.

King Arthur's 112 g figure is the outlier in the sense that most published converters use USDA's 128 g, but King Arthur's chart is a first-hand source that names its own conditions of use, so the disagreement is real rather than a copy error. Nutrition-label servings for retail cornstarch (Argo, Clabber Girl, store-brand) declare 8 g per tablespoon or 30 g per portion at various fractions of a cup, but these round to the nearest whole gram on a small measure and cannot serve as sources under rule 3 for the reasons documented on cocoa powder. No standards body publishes a bulk density for cornstarch.

USDA's technical report on cornstarch handling (2025) covers organic classification and food safety rather than physical density, and Codex Alimentarius standards for starches (CXS 87-1981 and successors) specify moisture, ash, protein and sulphur dioxide limits rather than bulk density. Two sources only, and the range is published because no third source arbitrates between the spooned and unspooned figures.