Arrowroot Starch

One US cup of arrowroot starch weighs between 120 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 arrowroot, arrowroot flour, arrowroot powder, arrowroot starch.

TL;DR: Arrowroot starch weighs 120–128 grams per US cup depending on producer. Bob's Red Mill and USDA both measure it at 128 g/cup; Anthony's labels it at 120 g/cup, reflecting FDA RACC-rounded serving conventions rather than measured density.

Arrowroot starch (also sold as arrowroot flour or arrowroot powder — all three names refer to the same product) is a pure starch extracted from the root of Maranta arundinacea, a tropical tuber. Unlike wheat flour, it contains no protein, no fiber, and no bran, which is why its density is dominated entirely by the packing behavior of fine starch granules.

The producer label variance is a small but real quirk. FDA labeling regulations set a Reference Amount Customarily Consumed (RACC) of 30 grams for the flour and starch category, and manufacturers can choose to anchor their per-serving weight either to the RACC baseline or to the measured density of their specific product. Anthony's Goods uses 30 g per quarter-cup (yielding 120 g/cup); Bob's Red Mill uses 32 g per quarter-cup (yielding 128 g/cup), matching what USDA's Standard Release database records for arrowroot flour. Neither is technically wrong — Anthony's label is legal and reflects the regulatory serving size, while Bob's label reflects a slightly more literal density measurement. The 6.7% gap between them is comfortably within the tolerance a home baker should expect from any powdered starch scooped from a bag.

What this means in practice: for most cooking and baking applications, treating arrowroot as approximately 125 g/cup is more than accurate enough. The gap between 120 and 128 grams is smaller than the variance you would introduce by sifting versus not sifting, or by scooping versus spooning into the measuring cup.

Where it does matter is in arrowroot's role as a thickener. Arrowroot is significantly more powerful than cornstarch at low temperatures, and the ratio of starch to liquid controls the final texture with unusual precision. The standard ratio for a medium sauce is 1 tablespoon (roughly 8 g) of arrowroot per cup of liquid; 1.5 tablespoons yields a gravy consistency; 2 tablespoons produces a firm gel. A measurement error of even 4–5 grams per tablespoon — easily introduced by scooping damp or packed starch — can shift a sauce from medium-bodied to over-firm, and unlike a flour-thickened sauce, an over-thickened arrowroot sauce cannot be successfully thinned without collapsing the gel.

For accuracy in baking and precise thickening work, the recommendation is straightforward: weigh it. A kitchen scale eliminates producer label ambiguity entirely, and at roughly $15 for a decent unit, it pays for itself the first time you avoid ruining a batch of pastry cream. Failing that, spoon the starch into the measuring cup and level with a straight edge rather than scooping directly from the bag — the scooping motion packs the starch and can add 10–15% to the weight.