Cake flour

One US cup of cake flour weighs between 120 and 137 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 soft wheat flour, pastry cake flour, unbleached cake flour.

Cake flour is the first entry in this dataset where USDA explicitly names the measurement method on the portion itself, rather than recording an undetermined cup. USDA FDC 169723 (Wheat flour, white, cake, enriched) publishes its cup as 137 g measured unsifted and dipped, and this is a distinctly different state from the spooned figure King Arthur gives on the Ingredient Weight Chart for their Unbleached Cake Flour at 120 g per cup. The two figures do not disagree in the sense that flour or bread flour disagree; they describe two different ways of filling the cup with the same substance, and the entry splits into two states under rule 6 for the same reason the flour entry splits into three.

Cake flour is milled from soft wheat, which has lower protein content (typically 6 to 9 percent) than the hard wheats used for all-purpose and bread flours. It is milled extremely fine and often chlorinated (in bleached cake flour) to weaken the gluten proteins further, producing the tender, fine crumb that gives sponge cakes, chiffon cakes and angel food cakes their characteristic lightness. The finer particle size makes cake flour more susceptible to compaction and packing variability than coarser flours.

Dipping into a settled bag captures air-free flour tightly against the cup; spooning fills the cup with more air trapped between particles. The 14 percent gap between USDA's 137 g and King Arthur's 120 g is consistent with this difference and is not a measurement error on either side. A third state exists in traditional baking practice: sifted cake flour, which most classic cake recipes historically specified.

Sifting breaks up any clumps that have formed and traps additional air between particles, and sifted cake flour typically weighs about 100 g per cup measured after sifting. This state is not published here because no independent source with the required method attribution was found; converter sites that publish 100 g per cup for sifted cake flour trace back to Wilton and to older editions of Betty Crocker, which state the figure without describing measurement conditions in enough detail to qualify as a source under rule 3. If a source that documents sifted cake flour with a stated method turns up, this entry will gain a third state.

Recipe substitution is a common question for home bakers who lack cake flour. Removing two tablespoons of all-purpose flour per cup and replacing them with two tablespoons of cornstarch produces a workable substitute with approximately the right protein content, though the particle size distribution is coarser than true cake flour and the results tend to be slightly less tender. Bleached and unbleached cake flour have similar density; the bleaching process affects protein structure and color but not bulk density measurably.

Swans Down and Softasilk are the two dominant US bleached cake flour brands, both published in similar weight ranges to the USDA figure. FDA identity standards under 21 CFR 137.180 specify cake flour must be milled from soft wheat and enriched with specific nutrients, but do not specify density. Codex Alimentarius has no specific standard for cake flour.

To publish a single figure, each state needs two further sources that name its method and measure it. The entry stays at low confidence and publishes the range across its two states, 120 to 137 g per cup, until additional sources fill it out.