When it comes to volume measurements, flour is one of the most vexing ingredients. Because flour is easily compacted or aerated depending on how it's handled, its mass per cup is highly variable. There are two main approaches to measuring flour by volume: dipping and spooning.
The dipping method (often called "dip and sweep") is perhaps the more realistic. It rightly assumes that the average person is going to grab a cup measure, stick it in the bag, and scoop out some flour. In part, the amount of force used to scoop out the flour determines how much will be crammed into the cup, but this can also be influenced by the degree to which the flour has already compacted. For example, a brand-new bag of flour may be highly compact, but one that's half full will be slightly aerated from use. Bakers who pour their flour from a bag into a bin will have well-aerated flour, too.
Due to such variables, Kenji has found that all-purpose flour can clock in at anywhere from four to six ounces per cup. According to that data, Serious Eats has historically based recipe conversions from mass to volume on the average: a five-ounce cup of all-purpose flour.
The spooning method, requiring bakers to spoon their flour from bag to cup, isn't as intuitive, and it takes a bit more effort. But it also goes a long way toward eliminating the risks of compression, producing a fairly standard four-and-a-half-ounce cup. In culinary school, this method was taught as the most accurate way to measure flour by volume, and it's what you'll see in my forthcoming cookbook—in addition to mass measurements, of course!
When I first joined Serious Eats, I went along with the existing policy of converting mass to volume based on the dipping method, for a five-ounce cup of all-purpose flour. But, as I've grown more comfortable in my role as the pastry wizard here, I've felt compelled to fall back on my training and switch to the method that offers the most consistent results for volume measurements.
My first recommendation for measuring flour will always be to use a digital kitchen scale. For those who don't yet own one, though, my volume measurements for all-purpose flour will be based on the spooning method, and four and a half ounces per cup, going forward. Just as brown sugar is often listed as "1 cup, firmly packed," the method of measuring will be listed simply alongside the amount.
(Hopefully, all of you have a scale and can disregard this little update.)
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