There are a variety of weight-to-volume conversions online for flour, with considerable variation even among reputable sources:
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King Arthur Flour says 4.25oz/cup but in their measuring tips article says it's 4oz/cup when sifted, up to 5.5oz/cup when scooped, and yet that somehow 4.25oz/cup is "closer to what bakers actually measure volume-wise".
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The Kitchn says 4.5oz/cup.
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Cook's Illustrated says 5oz/cup, based on real testing: "…had dozens of volunteers measure out 1 cup, weighed the results, and took the average…"
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Serious Eats sort of agrees, with J. Kenji Lopez-Alt finding a 4-6oz/cup range from tests, and ultimately deciding on 5oz/cup as an average but with Stella Parks deciding to use 4.5oz/cup, with the cup measured by spooning flour into the measuring cup.
The full explanations with varying volume measurement methods and big ranges all seem pretty realistic, but of course leaves the issue of what number to actually use when confronted with a recipe that just has a volume, and no mention of a measurement method.
Of course, after testing a recipe we can just note weights and forget about volume, but if I'm starting from an arbitrary recipe and I want to measure by weight, how can I decide what initial guess to use?
Best Answer
This may seem to be an overly simplistic response, but if it's in a decent cookbook that deals with baking, the book often has an introduction or appendix or chapter that describes how to measure flour. If it does, I measure the flour according to their directions, which can give a weight estimate. Older cookbooks tend to advocate a lot of sifting before weighing (in my experience). Newer ones sometimes mention spooning then leveling. And I've seen at least a couple which just admit they scoop and sweep.
If the recipe is from a book without such instructions or an internet resource without such instructions, I generally avoid it unless it's a type of recipe where I know exact flour measurement is unlikely to have a big impact. If I end up using such a recipe with no guidance on flour measurement, I sometimes try to take the era of the source into account if I know. Otherwise, I'm not sure my personal practice matters, because I'm just guessing -- like anyone else who answers this question will likely be. (Frankly, I worry this question is likely to turn into a poll.)