There are a variety of weight-to-volume conversions online for flour, with considerable variation even among reputable sources:

  • 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".

  • The Kitchn says 4.5oz/cup.

  • 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..."

  • 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?

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    It can vary so much according to how you fill the cup and even the shape of the cup, but I would tend toward the King Arthur numbers in part to their caveats on how the got them. It is striking though just how much difference there is between sifted and scooped and yet many of us tend to just slop it and wonder why we get inconsistent results when using volume. – dlb Jan 26 '18 at 19:53
  • PS, I remember one of the first cooking demos I saw as a kid. The Demonstrator started by pointing out that flour should always be measured by sifting first, spooning, leveling with a knife. They then started the recipe, dipped a cup of floor, did not level it, and sifted it into the bowl with no remeasure. – dlb Jan 26 '18 at 19:59
  • @dlb I think it's more why many of us refuse to use volume at all. I honestly will avoid recipes that don't offer a conversion. I have a difficult time accepting that they can be repeatably good if they don't give me a weight measurement. – Catija Jan 26 '18 at 20:00
  • @dlb The King Arthur caveats are really weird, though - they claim their choice is realistic but then say that scooping (the most common method from home cooks I suspect) results in much more, and CI and Serious Eats confirm that. – Cascabel Jan 26 '18 at 20:19
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    When I was first learning, many of the recipes did also address it a little, by saying "sifted flour" or "flour sifted", but even that the wording difference was so subtle that it was often missed. And as an old coot that learned with volume, I hate when I have a recipe by weight. But I sure wish I had learned that way instead. If our grandmothers were still around we could have them measure it out for us and correctly translate those old recipes. – dlb Jan 26 '18 at 21:06

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.)

  • Thanks for the answer! Looking for more information in a cookbook is definitely a good way to approach this guesswork. Maybe along with that you could sneak in a quick note, e.g. "if spooning, 4.5oz, if scoop, 5oz" or similar? Similar for the second paragraph - how is it you're guessing based on era? (I know it's best to just avoid this if possible, but a lot of people are gonna make volume-only recipes at some point, might as well give them a head start on the guessing.) – Cascabel Jan 26 '18 at 22:38
  • @Jefromi - I'm not sure my guesses as what constitutes scooping vs. sifted or whatever converted to weight are any more accurate than what you cited in the question. They can vary from person to person too. Type of flour, how much moisture the flour absorbed during storage, etc. can make a difference here too. Honestly, if I'm really trying a recipe like this the first time, I generally just give in and measure by volume myself, using a specified method if available. If the recipe actually works and I want to reuse it, I'll try to quantify it by weight next time based on my experience. – Athanasius Jan 26 '18 at 23:05
  • Yeah, I know it varies, so there's no perfect solution, but I expect that these things are all distributions with more probability near the middle (i.e. if everyone gets 4-6oz/cup, average 5oz/cup, I suspect that more people get closer to 5oz than 4oz or 6oz) so there are probably guesses that will be better than others. And if you're one of those people who somehow gets 6oz you're probably better off not starting with a volume measurement :) – Cascabel Jan 26 '18 at 23:26

What to do

I think you have to forget the idea of a guess being somehow "better" in the sense of giving you the "least error" and treat all the values within some interval (which seems to be the 4 to 5 ounce interval) as equally likely to give you "the least error"*. So simply choose a value from the interval based on some other criteria (convenience for you, aversion to different types of error, suspicion that your own measurement errors might not be symmetrically distributed, throwing a dice, whatever) and stick with it as the best guess.

My personal preference is 4.23 ounces, but I don't claim that this is the best choice for everyone, and certainly not that it more frequently hits what the recipe author uses.

A sampling of what established authors use

This is what Shirley O'Corriher has to say on the topic:

I used to measure as I was trained: by placing the measuring cup on the counter, spooning flour into it, and levelling it off with a straightedge. Through years of teaching, I observed that my students measured by dipping the measuring cup into the flour and leveling it by pressing it against the inside of the bag. This is actually 1 to 2 tablespoons more flour per cup than I was getting. [... T]his is the way most home cooks measure [...]. If you prefer to work by weight, a cup of bread flour when I measure this way weighs 5.6 ounces.

Peter Reinhart gives both weight and volume for each recipe in Bread baker's apprentice and uses 4.5 ounces per cup.

Rose Levi Beranbaum gives both weight and volume for each recipe in The cake bible, and uses 3.5 ounces per cup of "sifted cake flour".

America's test kitchen has a conversion table in The new best recipe: 1 cup AP flour is 5 ounces.

Jeff Potter, in Cooking for geeks (not as influential as the other authors, but somebody who is obsessive about basing everything on solid data) uses 4.5 ounces per cup.

Simple statistics on that sample

We have an interval between 3.5 and 5.6 ounces, with a mean of (rounded) 4 ounces and median of 4.5 ounces. This is quite close to your own sampling of sources.

My recommendations

First, if you really want to go towards good guesses, you may want to start treating recipes from different sources differently. It seems that home cooks tend to use more flour per cup than professionals - the two high numbers in the range are from a book made with the explicit purpose to have as many home cooks as possible achieve consistently good results, and from an author who adjusted her measurements to suit home cooks' habits. So, you may want to settle on one number for recipes created by classically trained cooks and another one for recipes from those coming from a history of home cooking (even if they have turned into professional recipe authors).

Second, even though statistically, your best bet is to assume a normal distribution*, I would advise to not use the middle of the sampled range, but a lower number. Why? Tender baked goods such as cakes, crepes, etc. tend to taste richer with less flour, so erring on the side of little flour is going to give you a result which most eaters prefer, unless it is so reduced that it gives you structural problems (in which case, you simply have to redo the recipe with more flour - if you are too averse to such surprises, it is probably best to stop using volumetric recipes at all). Breads on the other hand are highly dependent on a number of parameters of the exact flour used which are never accessible for the home cook, so the ratio given in a recipe is just a starting point, you are meant to adjust it a bit if the dough texture is wrong. And adjusting towards more flour is always easier than adjusting towards more liquid.

So these numbers suggest that you should go for something between 4 to 4.5 ounces per cup. I personally like using 120 g because it is easy to do gram-based math in my head with that number, and it makes for easy round ratios with liquids converted to 240 grams per cup. These 120 g happen to be 4.23 ounces. I don't think I have had too many recipes fail from bad flour amounts.

* Technically, what I am saying is that apparently our measurement error (of authors' intents and home cooks' measuring results, not of ounces of flour per cup) is so large that there is not enough information in the world to give you a sensible point estimate, and what you have to work with is an informal confidence interval. Which goes against many of our expectations, but is actually quite a common occurrence. And it is not somehow less scientific than working with "the right number".

* There probably are several such distributions, one for e.g. home cooks scooping bread flour and leveling it against the bag, another for pastry professionals scooping sifted flour and leveling it with a straightedge, etc., and then the sum of those should be normally distributed. And I see no reasons to assume a skewed distribution for any of the one-context distributions.

  • But including 3.5 oz seems wrong - if a recipe calls for sifted flour, it's specifically aerated, so you should expect it to be low... but if the recipe specifically calls for unsifted flour (which most do), you wouldn't use a conversion of 3.5 oz. Also worth noting that Cooks' Illustrated measures cake flour at 4 oz/cup instead of 5 oz/cup, though King Arthur measures them out the same (4-1/2)... so I'm not convinced that comparing cake flour to AP flour is correct, either. Can you explain why this shouldn't matter? – Catija Jan 28 '18 at 1:41
  • It shouldn't matter because, if there were a starting point which is obviously superior over all others, the cited sources would have found it and would agree. It seems that our uncertainty in this kind of calculation is so high, that using different models for calculation puts the optimal point anywhere in the range of 4-5 oz. Since we cannot know which model is more correct, I am saying drop the hope that we will have a perfect mathematical point estimate and just pick a point by other criteria. – rumtscho Jan 28 '18 at 9:32
  • I really think there's some letting perfect be the enemy of good going on here. Of course there's no perfect answer, no obviously superior answer, but that doesn't mean it's best to use "whatever you personally feel is best from 4-5oz" as the "other criteria." – Cascabel Jan 28 '18 at 15:42

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