This is quite an interesting discussion, and I include the slight deviations off topic. Reading a recipe that has measurements in cups is actually quite difficult due to the different definitions of a cup in different locations around the world. For instance, a cup in America is defined as 8 fluid ounces, which is equal to 236ml (as close as necessary for this discussion). However, a metric cup is defined as 250ml. Then we had (still have?) the Imperial cup, which is defined as 284ml.
So, I have a recipe that I know comes from the US and thus I can figure out how to convert it to weight. I can also get an old recipe book and see that it originated in the UK, and be able to convert to weight.
But, we have this fantastic electronic system called the Internet, which is helping screw up recipes. Let me take an example. A food magazine hires a couple of "graphic artists", fresh out of collage. These kids are good in what they do and can manipulate and mockup great looking pages in a magazine, with a couple clicks of a mouse. So, I tell them that they need to get me two recipes each for chocolate cakes. And off they go and search the Internet and return with a couple recipes each, together with a bunch of shutterstock photographs. Does the magazine actually test all the recipes? Actually, most appear not to. They may do one or two to get some good photographs. Have the recipes been converted - I mean the recipes may have come from a French site using metric cups and the youngster (who may not even know how to boil an egg, never mind bake a cake), does not know that the size of an American cup is smaller than a metric cup. You then come along and think the recipe sounds great and bake the cake and it is a flop.
Shel_B state up thread:
However, today, while looking through some recipes to decide what to bake this weekend, I discovered that using ATK's and CI's recipes, a cup of AP flour came in with widely varying weights: 5 recipes gave the weight of a cup of flour at 4.25-oz, 5.5-oz, 4.5-oz, 4.16-oz, and 5.0-oz. That's a pretty big variation of weight for a cup of flour. And KA flour came in with two weights for a cup of their AP flour: 4.16-oz and 4.25-oz.
Maybe the difference in the weights in these recipes was due to the source of the recipes - some from the US, Canada, England, France, Australia, etc. and the writer of the recipe did the conversion using the known cup size used in that country and actually tested the recipes.
What I am rambling on about, is that the source of a recipe needs to be taken into consideration when converting a "cup" to weight.
Also, Lisa Shock's comment is spot on:
The metric system rocks. You never have to calculate 7/5ths of 16. All the math flows easily in your head.
It really is high time that all publications of recipes publish by weight, and preferably using the metric system. All publications I come across out of the US that are for the professional chef are in metric weight, and have been for as long as I can remember. A kitchen scale is, in this day and age, a necessity in every kitchen. Publishers, especially in the US, need to get "with it" and evolve with worldwide trends.
So, to get back to the topic heading, measuring flour, the quantity and thus the weight of a cup of flour depends on where the recipe originally comes from.
And just to add a bit of humour, I assisted in a "test kitchen" for a magazine once where I was presented with a recipe for a wonderful chocolate almond tort. I baked the tort and the centre sunk when it cooled. It was pretty bad, but the magazine had their photographer there and when I was going to dump the tort I was told to stand aside and let the presentation team take over. I still remember the photograph of the absolutely wonderfully smooth ganache covered cake appearing in the publication, knowing that the centre contained a wad of crumpled toilet paper to prevent the weight of the ganache covered cardboard disk from sinking. I later found the original recipe for the tort and did the correct conversions to weight and still use it today. It was originally in cups and the young lady who "converted" it used the incorrect cup sizes and thought almond flour weighed the same as cake flour.
Cape Town - At the foot of a flat topped mountain with a tablecloth covering it.