The most accurate way to measure flour is to weigh it. Unlike white sugar or a jar full of ball bearings, flour can easily be compacted. All you have to do is press down a bit or tap on the container and the flour settles and you can put more into the container. Because of this, a cup of flour might weigh anywhere between 4 and 6 ounces per cup, depending on how you handle it.
Sifted flour - meaning that you sift it into the cup - can weigh 4 ounces per cup because it has a ot of air between the grains of flour. It's very fluffy. But if you really try, you can jam nearly 6 ounces into the cup by pressing down on the flour and tapping the container.
Realistically, no one is going to go to either of those extremes when measuring flour for bread. For some cakes, you do want to sift. For bread, not really. And most people know that they're not supposed to mash down on the flour like it's a cup of brown sugar. So most reasonable measurements fall between the 4 1/4 ounces per cup that King Arthur Flour says is correct, up to the 5 1/2 ounces that Rose Levy Berenbaum says is correct.
There are plenty of authorities in the middle. Both Peter Reinhart and Shirley Corriher say that flour weighs 4 1/2 ounces per cup. The Culinary Institute of America says that all purpose weighs 4.4 ounces per cup and bread flour weighs 4.8 ounces per cup. They, by the way, were the only reference I could find that differentiated between AP and bread flour. The USDA calls 4.83 ounces a cup, and Jeffrey Hamelman uses 4.8 ounces.
Corriher explained that the 4 1/2 ounce weight is average when someone spoons the flour into a cup and then levels it off, and of course other methods will yield different weights. Most of the cookbooks I've seen that discuss the subject suggest that spooning and leveling will be more precise than any other method. It might not be more precise, but it's just about average.
When I decided that I needed to use both weights and cup measures in my recipes, I decided to use 4 1/2 ounces as my standard cup weight. There were three reasons. First, that was pretty close to the average measurement in my kitchen when I tested every possible measuring method. Second, it was just about in the middle of the range for what I found listed in print and online. Third, it was the number endorsed by both Reinhart and Corriher. And last, it was easier to cut 4 1/2 ounces per cup into halves and quarters than if I went with the 4.8 ounces preferred by Hamelman and the USDA.
If you always weigh your flour, it doesn't matter how much a cup weighs... as long as you use recipes that specify weights. But when you're dealing with a recipe that only uses cup measures, you have to decide for yourself how much that cup will weigh, or how you will measure your cup. I've used 4 1/2 ounces per cup in recipes that didn't specify weights, and so far it hasn't let me astray. And that's the last reason why I decided to use the 4 1/2 ounce cup at the standard on Cookistry. If it works for other people's recipes, it's good enough for me.
If you don't weigh your flour and you're concerned about getting the right weight, you'll get close to the 4 1/2 ounces if you spoon the flour into the measuring cup and then level it off. If you make enough bread, you'll learn what a good dough should feel like, so you can adjust by adding more flour or water to get the right consistency.
When I'm making my everyday bread, sometimes I don't bother measuring the flour at all. Blasphemy, right? I start with a known amount of liquid and yeast and add flour until it feels right. Our great-grandmothers didn't weigh and gnash their teeth over ounces and grams, they knew what the bread was supposed to feel like. We can bake like that, too. But it's not an accurate way of sharing recipes, which is what this site is all about. I'd like you to be able to get the same results I do.
My decision about what a standard cup should weigh was working just fine when I was only writing for my own blog, but when I'm writing recipes that other people will publish, sometimes I have to adjust to their rules. Just like I need to adjust comma usage and grammar quirks to other people's writing styles, I also have to comply with other people's guidelines for recipes.
The first time I ran into this issue was when Serious Eats adopted a 5.5-ounce cup for bread flour in September, 2010. The good news is that I start by weighing the flour, so the weights I give are always going to be accurate. And I make every effort to use weights that will convert cleanly into even cup amounts. The problem is that my even cups become uneven on Serious Eats. For example, 9 ounces of flour is 2 cups on Cookistry. But that same 9 ounces is 1.63 cups on Serious Eats. That's going to be a bit...strange.
The reason that Serious Eats is standardizing on the 5.5-ounce cup is that it's the weight people get when they scoop the measuring cup into the flour, and then level it off. It's a valid measurement, and it's the way that a lot of people measure flour. I can't argue with that.
But this leads me to a dilemma. Should I continue using the 4.5-ounce cup as my standard when I create recipes, or should I convert all my future recipes to using the 5.5-ounce cup? Should I develop separate versions of the same bread for both sites? Should I make breads exclusively for Serious Eats using their measurements, and not publish them here?
I haven't decided what I'm going to do yet, but I'm leaning towards keeping the 4.5 ounces as my personal standard when writing recipes. It's what I'm used to, and if I add 2 1/2 ounces of flour to my basic bread dough, I'll have to adjust all of the other ingredients to make the bread work the way I want it to. As far as the rest of the questions, I still haven't answered them for myself.
What Cookistry defines as a cup isn't going to change the nature of the bread-baking universe, but it is important to me that my recipes are accurate and accessible. I like the idea that people will know that a cup of flour in the Cookistry universe always weighs the same amount, no matter when the recipe was posted.
My last reason for choosing the 4 1/2 ounce cup as my standard is probably the most egotistical of all. I'd like to think that if I ever publish a cookbook I'll be in the company of Reinhart and Corriher, in the neighborhood of Hamelman, and not too far from King Arthur flour.
So, if you read my recipes here and at Serious Eats and you see the difference in cup measures, now you know why. If you weigh your flour, you'll have no trouble. If you measure using volume rather than weight, use the dip-and-level method with the Serious Eats recipes. If you're using Cookistry recipes, spoon the flour into the cup, then level it off. That way, you'll be close to the correct weight wherever you read my recipes.