Hi Abel and Jeff, you’re right! There is a difference, and we have an explanation for you. Pour yourself a warm cup of tea and get comfortable. This one 
gets a little nitty gritty. 

Here at King Arthur, we use an imperial ounce (28.35 g), as the weight of an ounce of water, so our cup of water is 8 imperial ounces or 227 g. A customary US cup of water weighs 8.3214 imperial ounces or about 236 g, about 4% heavier. Our recipes are tested by weight and volume to be sure they work both ways.

A fluid ounce is a volume measurement, so a US customary fluid ounce of cream, melted butter, molasses, honey, egg, sourdough, oil, or other liquid used in baking will have a different weight that the US customary fluid ounce of water. This means the conversion factor for a cup of liquid will vary in our recipes.

Using an imperial ounce for water and milk means that an ounce of water in our weight chart weighs what an ounce of any other ingredient weighs. Unless someone is familiar with the weight of a fluid ounce of water, we find this to be less confusing. (The history of ounces is long and fascinating. There are several different kinds, some volume, some mass, all having different weights when the substance being measured is the same. For instance, the FDA uses a fluid ounce that weighs 30 g for measuring beverages.)

If we changed to a US customary fluid ounce (the official name of the kind of fluid ounce used in the US), there would still be lots of reasons hydration may need to be adjusted. Some examples being that flour varies significantly in water content depending on the ambient humidity, eggs vary in size, all the oil doesn’t come out of the 
measuring cup, sugar varies in granulation, milk varies in density depending on the fat %, the water content of butter and cream can vary by season and brand, or the manufacturer’s markings on the measuring cups may not be precise. If someone is baking by volume, there can be significant variability in the actual weight of ingredients. Our team has found 4% to be within that variability and a small factor in determining if hydration needs to be adjusted. 

In our recipes where small variations in the amount of water or milk are important, like bread, biscuits, and pancakes, we know that all the factors listed above may change how much water or milk is needed, so we write hydration adjustments into the recipe. We advise beginning with the smallest amount of water that can work, depending 
on the conditions, then adding more if needed. You can’t take it out of a mixture once it’s in there. 

The good news here is that we test our recipes by weight and volume, so you're covered either way! If measuring accuracy is a major concern for you, we strongly recommend weighing your ingredients according to the gram weights listed in our recipes. 

• Reply