When I first started making bread, I read a lot of different bread blogs. One of the things I noticed was that people talked about the percentage of the hydration of the dough, but I had no idea what that meant and I had trouble finding a coherent explanation, even though I am sure they were right under my fingertips. I also thought using grams rather than volume measurements to weight the dough was a ridiculous waste of time. Now I know better.
The clearest explanation I have ever found comes from Lisa Rayner's Wild Bread, which I have discussed in previous posts (Rayner 56-57). First off, baking percentages are based on grams. They are not particularly useful if you are only using volume measurements (although if you're willing to do a lot of math it could help you out).
So flour is always considered to be 100%. Let's say that you are using 500 grams of flour for your recipe, 100% is 500g. Water, or "hydration percentage," is calculated by it's comparision to the flour. For example, 375g means that to get the "hydration percentage" you divides 375g/500g=75% hydration. At this point the bread itself is 175% hydration. For most breads, salt should be between 1.8-2%, therefore between 9-10g of salt.
As a note, this flour and water includes any flour and water in your poolish or your sourdough starter. For example, if you use 100% hydration sourdough starter (which is what I have, meaning that I feed it with equal parts flour and water), then you can divide the weight of your starter in half. For example, if you use 200 grams of sourdough starter at 100% hydration, this means you consider it in baker's percentages as 100g water and 100g flour. So if you were to add 200g starter to your 500g flour and 375g water, then your bread would be (375+100g water)/(500+100g flour)=79.1%. The overall dough was 180% (if we use 1.9% salt). This would be a very wet dough-- maybe a ciabatta or focaccia. Lean European white doughs are around 65% hydration and whole wheat lean doughs are closer to 75% (because whole wheat flour absorbs more water).
If you are interested in using volume measurements but are willing to do the math to use baker's percentages, I found this tool to convert flour. Also, for water, 1g=1ml and ml can be converted into other volume measurements.
Today, while I try to translate Horace, Medea, and do my math homework, I am making two recipes using weights (in grams) instead of volumes trying to increase my understanding of baker's percentages. Wish me luck!