Hi mark, thanks for commenting! When we talk about hydration percentages, we speak in terms of mass (grams) not volume.

It's more convenient to measure (for some people at least) in cups.

In this case, we set the flour mass to be 100% (lets say we have 500 grams of flour), and then we measure the dough hydration relative to the flour mass (e.g. 250 grams of water = 50% hydration, 500 grams of water = 100% hydration). We often aim for a hydration of 70-80% for no-knead doughs.

What a hydration of 75% in cups might look like, however, is:

3 cups of flour

1.5 cups of water