What is hydration in bread dough?

The answer

Hydration is the weight of water divided by the weight of flour, written as a percentage; 350 g of water with 500 g of flour is 70% hydration. Higher hydration gives a softer, stickier dough and a more open crumb, while lower hydration gives a firmer dough that is easier to handle and a tighter crumb. Bagels sit toward the low end, sandwich loaves in the middle and rustic loaves higher. Weighing ingredients, rather than using cups, is the most reliable way to control it.