Flour as the baseline
Baker's percentage expresses every ingredient in a formula as a percentage of the total flour weight, which is always defined as 100 percent. A recipe with 1000 grams of flour, 650 grams of water, 20 grams of salt, and 10 grams of yeast is written as 100 percent flour, 65 percent water, 2 percent salt, 1 percent yeast.
Because the percentages sum to more than 100, the figure is a ratio to flour rather than a share of the whole, which is the common point of confusion for newcomers.
Why bakers use it
The notation makes recipes scale cleanly: pick any flour weight and multiply through. It also lets bakers compare formulas at a glance, since hydration (water as a percentage of flour) and salt level are immediately visible regardless of batch size. A 65 percent hydration dough behaves the same whether you mix one loaf or fifty, which is why it is the standard language in professional bakeries.