Sourdough Hydration and Bakers' Percentages

Bread bakers talk a strange language of percentages: "this loaf is 78% hydration", "feed the levain at 100%". The numbers look like nutrition labels but they are geometry — a compact way of writing a dough formula so it scales to any batch size, and the single best predictor of how a dough will feel in your hands and look on the inside of the loaf.

Bakers' percentages: one denominator

Bakers' percentage is a convention where every ingredient is expressed as a percentage of the flour weight, and the flour is always 100%. If a formula calls for 1,000 g of flour, 700 g of water, 20 g of salt and 200 g of starter, it is written as 100% flour, 70% water, 2% salt, 20% starter. The elegance is that the formula scales to any size without new numbers: halve the flour to 500 g and every other line halves with it. It is the same scaling instinct behind any recipe scaler, just with flour pinned as the reference.

Hydration is the most-quoted of these percentages: total water as a share of total flour. A "70% dough" carries 700 g of water per kilogram of flour — and the crucial word is total, which is where home bakers go wrong.

The starter's hidden payload

Sourdough starter is not a neutral flavoring; it is flour and water in a fixed ratio, plus the culture. A starter maintained at 100% hydration is exactly half flour and half water by weight, so 100 g of it contributes 50 g of flour and 50 g of water to your dough. A stiff levain at 50% hydration is two-thirds flour and one-third water; a liquid one at 125% is 44% flour and 56% water. Skip this accounting and your real hydration drifts several points from the formula — a "75%" recipe that behaves like an 80% one, slumping on the counter and baking flatter than the photos promised.

The arithmetic is small: for a starter of S grams at H% hydration, flour added is S × 100 ÷ (100 + H) and water added is S × H ÷ (100 + H). Then hydration equals (your water + starter water) ÷ (your flour + starter flour). A dough with 500 g flour, 350 g water and 100 g of 100%-hydration starter works out to 400 g of water over 550 g of flour — about 72.7%, not the 70% the ingredient list alone suggests.

What the number predicts

Hydration is a handling dial. Around 60–65% the dough is firm — bagel and sandwich-bread territory, easy to shape, tight crumb. The 68–74% band is the sourdough mainstream: cohesive enough to shape with lightly floured hands, open enough to bake into a lofty loaf. Push to 75–85% and the dough becomes sticky and slack, demanding confident folds and a bench scraper, but rewarding you with the glossy, irregular, custard-like crumb that fills Instagram. Above that lies focaccia and ciabatta, which are less shaped than folded and poured.

Flour type bends the same number in different directions. Whole wheat and rye absorb more water than white bread flour, so a 75% whole-wheat dough often handles like a 72% white one — bakers typically add 3–5 hydration points when swapping in whole grains, and give the dough a rest (an autolyse) so the thirsty bran catches up.

Hitting a target

The common workflow runs forward, then back. First compute where you are: flour, water, starter, and its hydration, yielding your actual hydration and total dough weight. Then, if you are chasing a specific target — 78% is a popular "open crumb but shapeable" goal — solve for the water that hits it given the starter's contribution: target × total flour, minus the water the starter already brings. The result is usually a nudge of 10–20 grams, not a redesign. Because everything is percentages, the same formula holds when you double the bake for a gift loaf; scale the list with a recipe scaler and the hydration stays put.

Try it yourself

The Sourdough Hydration Calculator does both halves: enter flour, water and starter (with its own hydration) to see total flour, total water, true hydration and dough weight — or set a target percentage and read off exactly how much water to add. For the liquid measurements in cups instead of grams, the Cooking Converter translates; but bake with the scale where you can, because hydration math is weight math.