The flour hiding in your starter

Take a recipe at 500 g flour, 350 g water and 100 g of starter. It is written as a 70% hydration dough, and most hydration calculators will agree.

The dough you actually mix is at 72.7%.

Starter is not an ingredient, it is two

A starter at 100% hydration is equal weights of flour and water that happen to be fermenting. So 100 g of it is 50 g of flour and 50 g of water, and both belong in the totals.

Add them in and the flour becomes 550 g, the water becomes 400 g, and 400 divided by 550 is 72.7%. Not a dramatic difference on its own, but it is in the direction of a wetter, slacker dough than the number on the page implies, and it compounds with every other variable in the room.

A stiff starter moves it the other way

The same 100 g of starter at 50% hydration is 66.7 g of flour and 33.3 g of water. Totals become 566.7 g of flour and 383.3 g of water, which is 67.6%.

So the identical recipe is either 72.7% or 67.6% depending on nothing but how you keep your starter. That five point swing is the whole explanation for why a recipe that works for the person who wrote it can behave quite differently in your kitchen while you follow it exactly.

This is also why the advice to simply add more flour when a dough feels too wet so often makes things worse. The dough was never at the hydration the recipe claimed, so you are correcting from a number that was not true to begin with.

Scaling makes it visible

The error scales with how much starter you use. At a 10% inoculation the gap is small. At 30%, which plenty of same-day recipes use, it is large enough to change how the dough handles from the first fold.

The sourdough calculator reports both figures, the recipe hydration and the true hydration with the starter’s flour and water folded in, and it will scale a formula to whatever flour weight you want without losing either one.

What the number is actually for

Hydration is not a score. A higher number is not a better loaf, and chasing one is how people end up with a puddle on a baking stone.

What it does is predict handling. Below about 65% a white dough is firm, easy to shape and gives a tight, even crumb. Between 65 and 75% it is softer, more open and still manageable. Past 80% it is a slurry that needs technique, a strong flour and a confident hand, and it will punish any of the three being absent.

Flour decides where those bands sit. Wholemeal and rye absorb far more water than white flour, so the same true hydration feels drier, which is why wholemeal recipes carry higher numbers without being harder to handle.

Two doughs at the same number can behave differently

Protein content, milling, how long the dough has rested and even the age of the flour all shift absorption. A strong bread flour at 72% can feel firmer than a supermarket plain flour at 68%.

So the practical use of true hydration is as a repeatable record of what you did, not as a target to hit. Once you know your flour and your starter, the number tells you how to reproduce a loaf you liked, and that is worth far more than knowing whether 75% is fashionable.

Keep the starter consistent and the maths stays quiet

Most of this disappears if the starter is fed the same way every time. Equal weights of flour and water, by scale rather than by volume, and the 100% assumption that every recipe quietly makes becomes true for you.

Feeding by cups is where stiff starters appear by accident, because a cup of water weighs roughly twice what a cup of flour does. Nothing goes wrong with the starter itself, but every recipe you follow is then being run several points drier than it was written, which is a frustrating thing to chase when you do not know it is happening.