Sourdough Hydration: The Percentage Your Starter Is Hiding From You
Your recipe says 75%. Your starter has other plans.
While researching this one, I worked through a stack of bread books written by professional bakers — the kind of thick, serious reference books people keep on a kitchen shelf for years. Almost none of them explain baker’s percentages. What you get instead is a recipe for one loaf: a list of ingredients, a list of weights, done. For a sourdough loaf, that usually reads as something like “200g of starter” and nothing more.
One book did better than the rest — it gave a hydration percentage that actually included the water sitting inside the starter, not just the water added separately. That’s more than most manage. But it was still built around one specific loaf, at one specific size. Which is fine, right up until you want to bake two loaves instead of one, or a bigger batch, or swap in a different flour — at which point a fixed recipe stops being useful and you’re back to guessing.
Your starter isn’t one ingredient. It’s two.
Baker’s percentages work on a simple rule: flour is 100%, and everything else — water, salt, whatever else is in the bowl — is a percentage of that. I got into the basics of this in Baker’s Percentages Explained. The rule holds for sourdough too, with one catch people miss constantly: your starter is not a single mystery ingredient. It’s flour and water that have already been mixed together, and both of those need to be counted in your totals — not set aside as if the starter doesn’t count.
A starter kept at 100% hydration is exactly half flour, half water by weight. So 200g (7oz) of that starter isn’t “200g of starter” for hydration purposes — it’s 100g (3.5oz) of flour and 100g (3.5oz) of water, and both amounts belong in your running totals alongside everything else you’re adding to the bowl.
Where this quietly goes wrong
Say you’re aiming for a 900g (2 lb) dough at 75% hydration, using a 200g (7oz) starter at 100% hydration. Total flour needs to land at 900g, so with 100g of that already coming from the starter, you add 800g (28oz) separately. Total water needs to be 75% of 900g, which is 675g — but 100g of that is already sitting in the starter, so you only add 575g (20oz) of water on top.
Skip that step, and it’s an easy trap to fall into: add 800g of flour, then add 675g of water because that’s what “75%” told you to add, forgetting the starter is already carrying 100g of each. Add up the real totals and you’ve built a dough at roughly 86% hydration — noticeably wetter than planned, and stickier than your hands were expecting.
Tip: this changes again if your starter isn’t kept at 100% hydration. A stiffer starter (say, 50%) carries more flour than water for the same weight, so a recipe written around a 100% starter won’t behave the same way if you feed yours differently. If you keep a non-standard starter, that’s one more reason to work in percentages rather than copying someone else’s fixed weights.
Why this matters more once you scale
A fixed, one-loaf recipe hides this problem reasonably well, because someone already did the arithmetic for you, once, for that one size. The moment you want a second loaf, or a different size entirely, you’re not scaling a recipe anymore — you’re scaling ratios, and the starter’s hidden flour and water have to scale right along with everything else. I got into the wider version of this problem in Scaling Recipes Without Losing Your Mind (or Your Ratios).
One honest disclaimer
Sourdough isn’t something I bake regularly myself — it came up briefly in the theory portion of my pizzaiolo training and not much beyond that. What’s above comes from working through the books and getting the arithmetic straight, not from years of feeding a starter. If you do keep one, you already know more about how yours behaves than any general rule can tell you — the math above is the part that’s easy to get wrong regardless of experience, not a substitute for it.
Once you’ve folded your starter’s hidden flour and water into the totals — the way we just did above — the rest is just baker’s percentages, same as any other dough. Run those adjusted numbers through our baker’s percentage calculator and you’re back on solid, predictable ground, no matter how many loaves you’re scaling to.
— Rob