How to Calculate Sourdough Hydration (Count the Starter)
September 28, 2026
You followed the recipe to the gram: 500 g flour, 375 g water, 100 g of bubbly starter. It said 75% hydration, a number you’ve handled before. But the dough spreads across the bench like pancake batter, won’t hold a shape, and bakes into a flat, wide loaf. You didn’t add too much water. The recipe just didn’t count the water in the starter, so it gave you the wrong hydration.
The answer: count everything that goes in the bowl
Sourdough hydration = total water ÷ total flour × 100, and “total” includes the flour and water inside your starter.
King Arthur Baking’s Martin Philip is blunt about it in King Arthur’s guide to bread hydration: “Both the flour and water amount in a sourdough preferment or sourdough culture should be considered when factoring hydration.” The flour doesn’t know which jar it came from. Everything in the loaf going into the oven is part of the loaf.
A starter fed equal weights of flour and water (100% hydration, the most common kind) is half flour and half water. So 100 g of it adds 50 g flour and 50 g water. Now redo the recipe above:
- Total flour: 500 g + 50 g = 550 g
- Total water: 375 g + 50 g = 425 g
- Hydration: 425 ÷ 550 = 77%, not 75%
That’s the same worked example King Arthur uses, and it’s why the dough felt wetter than the number on the page. Two points may not sound like much, but it’s the difference between a dough that holds a shape and one that doesn’t, especially if you’re new to shaping.
Chart: what a “75%” recipe really gives you
Here’s the gap for a recipe written as 500 g flour + 375 g water, with the starter treated as a separate ingredient and left out of the hydration math:
| Starter added | Starter hydration | Hidden flour + water | True hydration |
|---|---|---|---|
| 50 g (10%) | 100% | 25 g + 25 g | 76% |
| 100 g (20%) | 100% | 50 g + 50 g | 77% |
| 150 g (30%) | 100% | 75 g + 75 g | 78% |
| 100 g (20%) | 50% (stiff) | 67 g + 33 g | 72% |
Two things stand out. First, the more starter a recipe uses, the bigger the gap, so high-inoculation recipes are the ones that surprise you most. Second, look at the last row. A stiff starter pushes the error the other way. A 50% hydration starter is two parts flour to one part water, so it adds more flour than water and pulls the dough drier than the recipe says. Stiff levains usually sit around 50–65% hydration. The Perfect Loaf’s Maurizio Leo baked with one at about 65% for years before switching to a 100% liquid levain. Either works, he says, as long as you adjust the total water to match. If you’ve swapped a recipe written for a liquid starter over to your stiff one and the dough feels tight, this is why.
Doing it the other way: from target hydration to grams
The formula above checks a recipe. When you’re building one, run it backwards: decide the final hydration, then subtract whatever the starter already brings. These are the steps our sourdough calculator follows, with its default loaf of 500 g total flour, 75% hydration, 20% starter and 2% salt:
- Starter = 20% × 500 g = 100 g
- Flour inside it = 100 g ÷ (1 + 100%) = 50 g. Water inside it is the other 50 g.
- Total water for 75% = 75% × 500 g = 375 g
- Flour to add = 500 − 50 = 450 g. Water to add = 375 − 50 = 325 g
- Salt = 2% × 500 g = 10 g
| Ingredient | 100% starter | 50% (stiff) starter |
|---|---|---|
| Flour to add | 450 g | 433 g |
| Water to add | 325 g | 342 g |
| Starter | 100 g | 100 g |
| Salt | 10 g | 10 g |
| Total dough | 885 g | 885 g |
| True hydration | 75% | 75% |
Both columns come out at exactly 75%. They just split the flour and water differently to make up for what’s in the jar. Step 2 is the one people get wrong, because the “divide by one plus the starter hydration” part is easy to forget. It’s why the calculator asks for your starter hydration under More options instead of assuming it.
One thing to watch when you plug a published recipe in: the calculator expresses starter as a percentage of total flour, including the flour inside the starter. Many recipes express it as a percentage of the flour you add. A recipe calling for “200 g leaven per 1,000 g flour” is 20% by the recipe’s convention, but 200 ÷ 1,100 ≈ 18% of total flour.
Why some famous recipes leave the starter out
This isn’t only a beginner’s mistake. Some professionals do it on purpose. Chad Robertson’s Tartine Bread lists its Basic Country Bread at 75% hydration: roughly 1,000 g flour and 750 g water, plus 200 g of 100% hydration leaven. Count the leaven and bakers on The Fresh Loaf get 77%, the same two-point jump as above. Robertson has explained the choice directly: “I treat leaven as an ingredient component in the percentages. It is much simpler this way.”
For a bakery that always has a bucket of levain ready, that makes sense. The leaven is a stock ingredient, like salt. But it means a hydration number from one recipe can’t be compared with another until you know which convention the author used. Jeffrey Hamelman’s Bread, by contrast, lays out the pre-fermented flour and a total-formula breakdown alongside the final dough, and that total-formula approach is what King Arthur recommends.
There’s also a smaller third position. In The Perfect Loaf’s hydration guide, Maurizio Leo counts the levain he builds for a bake but leaves out the spoonful of mature starter used to seed it. For a home-sized loaf, he says, that’s a rounding error of well under a point. He’s right, and it’s a different question from leaving out a 20% levain, which is where the full two points come from.
The takeaway: when a recipe quotes a hydration, ask “including the starter?” before you trust the number. If it doesn’t say, add half the starter’s weight to both flour and water (for a 100% starter) and redo the division yourself.
What hydration should sourdough be?
Once you’re measuring it honestly, most everyday sourdough loaves land around 70–78%. That’s the range in our calculator’s spec table, and it sits inside the 60–80% that The Perfect Loaf gives for most bread doughs. King Arthur calls about 60% low hydration and more than 80% high, and it lists pain de campagne at 80%.
A few things move the right number for your loaf:
- Whole grains are thirstier. Both King Arthur and The Perfect Loaf note that whole wheat and rye absorb more water, because the bran and germ soak it up. A 75% whole wheat dough feels much stiffer than a 75% white dough. Add a few points as the whole-grain share goes up.
- Your flour matters. Strong bread flour holds more water than all-purpose. If you’re on all-purpose, start two or three points lower.
- Hold some water back. The calculator’s own advice: keep a little of the water aside and add it during mixing if the dough can take it. It’s much easier to add water than to take it out.
- Change one thing at a time. Raise hydration by two points per bake. That way you can tell whether the more open crumb came from the water or from a longer ferment.
Let the calculator do the subtraction
The math isn’t hard, but it’s fiddly, and it has to be redone every time you change the starter amount or its hydration. The sourdough calculator takes your total flour, target hydration, starter percentage and starter hydration, and gives you the flour and water to add in US or metric units.
If you’re converting a yeasted recipe, the baker’s percentage calculator turns any formula into grams, and our guide to baker’s percentages explains why flour is always 100%. Temperature controls how fast all that water and starter ferment: the dough temperature calculator tells you how warm your mixing water should be to reach a target dough temperature. If your recipe lists flour in cups, the cups to grams converter gets you to weights first. Hydration measured by the cup isn’t worth calculating.