Hydration is the weight of water divided by the weight of flour, written as a baker's percentage. For a yeasted dough it is a single division. Sourdough adds a wrinkle: the starter is itself a mixture of flour and water, and both end up in the bread. Leave them out and your hydration figure can be off by several points, which is enough to make one recipe look stiffer than another when it isn't, or to turn a manageable dough sticky when you switch starters. This guide shows how to count the starter properly, how to read levain and prefermented-flour percentages, how to convert a recipe to a different starter and how to handle milk, eggs and butter.
Total hydration includes the starter
The figure bakers quote for sourdough is the total hydration: all the water in the bread divided by all the flour, including what is inside the starter.
total hydration = (dough water + starter water) ÷ (dough flour + starter flour) × 100
To split a starter into its flour and water you need its own hydration. Using that hydration as a decimal (100% = 1.0, 50% = 0.5):
starter flour = starter weight ÷ (1 + starter hydration)
starter water = starter weight − starter flour
If you feed your starter equal weights of flour and water, it is at 100% hydration whatever the feeding ratio: 1:1:1 and 1:5:5 both give 100%. If the flour and water differ, work it out including the seed. A 1:5:4 feeding of a 100% starter (say 10 g starter, 50 g flour and 40 g water) gives (5 + 40) ÷ (5 + 50) = 81.8% hydration. For grams at any ratio and a rough time to peak, use the starter feeding calculator.
Worked example: a 100% hydration starter
A recipe calls for 450 g flour, 325 g water, 100 g ripe starter at 100% hydration and 10 g salt.
- Flour in the starter: 100 ÷ (1 + 1.0) = 50 g; water in the starter: 50 g
- Total flour: 450 + 50 = 500 g
- Total water: 325 + 50 = 375 g
- Total hydration: 375 ÷ 500 × 100 = 75%
Ignore the starter and you would calculate 325 ÷ 450 = 72.2%, nearly three points too low. The salt changes as well: 10 g is 2% of the 500 g total flour, not 2.2% of the 450 g in the final dough.
Worked example: a stiff starter
Now make the same recipe with a stiff starter kept at 50% hydration, a style many Italian bakers favor. 100 g of it contains 100 ÷ 1.5 = 66.7 g flour and 33.3 g water.
- Total flour: 450 + 66.7 = 516.7 g
- Total water: 325 + 33.3 = 358.3 g
- Total hydration: 358.3 ÷ 516.7 × 100 = 69.4%
The weights on the scale are identical, but the dough is more than five points drier and will feel noticeably firmer. The table shows how much the starter's own hydration moves this particular recipe:
| Starter hydration | Flour in 100 g starter | Water in 100 g starter | Total hydration of the example dough |
|---|---|---|---|
| 50% | 66.7 g | 33.3 g | 69.4% |
| 60% | 62.5 g | 37.5 g | 70.7% |
| 80% | 55.6 g | 44.4 g | 73.1% |
| 100% | 50 g | 50 g | 75.0% |
| 125% | 44.4 g | 55.6 g | 77.0% |
Two ways to express the levain
Formulas describe the amount of starter in one of two ways, and mixing them up can double or halve your starter by mistake.
Levain as a percentage of total flour
This is the weight of ripe levain divided by the total flour. In the first example, 100 ÷ 500 = 20%. It is easy to weigh and is what most home recipes mean by "20% starter". Some recipes divide by the final-dough flour instead (100 ÷ 450 = 22.2%), so check which base a recipe uses before copying the number.
Prefermented flour (PFF)
Professional formulas often state the prefermented flour instead: the share of the total flour that is inside the levain. In the first example that is 50 ÷ 500 = 10%. PFF is the better guide to fermentation because it does not depend on how wet the starter is. The two are linked by a simple formula:
PFF % = levain % ÷ (1 + levain hydration)
| Levain (% of total flour) | Levain hydration | PFF |
|---|---|---|
| 20% | 100% | 10% |
| 20% | 50% | 13.3% |
| 15% | 100% | 7.5% |
| 15% | 50% | 10% |
| 30% | 100% | 15% |
Notice the fourth row: 15% of a 50% hydration levain delivers the same 10% PFF as 20% of a liquid one.
Converting a recipe to a different starter hydration
The cleanest conversion keeps both the prefermented flour and the total hydration the same, so the bread ferments and feels much as the author intended. Here is the recipe above, written for a 100% starter, converted for a 50% stiff starter:
- Find the flour in the original starter: 100 g at 100% hydration = 50 g flour.
- Build the same amount of flour into your starter: 50 × (1 + 0.5) = 75 g of stiff starter, which holds 50 g flour and 25 g water.
- Keep the final-dough flour at 450 g.
- Adjust the final-dough water so the total stays at 375 g: 375 − 25 = 350 g.
The converted recipe is 450 g flour, 350 g water, 75 g stiff starter and 10 g salt: still 500 g total flour, 375 g total water, 75% hydration and 10% PFF. Going the other way works the same: a recipe calling for 80 g of starter at 60% hydration contains 50 g flour and 30 g water, so use 100 g of a 100% starter and reduce the final-dough water by 20 g.
If you would rather not change the starter amount, you can use the original 100 g of stiff starter and rebalance the dough instead: 433.3 g flour and 341.7 g water in the final dough keep the totals at 500 g and 375 g. The PFF rises to 13.3%, though, so watch the dough rather than the clock. Stiff starters are also often described as giving a more acetic tang than liquid ones, so even a perfect conversion may taste a little different.
Effective hydration with milk, eggs and butter
Enriched sourdoughs complicate things further, because milk, eggs and butter are only partly water. For a realistic hydration figure, count only the water they contain. Common approximations are milk 87% water, whole eggs (out of the shell) 75% and butter 16%.
Example: a sourdough sandwich loaf with 450 g flour, 100 g starter at 100% hydration, 250 g milk, 50 g egg, 30 g butter and 10 g salt.
| Ingredient | Weight | Water content | Water |
|---|---|---|---|
| Starter (100% hydration) | 100 g | 50% | 50 g |
| Milk | 250 g | 87% | 217.5 g |
| Egg | 50 g | 75% | 37.5 g |
| Butter | 30 g | 16% | 4.8 g |
| Total water | 309.8 g |
Total flour is 450 + 50 = 500 g, so the effective hydration is 309.8 ÷ 500 = 62%. Count every liquid at face value and you would get (50 + 250 + 50) ÷ 500 = 70%, which describes a very different dough. Even at the same effective hydration an enriched dough feels softer than a lean one, because fat and sugar tenderize the gluten. Our bread hydration chart lists typical figures for brioche, challah and sandwich bread.
Practical tips
- Write down your starter's hydration. It is the one number you need for every calculation on this page.
- When comparing two recipes, convert both to total hydration first. A "72%" recipe that ignores a 100 g starter may really be 75%.
- If you change the starter amount, recalculate. Adding 50 g more of a 100% starter to the first example adds 25 g flour and 25 g water and lifts hydration from 75% to 76.2%.
- Whole grain flours absorb more water, so a 75% dough with 30% whole wheat feels firmer than a 75% white dough.
The sourdough hydration calculator handles all of this automatically, and you can see these ideas at work in our country sourdough formula. For the underlying math, start with baker's percentage explained.