Starter hydration is the ratio of water to flour in your sourdough starter, expressed as a baker's percentage. If you feed your starter with equal parts flour and water by weight (for example, 50 g flour and 50 g water), your starter is at 100% hydration. If you feed it with 50 g flour and 25 g water, it is at 50% hydration.
The formula is the same as for any hydration calculation:
Starter Hydration (%) = (Water in Starter / Flour in Starter) x 100
Starter hydration matters because your starter is not just a flavoring or leavening agent. It is a mixture of flour and water, and when you add it to your dough, that flour and water become part of the dough's total flour and water. If you do not account for this, your actual dough hydration will be different from what you intended.
Most bakers maintain their starter at one of a few standard hydration levels. Each has different characteristics, advantages, and typical uses.
| Hydration | Feed Ratio (Flour:Water) | Consistency | Characteristics | Common Names |
|---|---|---|---|---|
| 50% | 2:1 | Very stiff, like a firm dough ball. Must be kneaded into the dough or dissolved in water before mixing. | Slow fermentation, mild acidity, more acetic acid (vinegary tang). Rises slowly but holds its peak longer. Tolerates neglect well. | Stiff starter, lievito madre, pasta madre |
| 60-65% | Approx. 3:2 | Firm but pliable. Holds its shape but can be scooped with a spoon. | Moderate fermentation speed. Good balance of acetic and lactic acid. Popular in Italian and French baking traditions. | Stiff levain, firm starter |
| 75% | 4:3 | Thick paste, similar to thick hummus. Scoopable but does not pour. | Moderate fermentation speed. Slightly more acetic than a 100% starter. Good compromise between stiff and liquid. | Medium starter |
| 100% | 1:1 | Thick batter, like pancake batter. Pours slowly. Doubles in volume at peak. | Standard fermentation speed. Balanced flavor with both acetic and lactic acid. Easy to mix into dough. | Standard starter, liquid levain |
| 125% | 4:5 | Thin batter, pours easily. Very bubbly and active at peak. | Faster fermentation. More lactic acid (creamy, yogurt-like tang). Passes the peak quickly; timing is more critical. | Liquid starter, liquid levain |
| 166% | 3:5 | Very thin, almost like a slurry. Extremely bubbly. | Very fast fermentation. Strongly lactic flavor. Must be used promptly at peak or it will exhaust its food supply quickly. | Very liquid starter |
The vast majority of home bakers use a 100% hydration starter because it is the simplest to maintain (equal parts flour and water) and the easiest to do math with (the starter is exactly half flour and half water). Most recipes you find online assume a 100% hydration starter unless stated otherwise.
This is the critical concept. When you add starter to a recipe, you are adding both flour and water. The amount of each depends on the starter's hydration.
The formulas are:
Or, more simply: once you know the flour in the starter, the water is just the remainder.
Let us say you are adding 100 g of starter to your recipe. Here is how the flour and water break down at different hydration levels:
| Starter Hydration | Starter Weight | Flour in Starter | Water in Starter |
|---|---|---|---|
| 50% | 100 g | 100 / (1 + 0.50) = 66.7 g | 100 - 66.7 = 33.3 g |
| 75% | 100 g | 100 / (1 + 0.75) = 57.1 g | 100 - 57.1 = 42.9 g |
| 100% | 100 g | 100 / (1 + 1.00) = 50.0 g | 100 - 50.0 = 50.0 g |
| 125% | 100 g | 100 / (1 + 1.25) = 44.4 g | 100 - 44.4 = 55.6 g |
| 166% | 100 g | 100 / (1 + 1.66) = 37.6 g | 100 - 37.6 = 62.4 g |
Notice the dramatic difference: 100 g of a 50% starter contributes 66.7 g of flour and only 33.3 g of water, while 100 g of a 125% starter contributes just 44.4 g of flour and 55.6 g of water. If you swap one for the other without adjusting, your dough will be significantly different.
The hydration of your dough is determined by the total water divided by the total flour. Since your starter contributes both flour and water, its hydration directly affects the effective hydration of the finished dough.
Consider this base recipe: 450 g bread flour, 315 g water, 100 g starter, 10 g salt. Let us see how the effective dough hydration changes depending on what hydration starter you use.
| Starter Hydration | Flour in Starter | Water in Starter | Total Flour | Total Water | Effective Dough Hydration |
|---|---|---|---|---|---|
| 50% | 66.7 g | 33.3 g | 516.7 g | 348.3 g | 67.4% |
| 75% | 57.1 g | 42.9 g | 507.1 g | 357.9 g | 70.6% |
| 100% | 50.0 g | 50.0 g | 500.0 g | 365.0 g | 73.0% |
| 125% | 44.4 g | 55.6 g | 494.4 g | 370.6 g | 75.0% |
| 166% | 37.6 g | 62.4 g | 487.6 g | 377.4 g | 77.4% |
The difference between using a 50% starter and a 125% starter is a full 7.6 percentage points of effective dough hydration. That is the difference between a manageable, easy-to-shape dough and a wet, sticky, high-hydration dough. If you follow a recipe that assumes a 100% hydration starter and you use a 50% stiff starter instead, your dough will be noticeably drier and stiffer than intended.
Let us say you have a recipe that uses a 100% hydration starter and you want to use your 75% hydration stiff starter instead, while keeping the effective dough hydration the same.
Effective hydration: Total flour = 450 + 50 = 500 g. Total water = 315 + 50 = 365 g. Hydration = 365 / 500 = 73.0%.
You want to keep the same amount of fermentation activity, so you will use the same weight of starter: 100 g. But now it is 75% hydration instead of 100%.
The original 100% starter contributed 50 g flour and 50 g water. The new 75% starter contributes 57.1 g flour and 42.9 g water. That means the new starter adds 7.1 g more flour and 7.1 g less water than the original.
To keep the same effective hydration, you need to add back the missing water and remove the extra flour:
| Ingredient | Original (100% starter) | Converted (75% starter) |
|---|---|---|
| Bread flour | 450 g | 443 g |
| Water | 315 g | 322 g |
| Starter | 100 g (at 100%) | 100 g (at 75%) |
| Salt | 10 g | 10 g |
| Effective hydration | 73.0% | 73.0% |
The adjustments are small (7 g of flour and water), but at higher starter percentages or more extreme hydration differences, the adjustments become significant. The sourdough calculator handles this conversion automatically. Enter your starter hydration and target dough hydration, and it calculates the exact flour and water amounts.
In some situations, you do not need to worry about starter hydration at all:
If a recipe is written for a 100% hydration starter and you use a 100% hydration starter, no conversion is needed. The recipe's listed hydration is already correct for your situation. Most online recipes assume 100% hydration, and most home bakers maintain 100% starters, so this is the most common case.
Some professionally written recipes, especially those using baker's percentage, already decompose the starter into its flour and water components in the formula. In these recipes, the flour percentage already includes the flour from the starter, and the water percentage already includes the water from the starter. The starter is not a separate line; it is built into the totals. In this case, you simply use whatever flour and water the recipe calls for, plus however much active starter you need for leavening.
If your recipe uses a very low percentage of starter (say, 5% or less), the flour and water it contributes are so small that the effect on overall hydration is negligible. At 5% starter (25 g in a 500 g flour recipe), the difference between a 100% and a 50% starter is about 4 g of flour and 4 g of water. That is within the margin of error of most kitchen scales and will not meaningfully change the dough.
Many experienced bakers do not calculate the precise effect of their starter. Instead, they know what their target dough should feel like and add or withhold a splash of water during mixing to achieve the right consistency. This works well once you have enough experience to judge dough feel, but it is less reliable for beginners or when trying a new recipe for the first time.
Beyond the math, starter hydration has a real impact on the flavor profile of your bread. This is because the yeast and bacteria in your starter behave differently depending on the water content of their environment.
A stiff environment favors acetic acid production. Acetic acid is the sharp, vinegary tang that many people associate with San Francisco-style sourdough. Stiff starters ferment more slowly, which means longer rise times, but the resulting bread tends to have a more pronounced, assertive sour flavor.
Stiff starters are also more resilient. They can go longer between feedings without becoming over-fermented, which makes them convenient for bakers who do not bake every day. In Italy, the traditional lievito madre (mother yeast) is a stiff starter maintained at around 50% hydration, and it can go 2-3 days between feedings if refrigerated.
A wetter environment favors lactic acid production. Lactic acid is the milder, creamier tang found in yogurt. Breads made with a liquid starter tend to have a gentler sourness that is more dairy-like than vinegary. The fermentation is faster, which means shorter rise times but also a narrower window before the starter becomes over-fermented.
Liquid starters are easier to mix into dough and easier to handle, which is why the 100% hydration starter has become the default for home bakers. The faster fermentation also means they are more visually active, with lots of bubbles and a dramatic rise and fall, making it easier to judge when the starter is at peak activity.
A 75% hydration starter sits between the two extremes. It produces a balanced mix of acetic and lactic acid, giving a moderately sour flavor that is complex without being overpowering. Some bakers prefer this hydration because it offers the flavor complexity of a stiff starter with the easier handling of a wetter one.
| Starter Hydration | Dominant Acid | Flavor Character | Fermentation Speed | Feeding Frequency |
|---|---|---|---|---|
| 50% | Acetic acid | Sharp, vinegary tang | Slow | Every 24-48 hours (fridge: 2-3 days) |
| 75% | Balanced | Complex, moderately sour | Moderate | Every 12-24 hours (fridge: 2 days) |
| 100% | Lactic acid | Mild, creamy tang | Moderate-fast | Every 12-24 hours (fridge: 1-2 days) |
| 125%+ | Lactic acid | Very mild, dairy-like | Fast | Every 8-12 hours (fridge: 1 day) |
If you want to experiment with different starter hydrations, here is how to convert your existing starter and maintain it at the new level.
You do not need to start over. Simply change the ratio of flour to water at your next feeding. The microbes in your starter will adapt to the new environment within 2-3 feedings.
To convert a 100% starter to 50% hydration:
To convert a 100% starter to 125% hydration:
Some bakers keep two starters: a stiff one for more assertively sour breads and a liquid one for mild everyday loaves. This is practical if you bake frequently, but it doubles your feeding obligations. A more practical approach for most home bakers is to maintain one starter at 100% hydration (for convenience) and build a levain at a different hydration the night before baking.
For example, if you want to bake with a stiff levain:
This way, you only maintain one starter daily but can use any hydration of levain in your recipes.
The float test (dropping a spoonful of starter into water to see if it floats) is often used to check if a starter is ready to use. Starter hydration affects the reliability of this test.
In general, the float test is a useful tool for 100% hydration starters but should be used with caution at other hydrations. Visual cues (doubling in volume, lots of bubbles, domed top) and timing (knowing your starter's peak after feeding) are more reliable across all hydration levels.
The most common mistake is treating the starter as a single ingredient and not accounting for the flour and water it adds. This leads to inaccurate hydration calculations, especially at higher starter percentages. Always decompose the starter into its flour and water components when calculating effective hydration.
If a recipe was developed with a 100% hydration starter and you switch to a 50% starter without changing the flour and water amounts, your dough will be 5-8% drier than intended. Always recalculate or use the sourdough calculator to adjust.
A cup of stiff starter and a cup of liquid starter contain very different amounts of flour and water. Always measure your starter by weight on a scale, not by volume.
While most home baking recipes assume 100% hydration, many professional and European recipes use stiff starters (50-65%). If you see a recipe from an Italian or French bakery that calls for "levain" or "lievito madre" without specifying a hydration, check the context. Traditional French levain is often 60-65% hydration, and Italian lievito madre is typically 50%.
If you feed your starter with slightly different ratios each time (sometimes 1:1, sometimes a bit more water, sometimes less), its hydration drifts and your recipe calculations become unreliable. Use a scale and be consistent with your feeding ratios. Pick a hydration and stick with it.
| What You Want to Calculate | Formula |
|---|---|
| Starter hydration | (Water in starter / Flour in starter) x 100 |
| Flour in starter | Starter weight / (1 + Hydration / 100) |
| Water in starter | Starter weight x (Hydration / 100) / (1 + Hydration / 100) |
| Total dough flour | Recipe flour + Flour in starter |
| Total dough water | Recipe water + Water in starter |
| Effective dough hydration | (Total dough water / Total dough flour) x 100 |
Your starter's hydration determines how much flour and water it contributes to your dough. A 100% hydration starter is half flour and half water. A 50% stiff starter is two-thirds flour and one-third water. A 125% liquid starter is less than half flour and more than half water. These differences directly affect the effective hydration of your dough, and ignoring them can lead to a dough that is wetter or drier than you intended.
When switching between starter hydrations, adjust your recipe's flour and water to compensate. When starter hydration matches what the recipe assumes (usually 100%), no adjustment is needed. The sourdough calculator handles all of this math automatically, and the baker's percentage guide explains the broader system that makes these calculations possible.
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