A starter’s hydration describes its water relative to flour. Equal fresh additions do not make a whole build 100% hydration if the seed is stiff or liquid.
- Split seed starter into flour and water.
- Include seed contributions in the next build.
- Use weight ratios for reproducible comparisons.
Hydration describes composition
Starter hydration is water weight divided by flour weight, multiplied by 100. A starter containing 60 g flour and 60 g water is at 100% hydration. It weighs 120 g in total. The percentage is not a statement that the starter is entirely water; it expresses how much water there is relative to flour.
This guide treats starter as flour and water for recipe bookkeeping. That is the same practical convention used when calculating sourdough dough hydration. It does not measure microbial activity, acidity, or readiness for baking. A precise starter weight and hydration allow a calculator to account for ingredients, while the starter’s condition still requires separate observation and an appropriate feeding routine.
Feeding ratio and hydration are different numbers
A feeding ratio commonly lists seed starter, added flour, and added water by weight. Under that ordering, 1:2:2 means one part seed, two parts flour, and two parts water. With 20 g seed, add 40 g flour and 40 g water for a 100 g build. Always verify the order because some instructions write water before flour.
If the seed is already at 100% hydration, equal fresh flour and water preserve 100% hydration. Increasing the ratio from 1:1:1 to 1:4:4 does not make the starter wetter when the last two quantities remain equal. It changes the seed’s share of the build. King Arthur Baking’s feeding-ratio explanation supports this distinction between the quantities used at feeding and the behavior of the resulting culture.
The seed brings its own flour and water
The old starter is not an empty container for the new feeding. It contributes flour and water that must be included when the starting hydration differs from the target. Consider 30 g seed at 50% hydration. It contains 20 g flour and 10 g water. Adding 60 g flour and 60 g water produces 80 g total flour and 70 g total water.
That build weighs 150 g and has 87.5% hydration, even though its fresh additions were equal. Repeating equal flour-water feedings moves its composition toward 100%, but a single feeding does not make the inherited imbalance disappear. This is why a recipe that says feed equal parts needs the seed’s hydration as an assumption if you want exact mathematical consistency.
Split starter into flour and water
Let M be starter mass and h be hydration expressed as a decimal. Starter flour is M ÷ (1 + h). Starter water is M − starter flour. For 180 g starter at 80%, calculate 180 ÷ 1.8 = 100 g flour; the remaining 80 g is water.
For a 100% starter, division by two is the convenient special case. For a 50% starter, division by 1.5 gives the flour, not half the mass. Write the hydration beside every stored starter quantity in a recipe notebook. Without that label, another baker cannot reliably reconstruct the dough’s flour and water just from a line that says 180 g starter.
Build a target amount at a target hydration
Suppose you want 200 g starter at 100% hydration and have 30 g seed at 50%. The final build must contain 100 g flour and 100 g water. The seed already supplies 20 g flour and 10 g water, so add 80 g fresh flour and 90 g fresh water.
The resulting total is 30 + 80 + 90 = 200 g. Checking the components gives 20 + 80 = 100 g flour and 10 + 90 = 100 g water. This general approach works for any feasible target: determine final flour and water, calculate what the seed contributes, and subtract. It is more reliable than applying an equal-parts feeding automatically to every starter you encounter.
| Component | Flour contribution | Water contribution | Combined weight |
|---|---|---|---|
| 30 g seed at 50% hydration | 20 g | 10 g | 30 g |
| Fresh additions | 80 g | 90 g | 170 g |
| Completed build | 100 g | 100 g | 200 g |
Convert a liquid seed into a stiff build
For a 160 g build at 60% hydration, final flour is 160 ÷ 1.6 = 100 g and water is 60 g. Starting with 20 g seed at 100% hydration contributes 10 g flour and 10 g water. Add 90 g flour and 50 g water to reach the target composition.
The mathematics describes the ingredients immediately after mixing; it does not promise how the starter will develop or when it will be ready. A change in flour, hydration, or feeding routine can change the behavior you observe. Keep the new build labeled and record its progression instead of assuming a previous clock-based schedule transfers unchanged to the new composition.
Recognize impossible build inputs
A target is impossible by addition alone when the seed already contains more flour or water than the planned final build. For example, 100 g seed at 100% hydration contains 50 g water. A target of 120 g starter at 50% hydration would contain only 40 g water and 80 g flour.
The required fresh-water addition would be negative 10 g. That is a useful warning, not an instruction to remove water from a mixed starter. Use less seed, choose a larger final build, or reconsider the target hydration. A well-designed calculator should explain which component exceeds the target rather than outputting a negative ingredient and leaving the baker to interpret it.
Calculate a feeding ratio from a desired yield
For a 1:3:3 feeding with seed, flour, and water in that order, the total contains seven parts. To make 210 g using a 100% hydration seed, divide 210 by seven to obtain 30 g seed, then add 90 g flour and 90 g water. The final hydration remains 100% because both the seed and additions have equal flour and water weights.
To make 150 g with the same ratio, the theoretical quantities are approximately 21.429 g seed and 64.286 g each flour and water. Round sensibly for your scale and record the actual amounts. Feeding ratios are helpful planning language, but they do not require pretending that every home scale can measure a thousandth of a gram.
Plan what goes into bread and what stays behind
A starter build can serve two purposes: supplying the dough and preserving an amount for future maintenance. If a dough needs 120 g and you want to retain 20 g, plan at least 140 g before allowing for any starter left on the jar or utensils. King Arthur Baking’s smaller-starter guidance explicitly accounts for retaining starter after a build.
Keep the bread allocation separate from the maintenance reserve. Only the 120 g that enters the dough belongs in the dough’s flour, water, and weight totals. Counting the whole 140 g build in the bread formula would overstate what was mixed. A build worksheet and a dough worksheet may share ingredients, but they should not silently share the same yield field.
Change starter hydration while preserving a dough
Take a dough target of 600 g total flour, 432 g water, and 60 g prefermented flour. With a 100% hydration starter, use 120 g starter and add 540 g fresh flour plus 372 g fresh water. The starter carries 60 g flour and 60 g water.
With a 60% hydration starter, the same 60 g prefermented flour requires 96 g starter containing 36 g water. Fresh flour remains 540 g, but fresh water becomes 396 g. Both versions have 72% dough hydration and 10% prefermented flour. Their ingredient distribution differs, and identical overall ratios do not establish identical fermentation behavior. The calculation simply isolates the composition variables you chose to hold constant.
Starter mass percentage versus prefermented flour
Starter mass percentage divides the whole starter weight by total dough flour. Prefermented flour percentage divides only the starter’s flour by total dough flour. In the preceding example, the liquid starter is 20% by mass and the stiff starter is 16% by mass, yet both supply 10% prefermented flour.
This distinction is essential when comparing recipes. A statement such as use 20% starter is incomplete unless the denominator and starter hydration are known. Some recipes compare starter with fresh flour rather than total flour, introducing another difference. Preserve the original convention when transcribing a recipe, then convert it explicitly if you want to compare its formula with another one.
Multiple builds and retained portions
When a recipe uses several builds, calculate each stage from what actually moves into the next. Suppose one build produces 200 g starter but the next stage uses only 50 g. Only one quarter of the first build’s flour and water moves forward, assuming the starter is thoroughly combined before taking that portion.
The remaining 150 g has not vanished, but it is outside the later build’s ingredient accounting. Likewise, when only part of a final levain enters the bread, include only that used portion in the dough totals. Drawing a short sequence of containers and transfer weights can make a complex recipe easier to audit than a long list of ingredient additions without any record of what was retained or discarded.
Hydration cannot tell you maturity
A freshly mixed starter and a mature starter can have the same recorded hydration. The number does not tell you whether either is suitable for a particular recipe. King Arthur Baking’s feeding resources describe feeding and visible starter activity as parts of ongoing maintenance; they do not reduce readiness to a flour-water percentage.
Keep observations separate from the arithmetic: note when the build was mixed, its environment, and how its appearance and expansion changed. Follow the readiness cues in the recipe you are using rather than turning a calculated feeding ratio into an exact deadline. When diagnosing inconsistent results, a reliable record of composition and condition is more useful than assuming that every 100% hydration starter behaves alike.
A practical starter worksheet
For a build, write down seed mass, seed hydration, target final mass, target hydration, fresh flour, fresh water, and the amount reserved after mixing the dough. For the bread, record only starter used and its hydration. These few entries make it possible to repeat the arithmetic without reconstructing several days of feeding from memory.
As a final check, the build’s ingredient weights should add to its target mass, its water divided by flour should match target hydration, and no required addition should be negative. Then confirm that the usable amount covers the recipe and the planned reserve. Those checks establish a consistent ingredient plan. They leave fermentation decisions where they belong: with the recipe method, observed starter condition, and the actual environment.
Sources & further reading
This guide combines published references with our own explanations and decision aids. Numerical examples are illustrative; they are not records of Sourdough Bench test bakes.
Put it into practice.
Keep the recipe, observations and next change together in your downloadable bake log. For a symptom-led starting point, use the loaf troubleshooting guide.