What a levain build is, and what the ratio actually means
A levain is the specific batch of starter you build for one bake. You seed it with a spoonful of your mother culture, feed it flour and water, and let it ripen on a timetable that lines up with your mixing. The feeding ratio — written 1:5:5, 1:3:3, 1:2:2 and so on — is the proportion of seed to flour to water by weight, and it is the single control you have over how long the levain takes to ripen and how sour it ends up.
Read the ratio as parts, not as grams. A 1:5:5 build is one part mature starter, five parts flour, five parts water: eleven parts in total, of which the seed is 1/11, or 9.1%. Scale those parts to whatever total you need. That is all this calculator does on the arithmetic side, but it also handles the part bakers get wrong: the seed is not neutral. It carries its own flour and water, so a 1:5:5 build seeded from a stiff 50% mother is not at 100% hydration, and a levain that looks like 10% of your formula flour by weight is contributing something quite different in prefermented flour.
Why does the seed ratio matter so much? Because it sets the dilution. A small seed in a lot of fresh flour means a long ripening time, a mild flavour and a levain that is at peak strength when you use it. A large seed means a fast build and a sharper, more acidic culture. Bakers move between them by season: bigger seed in winter to keep the timing, smaller seed in summer so the levain is not screaming ripe by dawn.
The formula: sizing the build, then splitting it
Two steps. First decide how much levain you need. Second, split that mass by the ratio.
Step one depends on which convention your recipe uses, and they are not interchangeable. Home recipes almost always mean levain weight as a percentage of formula flour: 20% levain on 1,000 g of flour is 200 g of levain. Professional formulas, and most bakery textbooks, mean prefermented flour: the flour inside the levain as a percentage of the formula's total flour. Because a 100% hydration levain is only half flour by weight, 20% prefermented flour on 1,000 g needs 400 g of levain, not 200 g. Choosing the wrong basis doubles or halves your leavening.
To go from prefermented flour to levain weight you need the levain's flour fraction, which is not simply 1/(1+hydration) unless the seed happens to sit at the build hydration. Written out, the flour fraction is
k = (b + a/(1+h)) ÷ (a + b + c)
where a, b, c are the seed, flour and water parts and h is the seed hydration as a decimal. The seed contributes a/(1+h) parts of flour; the added flour contributes b. Then levain mass = prefermented flour target ÷ k.
Step two is proportional division. Multiply the build mass by a/(a+b+c) for the seed, by b/(a+b+c) for the flour, and by c/(a+b+c) for the water. Add a waste allowance on top — levain sticks to jars, spatulas and hands, and coming up 15 g short at mixing time is worse than throwing 20 g away.
Finally, the true hydration. Add the seed's water to the water you poured, add the seed's flour to the flour you fed, and divide. When the seed sits at the same hydration as the build, the two figures coincide and the arithmetic simplifies to c/b. When they differ — a stiff mother feeding a liquid levain, or the reverse — the finished hydration lands between the two, weighted by how large the seed is.
Worked example: 20% levain, 1:5:5, on a 1,000 g formula
These are the calculator's defaults, with a 100% hydration mother and a 10% waste allowance. Every figure is checkable on paper.
- Size the levain. The basis is levain weight, so 20% × 1,000 g = 200 g of levain goes into the dough.
- Add the waste allowance. 200 × 1.10 = 220 g to build.
- Count the parts. 1 + 5 + 5 = 11.
- Seed. 220 × 1 ÷ 11 = 20 g of mature starter.
- Flour. 220 × 5 ÷ 11 = 100 g.
- Water. 220 × 5 ÷ 11 = 100 g.
- Split the seed. At 100% hydration the 20 g of seed is 20 ÷ 2 = 10 g flour and 10 g water.
- True hydration. Water 100 + 10 = 110 g; flour 100 + 10 = 110 g; 110 ÷ 110 = 100%. It matches the build hydration here because the seed is also at 100%.
- Prefermented flour. The 200 g that goes into the dough is half flour, so 100 g of the formula's 1,000 g arrives prefermented: 10.0%.
Now change one thing: seed from a stiff 50% mother instead. The 20 g of seed becomes 20 ÷ 1.5 = 13.3 g flour and 6.7 g water. Total water is 106.7 g against 113.3 g of flour, so the levain lands at 94.1% rather than 100%, and the prefermented flour in the 200 g portion rises to 10.3%. Small, but it is the kind of drift that makes a formula behave differently in someone else's kitchen.
Change the basis instead, and the difference is large. Ask for 20% prefermented flour on the same 1,000 g at 1:5:5 from a 100% mother: the flour fraction k = (5 + 1÷2) ÷ 11 = 0.5, so you need 200 ÷ 0.5 = 400 g of levain — twice as much as the levain-weight reading of the same number.
Choosing a ratio and reading the result
Pick the ratio by the time you have, not by tradition. A 1:1:1 build has a third of its mass as ripe seed and will peak in a few hours in a warm kitchen; a 1:10:10 build has under 5% seed and will take most of a day. Between them the relationship is roughly geometric — each doubling of the flour-and-water side buys you meaningful extra time, though the exact hours depend on temperature far more than on the ratio.
Read the prefermented flour figure as your fermentation clock and the levain weight as a bookkeeping number. Most country breads run 10–20% prefermented flour. Below 10% the bulk stretches out and the flavour stays mild; above 25% the dough moves fast and the acidity climbs; above 40% you are close to baking the levain itself. When you change ratio or basis, watch this number rather than the levain weight, because it is what the dough responds to.
Read the true hydration figure as the one to carry forward into your dough calculation. If your mother is at a different hydration from the build, the finished levain sits between the two and the calculator says where. Feed that number into the sourdough starter hydration calculator along with your flour and water weights and you will get the dough's real hydration rather than an estimate.
One judgement the arithmetic cannot make for you: whether the levain is ripe. Volume roughly doubling to tripling, a domed then just-flattening top, a smell that has turned from sweet to tangy but not sharp, and a float test that passes are the usual signs. A build that has collapsed has passed peak; it will still leaven, but more slowly and more sourly than the schedule assumes.
Seed, flour and water for a 200 g levain at common feeding ratios
| Ratio (seed:flour:water) | Seed | Flour | Water | Seed share |
|---|---|---|---|---|
| 1:1:1 | 66.7 g | 66.7 g | 66.7 g | 33.3% |
| 1:2:2 | 40.0 g | 80.0 g | 80.0 g | 20.0% |
| 1:3:3 | 28.6 g | 85.7 g | 85.7 g | 14.3% |
| 1:4:4 | 22.2 g | 88.9 g | 88.9 g | 11.1% |
| 1:5:5 | 18.2 g | 90.9 g | 90.9 g | 9.1% |
| 1:10:10 | 9.5 g | 95.2 g | 95.2 g | 4.8% |
Every row is at 100% build hydration because the flour and water parts are equal. Stiff ratios such as 1:5:2.5 have a different part count, so read those from the calculator rather than adjusting a row here.
Mistakes that throw a levain build off
- Reading a percentage against the wrong basis. Levain weight and prefermented flour are different quantities. At 100% hydration the second is half the first, so mixing them up doubles or halves your leavening.
- Treating the seed as if it were nothing. A 1:2:2 build is a fifth seed by weight, and that seed's own flour and water shift both the hydration and the prefermented flour figure noticeably.
- Assuming the mother and the build share a hydration. They often do not, especially when bakers keep a stiff mother and build a liquid levain for a specific bread.
- Building exactly what the recipe calls for. Levain clings to everything. Ten per cent extra costs a few grams of flour and removes a whole category of mixing-day panic.
- Weighing a 3 g seed on a 1 g scale. At small build sizes the seed becomes the least accurate weight in the formula. Use a 0.1 g scale or build in two stages.
- Changing the ratio without changing the timing. A 1:2:2 and a 1:8:8 build cannot ripen on the same clock. If you swap ratios to fit a schedule, expect to re-time the bulk as well.
- Using discard as seed. Unfed discard from the fridge is weaker and more acidic than ripe starter. It works, but the build will run slow relative to what the ratio implies.
Key terms
- Seed
- The mature starter you carry over into a new build. Sometimes called the inoculum or the mother portion.
- Feeding ratio
- Seed to flour to water by weight, written a:b:c. It sets both the ripening time and the build hydration (c divided by b).
- Prefermented flour
- The flour inside the levain expressed as a percentage of the formula's total flour. The standard professional way of describing how much preferment a bread carries.
- Stiff levain
- A build at roughly 50-60% hydration. It ripens more slowly, produces relatively more acetic acid, and holds its peak longer than a liquid one.
- Discard
- Starter removed before a feed. Usable in pancakes and crackers, but weaker and more acidic than ripe levain if used as seed.
Fitting the build into the rest of the bake
The build is the first of three calculations in a sourdough day. Once you know the levain's weight and true hydration, the dough hydration follows — and because the levain is flour and water, you cannot get that right by dividing the water you pour by the flour you weigh. The sourdough starter hydration calculator takes the two figures this page produces and returns the dough's real hydration plus the water adjustment to hit a target.
The third calculation is temperature. Your levain goes into the mixer at its own temperature and counts as a fourth factor in the desired dough temperature formula, so the water temperature you need depends on how warm the build is when you use it. Run the desired dough temperature calculator with the levain probed rather than assumed; a ripe build often reads two or three degrees above the room.
If you are comparing a levain against commercial yeast — deciding whether to spike a slow sourdough or converting a yeasted formula — the useful common currency is baker's percentage of flour, which the yeast conversion calculator reports alongside the weight conversions between fresh, active dry and instant. And for pizza, where dough is portioned by pan area rather than by loaf, the pizza dough thickness factor calculator sets the ball weight while the levain percentages here still govern the ferment.
What this calculator does not do is predict ripening time. That depends on temperature, on the vigour of your particular culture, on flour type and on how long the mother has gone unfed, and no formula built from a feeding ratio alone can capture it. Use the ratio to set a plan, then judge ripeness by the levain itself.