Build backward, not forward
Most people feed a starter forward: take what is in the jar, add some flour and water, see what happens. That works until a formula asks for 400 g of levain and you are holding 30 g of seed, and then you are guessing. Working backward is arithmetic. Decide what you need at the mixer, add what you are keeping back and a few percent for what sticks to the jar, then divide by the growth factor of your feed as many times as you plan to feed.
The growth factor is the whole ratio divided by the seed part. A 1:5:5 feed is eleven parts out for one part in, so the mass multiplies by 11. A 1:1:1 feed multiplies by 3. A 1:2:2 by 5. Two builds at 1:5:5 is 121 times, which is how 4 g of seed becomes half a kilo of levain in two feeds without any of the intermediate amounts being awkward to mix.
The part everyone gets wrong once
A levain is not an ingredient sitting outside the formula. It is flour and water that you happened to mix early. If your levain is kept at 100% hydration — equal weights of flour and water — then 200 g of it is 100 g of flour and 100 g of water, and both belong in the formula totals.
Here is the failure in numbers. A formula says 500 g flour, 75% hydration, so 375 g of water, plus 200 g of levain. Weigh out all three and mix. You now have 600 g of flour and 475 g of water in the bowl, which is 79.2% hydration, not 75%. The dough is four points wetter than the recipe you were following, and if it spread on the bench you will blame your shaping. Deduct the levain first and you weigh 400 g of flour and 275 g of water at the mixer, which with the levain gives exactly the 500 g and 375 g the formula asked for.
The same deduction gives you the prefermented flour figure — the share of the total flour that has been through the levain. In that example it is 100 of 500, so 20%. It is the number bakers actually compare formulas on, far more than the levain weight, because 200 g of levain means nothing until you know how much flour the bread has.
Stiff starters carry less water than you think
| Starter hydration | 200 g of it is | Water it adds |
|---|---|---|
| 50% — stiff, lievito madre style | 133 g flour + 67 g water | 67 g |
| 75% | 114 g flour + 86 g water | 86 g |
| 100% — the common liquid starter | 100 g flour + 100 g water | 100 g |
| 125% | 89 g flour + 111 g water | 111 g |
Switching a formula from a liquid starter to a stiff one and changing nothing else takes 33 g of water out of that 200 g and puts 33 g of flour in, which moves a 500 g formula by more than six points of hydration in the wrong direction. That is a different bread, not a small adjustment. The calculator above asks for the seed hydration for exactly this reason, and the dough hydration adjustment calculator works out the correction if you find out after the fact.
Discard, and why the ratio decides how much of it there is
Discard is not a mistake in the method, it is a consequence of the ratio. Feeding 1:1:1 means you must throw away two thirds of the jar every time or the jar triples. Feeding 1:5:5 means you only keep back a small seed and the discard is proportionally larger but happens from a smaller jar. Keeping 20 g of seed and feeding it 1:5:5 gives 220 g and no discard at all if you are using 200 g of it for bread. Sizing the maintenance feed to what you actually bake is the way to stop generating discard, rather than finding recipes to use it up.
What this page will not tell you
It will not tell you when the levain is ready, how warm to keep it, or how long any of it takes. Those depend on your flour, your culture, your kitchen temperature and how the starter has been kept, and any single number offered for them would be wrong for most readers. If you want the temperature side of the arithmetic, the desired dough temperature calculator handles the water temperature that lands your dough where you want it. For the formula itself in percentages, the bakers percentage calculator converts between percentages and weights, and the baking basics guide covers the vocabulary.
Questions people ask
How much starter do I need to end up with 400 g of levain?
Divide by the growth factor of your feed, once per build. At 1:5:5 the factor is 11, so one build needs about 36 g of seed and two builds need about 3.3 g. Add whatever you are keeping back for next time and a few percent for what stays stuck to the jar before you divide, or you will come up a little short at the mixer — which is the annoying kind of short, because there is no way to fix it in the twenty minutes before you mix.
Does the levain count toward the flour and water in my recipe?
Yes, always, and forgetting it is the most common arithmetic error in sourdough baking. A levain is flour and water that fermented early. At 100% hydration, half its weight is flour and half is water, and both belong in the formula totals. If the recipe lists levain as a separate line on top of a flour weight and a water weight, read carefully to see whether the author has already deducted it; good formulas say so explicitly, and plenty do not.
What is prefermented flour percentage?
The share of the total flour in the bread that went through the levain first, expressed against the total flour. If a formula has 500 g of flour altogether and 100 g of that arrived inside the levain, the prefermented flour is 20%. It is the figure bakers use to compare formulas, because the levain weight on its own tells you nothing without knowing the size of the dough. Two breads at the same prefermented flour percentage behave much more alike than two breads that happen to use the same weight of levain.
Should I feed 1:1:1 or 1:5:5?
They are different tools rather than better and worse. A small feed like 1:1:1 leaves a lot of mature starter in the mix, so the culture is dense and the build finishes sooner, but the jar triples every time and the discard piles up. A big feed like 1:5:5 dilutes the seed heavily, takes longer to come up, and lets you get from a teaspoon to a kilo in two builds. Bakers on a schedule pick the ratio that makes the build finish when they want to mix, which is a question about your day rather than about the culture.
Why does the calculator ask about loss to the jar?
Because levain is sticky and a meaningful fraction of it never comes out. On a 200 g build, four percent is eight grams and nobody notices. On a five kilogram build for a bakery day it is two hundred grams, which is a whole loaf. The field is there rather than baked in as a constant because it depends entirely on your container and your scraper: a straight-sided tub with a bowl scraper loses far less than a jar with shoulders.