From starter to levain · Free calculator

Leaven Calculator

Most recipes call for a set amount of leaven at a set hydration, but your jar holds a seed starter that’s some other amount at some other hydration. This calculator does the conversion: tell it what you have and what the recipe needs, and it returns exactly how much flour and water to feed.

No more mixing double what you need "to be safe" — build the leaven the bake actually calls for.

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? Leaven Calculator

? Calculate how much flour and water to add to a seed starter to build a target leaven.

Parameter	Value
? Target Leaven Amount
? Target Leaven Hydration	%
? Seed Starter Amount
? Seed Starter Hydration	%
? Water to Add	--
? Flour to Add	--

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Leaven Calculator

Building a Leaven From a Seed

You keep a small "seed" of mature starter and grow it into the larger leaven a recipe needs. The trick: your seed already contains flour and water, so to hit a target leaven at a target hydration you must add flour and water in the right amounts — accounting for what's already in the seed. That's the whole job of this calculator.

seedFlour = seedStarter ÷ (1 + seedHydration/100) seedWater = seedStarter − seedFlour targetFlour = targetLeaven ÷ (1 + targetHydration/100) targetWater = targetLeaven − targetFlour flourToAdd = targetFlour − seedFlour waterToAdd = targetWater − seedWater

Leaven Hydration Guide

Leaven hydration changes far more than texture — it shifts fermentation speed, acidity balance, and which breads the leaven naturally suits. Lower hydration favours acetic (vinegary, sharp) acid and slower, cooler-tolerant ripening; higher hydration favours lactic (yogurt-tangy, mild) acid and faster ripening. Here is what each band typically means and makes:

Leaven Style	Hydration	Breads normally made with it	Why bakers choose it
Lievito madre	~45–50%	Panettone, pandoro, colomba and other enriched Italian celebration breads.	A very firm, sweet-leaning culture that stays mild and strong enough to lift heavy butter-and-egg doughs. Kept stiff to restrain acidity so the crumb tastes delicate, not sour.
Stiff leaven	50–65%	Rye-blend loaves, bagels, some baguettes, doughs wanting extra structure.	Slower, more controlled ripening and a stronger gluten build. Tends toward a rounder, more acetic tang and gives a longer window before over-ripening — handy in warm kitchens.
Medium leaven	70–85%	Everyday pan loaves and mixed-flour sourdoughs.	A practical compromise: easier to mix than a stiff build, less quick-souring than 100%. Balanced flavour and a forgiving schedule.
Liquid leaven	100%	Classic country boules, pain au levain, most rustic open-crumb sourdough.	Equal flour and water — trivial to maintain, mix, and do baker's math with. Ripens at a moderate pace with a balanced lactic/acetic profile. The default for most home bakers.
High-hydration leaven	110–125%+	High-hydration ciabatta, focaccia, very open-crumb breads.	Loose and batter-like; ripens fast and leans mild/lactic. Adds extensibility (dough stretches easily) but the timing window is short, so it suits bakers watching the clock closely.

Sourjoe note: hydration is not "better" or "worse" by itself. As a rule of thumb: stiffer → slower, sharper (acetic), more structure; wetter → faster, milder (lactic), more extensible. Match the leaven to the bread and the schedule you actually have.

Worked example

Sourjoe's standard single-loaf build: take 15 g of refrigerated starter and add 60 g flour and 60 g warm water — 135 g of leaven at 100% hydration, comfortably covering the 100–125 g most single-loaf recipes call for. Now work it backwards above: tell the calculator you need 250 g at 80% hydration for a two-loaf bake, and it returns the exact flour and water to add to your seed.

Where it fits in the bake

The leaven build is the true first step of the Baker's Process — it starts 6–8 hours before your hands ever touch flour, which is why the Schedule Builder puts it at the top of every timeline. Build it here, and use the float test to know when it's ready to mix.

Leaven Calculator FAQ

What is the difference between a starter and a leaven?

The starter is the master culture living in your fridge; the leaven (or levain) is a fresh offshoot built from a small scoop of it, brought to peak activity for one specific bake. The starter is the seed bank — the leaven is the crop.

How much leaven do I need for one loaf?

Most single-loaf recipes call for 100–125 g. Sourjoe's standard 15 g + 60 g + 60 g build yields 135 g, leaving a little margin for what sticks to the bowl. Doubling every input covers two loaves.

How long before mixing should I build my leaven?

At room temperature, 6–8 hours to peak — so an evening build is ready for a morning mix. It's ripe when it has roughly doubled, is visibly aerated, and smells sweet-tart like yogurt or overripe fruit.

What hydration should my leaven be?

Match the calculator's default 100% unless the recipe says otherwise — and if it does, that's exactly what the hydration input is for. Stiffer leavens (60–80%) ferment slower and trend milder; the calculator recomputes the flour/water split instantly.

How do I know my leaven is ready?

The float test: drop a teaspoon into water. If it floats, it has trapped enough gas to raise your dough. One caveat — heavy whole-grain leavens can be ready yet still sink, because bran pops the bubbles, so pair the test with the doubled-and-bubbly visual check.

Can I bake with starter straight from the fridge instead of building a leaven?

You can, but you’re trading control for convenience: cold, unfed starter is acidic and sluggish, so the rise runs slower and sourer than the recipe expects. The 6–8 hour leaven build exists to hand your dough a culture at peak power — it’s the cheapest quality upgrade in sourdough.

Want to know why this works? Every model and constant behind this calculator is explained, derived, and put to work in The Book of Sourjoe — the interactive book this calculator was built for.

These calculators are just the crust. Check out the Public Preview of the Sourjoe App — The Book of Sourjoe, recipes, and every baking tool in one place.

Questions? Comments? Send us a note at info@sourjoe.com.