Sourdough Calculator

Background.

A sourdough formula has one complication that a commercial-yeast formula does not: the leaven is itself flour and water, so it changes both of the numbers that hydration is computed from. King Arthur states the rule plainly — "everything that comprises the loaf you place in the oven should be considered as part of the loaf" — and gives the arithmetic: a 150 gram starter at 100% hydration contributes 75 grams to the flour total and 75 grams to the water total. This calculator does that decomposition. You tell it the total flour, the overall hydration you want, how big the levain is as a percentage of total flour, and how you maintain your starter, and it hands back the four weights that actually go in the bowl.

That is the difference between overall hydration and mix hydration, and it is worth being precise about because published formulas mean the first and quick mental arithmetic produces the second. Take a 1,000 gram flour formula at 75% overall hydration with a 20% levain at 100% starter hydration. The levain weighs 200 grams and splits into 100 grams of flour and 100 grams of water. So the flour you weigh into the bowl is 900 grams, not 1,000, and the water is 650 grams, not 750. The dough in the bowl is at 650 ÷ 900 = 72.2% — three percentage points below the 75% the formula specifies. Both numbers are correct; they measure different things, and this page reports both so you can tell which one a recipe you are reading actually means.

The page also reports prefermented flour percentage, which is what professional formulas specify instead of a levain percentage. It is the share of the formula's total flour that is already fermenting when you mix — in the example above, the levain holds 100 grams of the 1,000 gram total, so prefermented flour is 10% while the levain percentage is 20%. The two are always different, and the gap widens as the starter gets wetter: at 125% starter hydration a 20% levain carries only 8.9% of the flour. Jeffrey Hamelman's formulas in Bread specify this figure rather than a levain weight, which is why a page that only reported levain percentage would leave you unable to follow them.

There is a third thing this arithmetic buys you, and it is a guard rather than a number. A levain can be so large, or so wet, that the formula becomes impossible — the levain alone holds more flour than the total, or more water than the hydration allows. That happens at exact, calculable points: with a 100% hydration starter and a 75% hydration dough, the water runs out at a levain of 150% of total flour, and the flour runs out at 200%. Below those points the calculator returns weights; at the water edge it returns a legitimate zero, because the levain supplies every gram of the water; above them it refuses and tells you which input to change rather than printing a negative weight.

Five scope limits belong beside the answer rather than buried below it. This page resolves quantities, not fermentation: dough temperature, bulk time, proof time and starter ripeness are process, and a correct set of weights does not make a loaf rise. It does not model the small seed of mature starter carried forward into a levain build, so its levain is flour plus water exactly — a choice recorded in the dossier along with the alternative, because at real seed sizes the effect on overall hydration is a fraction of a point. Salt is taken on total flour, levain flour included, which is the published convention. Hydration counts added water only; 21 CFR 137.105 permits flour to hold up to 15% moisture by law and no hydration figure anywhere includes it. And every input is a mass, because a starter cannot be measured by volume — King Arthur's own chart puts a cup of it at anywhere from 227 to 241 grams.

Below the calculator the arithmetic is written out in full, King Arthur's published example is reproduced step by step, and the FAQs answer the questions this notation raises: why your dough feels drier than the hydration says, why prefermented flour is not the levain percentage, what to do with a stiff starter, and why the dough weight has no levain term in it.

What is sourdough calculator?

A sourdough calculator resolves a bread formula in which the leaven is a flour-and-water culture rather than a packet of yeast. The arithmetic is baker's percentage — flour is 100% and every other ingredient is a percentage of total flour weight — with one extra step: the levain is split into the flour and the water it contains, and those are subtracted from the totals to give what you actually weigh into the bowl.

The split follows from the definition of starter hydration. A starter at hydration q is q parts water to 1 part flour by weight, so a mass L of it is L ÷ (1 + q) flour and L x q ÷ (1 + q) water. At the common 100% hydration that is half and half. At 50% (a stiff starter) it is two-thirds flour. At 125% (Hamelman's liquid levain) it is 44% flour.

Three figures come out of it that a plain formula does not give you. Overall hydration is total water over total flour with the levain counted in both — the number published formulas mean. Mix hydration is what is in the bowl before the levain goes in, and it is lower. Prefermented flour percentage is the share of total flour already fermenting in the levain, and it is the figure professional formulas specify.

The range of validity is any single-build levain formula written by weight. It is exact arithmetic, not an estimate. What it does not cover: multi-stage builds where a levain is refreshed more than once at different hydrations, the seed of mature starter carried into a build, flours' own 15%-or-less moisture content, and every process variable — temperature, timing, ripeness, shaping, oven. Weights are necessary for a reproducible loaf and nowhere near sufficient.

How to use this calculator.

1. 01Choose what you are starting from. Total flour is the right choice when you are following a published formula. Target dough weight is right when you know the finished batch you want — two 885 g loaves, say. The levain you already have is right when you fed your starter last night and want to build the batch around what you actually made.
2. 02Enter the weight for whichever of those you chose. If it is total flour, include the flour inside the levain — that is what 'total' means here and it is the point King Arthur makes about counting everything in the loaf.
3. 03Set the overall hydration you want. This is total water over total flour with the levain included, which is what published sourdough formulas specify. It is not what you will read off the water and flour you weigh into the bowl; the calculator reports that separately as mix hydration.
4. 04Set the levain as a percentage of total flour. 15% to 25% is ordinary at 100% starter hydration. If your formula specifies prefermented flour instead, read the prefermented-flour output and adjust this figure until it matches — for a 100% starter, prefermented flour is exactly half the levain percentage.
5. 05Set your starter hydration honestly. Equal weights of flour and water is 100%. A stiff starter is around 50-60%. Hamelman's liquid levain is 125%. This single input controls how the levain is split, and a wrong value moves the overall hydration by several points.
6. 06Set the salt percentage. It is applied to total flour including the levain's flour, which is the published convention. Weigh salt rather than spooning it: table salt is 18 g per tablespoon and Diamond Crystal kosher is 8 g.
7. 07Read the summary line. It gives the four weights to put in the bowl, the batch weight and the per-loaf weight in one sentence, and the hydration check beside it shows the division so you can audit the number rather than trust it.
8. 08If the calculator refuses, read which field it names. A levain that is too large or too wet makes a formula arithmetically impossible, and the message tells you whether it is the flour or the water that has run out.

The formula.

Start from total flour F, because every percentage on the page is a percentage of it. Total water is W = F x h, salt is S = F x s, and the levain weighs L = F x p.

Now split the levain. A starter maintained at hydration q is, by definition, q parts water to 1 part flour by weight. So a mass L of it contains L ÷ (1 + q) of flour and L x q ÷ (1 + q) of water. At q = 1 — the '100% hydration' starter most home bakers keep — that is exactly half and half, which is the rule King Arthur states: a 150 g starter contributes 75 g flour and 75 g water. At q = 0.5, a stiff starter, it is two-thirds flour. At q = 1.25, Hamelman's liquid levain, it is 44.4% flour.

Subtract, and you have the mixing-bowl weights: flour into the bowl is F minus the levain's flour, water into the bowl is W minus the levain's water.

The dough weight is D = F + W + S = F x (1 + h + s), and it contains no separate levain term. That is not an omission. The levain's flour is already counted inside F and its water inside W, so adding L again would count the same grams twice and inflate the batch by the entire levain weight. The check that this is right is to add up everything physically placed in the bowl — flour + water + salt + levain — and confirm it equals F x (1 + h + s). For the worked example that is 900 + 650 + 20 + 200 = 1,770 g, and 1,000 x 1.77 = 1,770 g. The two routes agree, which they cannot do if the levain has been double-counted.

Running it backwards from a dough weight is one division: F = D ÷ (1 + h + s). Running it backwards from a levain you already have is another: F = L ÷ p. Both then follow the same path forward, which is why all three modes return an identical record for the same formula.

Rounding stage. Nothing is rounded at an intermediate step. Every quantity is carried in arbitrary-precision decimal arithmetic and rounded exactly once, at the point the result leaves the calculator, to ten decimal places. This matters in the dough-weight mode: if F = D ÷ (1 + h + s) were rounded to the nearest gram before the other rows were derived from it, the rows would no longer sum back to the batch weight you asked for.

Classification stage. This page classifies nothing into bands. Every output is a weight or a ratio, and no verdict word is attached to any of them, so there is no possibility of a band edge being decided from a rounded value. What it does have is two exact boundaries. Water into the bowl reaches zero when p x q ÷ (1 + q) = h — with a 100% starter and 75% hydration, at a levain of 150% of total flour. Flour into the bowl reaches zero when p ÷ (1 + q) = 1 — at 200%. Zero water is a legitimate answer, and the calculator returns it with a mix hydration of 0. Negative is not, and the calculator refuses and names the input that has to change.

A worked example.

Frequently asked questions.

References& sources.

1. [1]King Arthur Baking Company. "Hydration in bread dough, explained." Blog post dated 11 January 2023; retrieved 30 July 2026. The primary source for this page's central rule — "Everything that comprises the loaf you place in the oven should be considered as part of the loaf" — and for the levain split at 100% hydration: "a 150 g starter contributes 75 g flour and 75 g water". Also the source of the worked example this calculator reproduces: all-purpose flour 400 g + whole wheat 50 g + rye 50 g + water 375 g + a 100 g sourdough preferment consisting of 50 g flour and 50 g water, giving total water 425 g, total flour 550 g and hydration 425 ÷ 550 = 77%. The band figures quoted in the hydration hint (55-65% firm or stiff, 75-85% slack or sticky, above 80% high hydration) come from the same article. Independence: primary. Access: open.
2. [2]King Arthur Baking Company. "What is baker's math, and how can I use it in my everyday bread baking?" Blog post dated 28 April 2022; retrieved 30 July 2026. Source of the underlying notation this page extends: "the total weight of flour is always treated as 100%" with every other ingredient expressed as a percentage of it, and the total formula percentage as the factor converting a flour weight into a dough weight — the identity behind D = F x (1 + h + s) and its inverse. Independence: primary. Access: open.
3. [3]King Arthur Baking Company. Ingredient Weight Chart, current revision; retrieved 30 July 2026. Cited for the figure that forces this page to be mass-only: "Sourdough Starter — 1 cup: 8 to 8 1/2 ounces / 227 to 241 grams", a 6% spread inside a single published value. Also the source of the salt figures quoted in the hints (table salt 1 tablespoon = 18 g, Morton's kosher 16 g, Diamond Crystal kosher 8 g) and of all-purpose flour at 120 g per cup. Independence: primary. Access: open.
4. [4]U.S. Government Publishing Office / U.S. Food and Drug Administration. 21 CFR § 137.105, "Flour", 2023 annual edition; retrieved 30 July 2026. Quoted for the moisture provision — "Its moisture content is not more than 15 percent." — which is the basis of the page's stated limitation that hydration counts added water only: flour as purchased may legally carry up to 15% of its weight as water, and no hydration figure in any source includes it. Independence: primary. Access: open.
5. [5]U.S. Government Publishing Office / U.S. Food and Drug Administration. 21 CFR § 136.110, "Bread, rolls, and buns", 2023 annual edition; retrieved 30 July 2026. Cited for the point that expressing ingredients as a percentage of flour weight is the notation of federal law and not merely of recipe writing: every optional-ingredient limit in the standard of identity is written as parts "for each 100 parts by weight of flour used" — nonwheat flours at not more than 3 parts, monocalcium phosphate at 0.75 part, dough strengtheners and conditioners at 0.5 part. Independence: primary. Access: open.
6. [6]Hamelman, Jeffrey. Bread: A Baker's Book of Techniques and Recipes. John Wiley & Sons. Cited bibliographically for the prefermented-flour percentage metric — the share of a formula's total flour carried in the preferment — and for the subtract-the-levain method that this page's flour-into-the-bowl and water-into-the-bowl outputs implement, together with the 125% liquid-levain hydration referenced in the starter-hydration hint. PRINT reference: no online full text exists, so no URL is given rather than linking to a third-party reproduction. Independence: primary. Access: print.