Sourdough Calculator

How to use this calculator

1. Know your starter's hydrationA starter fed equal weights of flour and water is 100% hydration. A stiff starter fed 2:1 flour to water is 50%. It changes how much flour and water the starter contributes.
2. Scale from dough weight for a known tinA 900 g dough gives a standard large loaf. Scaling from flour weight is more useful when following an existing recipe.
3. Adjust hydration to your flourThe same 75% is stiff with wholemeal and slack with a weak white flour. Treat the number as a starting point and go by feel.
4. Weigh everything, including waterVolume measures for flour vary by 20% depending on how the cup was filled, which is more than enough to change the loaf.

The starter is flour and water too

Baker's percentages express every ingredient as a proportion of the total flour, which is always 100%. Water at 75% means 75 grams of water for every 100 grams of flour. Salt at 2% means 2 grams.

That convention is what makes bread recipes scale cleanly. Double the flour and everything else follows, with no recalculation and no rounding drift.

The complication in sourdough is the starter, and it is where most recipes are quietly wrong.

A starter is not a separate ingredient like salt. It is flour and water that has already been mixed and fermented. A starter fed equal weights of flour and water — 100% hydration, the usual kind — is half flour and half water.

So 200 grams of starter contributes 100 grams of flour and 100 grams of water to the dough. Both belong in the totals.

A recipe that lists 500 g flour, 375 g water and 200 g starter, and calls itself 75% hydration, actually has 600 g of flour and 475 g of water. That is 79% hydration, not 75.

Four percentage points does not sound like much until you handle the dough. It is the difference between a dough that holds its shape and one that spreads, and it explains a great many first loaves that came out flatter and wetter than the pictures.

The error compounds with higher starter percentages. At 30% starter the discrepancy is larger still.

This calculator works the other way round. You state the hydration you want, and it works out how much flour and water to add given what the starter already provides. The percentage you enter is the percentage the dough actually has.

A stiff starter at 50% hydration changes the split — it is two thirds flour and one third water — so the same 200 g contributes 133 g of flour and 67 g of water. That is why the starter hydration field matters and is not just a detail.

Worked example: one 900 g loaf at 75%

Take a 900 gram dough at 75% hydration, 20% starter at 100% hydration, and 2% salt.

The dough weight is the sum of flour, water, salt and starter. As proportions of flour that is 1 + 0.75 + 0.02 + 0.20 = 1.97.

So total flour is 900 ÷ 1.97 = 456.9 grams.

Starter at 20% of that is 91.4 grams. Being 100% hydration, it is 45.7 g of flour and 45.7 g of water.

Total water at 75% of 456.9 is 342.6 grams.

Subtracting what the starter provides leaves 411.2 g of flour and 296.9 g of water to weigh out, plus 9.1 g of salt and the 91.4 g of starter.

Adding those gives 808.6 g — and here is a useful check: that is less than 900 because the starter's own weight is already counted once. The calculator handles this consistently, so the figures always add to the dough weight you asked for.

Now see what ignoring the starter would do. If you weighed 456.9 g of flour and 342.6 g of water and then added 91.4 g of starter on top, the real totals become 502.6 g flour and 388.3 g water — 77.3% hydration rather than 75.

At 30% starter the same mistake takes it to nearly 79%.

Now scale up. Three loaves of the same size need 1,370.7 g of total flour, so 1,233.6 g to weigh plus 274.1 g of starter. Baker's percentages make that trivial — every ratio is unchanged.

And change the starter to a stiff 50% hydration one. The same 91.4 g of starter now contains 60.9 g of flour and 30.5 g of water, so you add more water and less flour to hit the same 75%. The finished dough is identical; only the weighing changes.

One practical note on the salt figure. Two per cent of 456.9 g is 9.1 g — call it 9 grams. Salt is worth weighing rather than measuring by spoon, because different salts have very different densities and a teaspoon of flaky sea salt weighs about half what a teaspoon of fine salt does.

Hydration and what it does

These assume a strong white bread flour. Wholemeal absorbs considerably more, so 75% wholemeal handles like about 65% white — which is why wholemeal recipes routinely specify higher hydration and still produce a stiffer dough. A weak plain flour goes the other way and cannot hold high hydration at all, however carefully it is handled.

Flour, temperature and the things percentages cannot tell you

Hydration is a number about water. It is not a number about how a dough feels, and the same percentage behaves completely differently depending on the flour.

Protein content is the main variable. Strong bread flour at 12 to 14% protein absorbs far more water than plain flour at 9 to 10%, and holds it in a gluten network that can support high hydration. A weak flour at 80% hydration is a puddle.

Wholemeal absorbs more again, because the bran takes up water. It also cuts the gluten network physically, so a wholemeal dough is both stiffer and weaker — more water and less structure at once.

Rye is different again and behaves almost nothing like wheat, relying on pentosans rather than gluten. Rye percentages are not comparable to wheat ones.

The practical response is to hold back some of the water, mix, and add the rest as the dough tells you. Bakers call this bassinage, and it is far more reliable than trusting a number.

Temperature controls the timing far more than the starter percentage does. Fermentation roughly doubles in speed for every 10°C, so a dough that takes four hours at 26°C takes eight at 18°C. Chasing a fixed schedule in a cold kitchen is the commonest reason a loaf is underproved.

The usual lever is the starter percentage rather than the clock: use less starter in summer and more in winter, and judge by how the dough looks rather than how long it has been.

Salt slows fermentation as well as adding flavour. Two per cent is standard; drop below 1.5% and the loaf tastes flat and ferments faster than expected, go above 2.5% and it slows noticeably.

On starter health: a starter should roughly double within four to eight hours of feeding and smell pleasantly sour rather than sharp or acetone-like. A sluggish starter will not raise a loaf however good the arithmetic, and no calculator can detect that.

Use it at or near its peak. A starter well past peak has consumed its food and lost gas-producing power, which is why the float test — a spoonful floating in water — remains a useful check.

Finally, weigh everything. Flour measured by volume varies by up to 20% depending on how the container was filled and whether the flour had settled. On a 500 g loaf that is 100 g of flour, which changes the hydration by more than any adjustment you would make deliberately.

Assumptions and limitations

• Assumes the starter hydration you enter is accurate
• Does not account for how a particular flour absorbs water
• Does not model fermentation time, which depends mainly on temperature
• Assumes salt is added to the main dough rather than held back
• Does not handle preferments other than a single starter
• Rye and other non-wheat flours behave differently from these percentages
• Does not assess starter health, which no calculation can detect

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