How the sourdough calculator works
The calculator uses baker's percentages. King Arthur's guide to baker's math puts it simply: the total weight of flour is always 100%, and everything else is a percentage of it. The catch with sourdough is the starter. It is part flour and part water, and King Arthur counts a preferment's flour and water in the total formula. Leave it out and the dough comes out wetter than the number on the recipe.
1. Find the total flour
A dough is flour, water and salt. The starter is just flour and water that were mixed earlier. So the dough weight is the total flour times (1 + hydration + salt). Divide your dough weight by that and you have the total flour. If you choose to size by flour instead, the flour you type is the total flour.
2. Work out the starter
Starter % is the weight of ripe starter as a percentage of total flour. A starter fed equal weights of flour and water is 100% hydration, so half of it is flour and half is water. A stiff starter at 50% hydration is two thirds flour.
3. Subtract what the starter already brings
The flour to add is the total flour minus the starter's flour. The water to add is the total water minus the starter's water. Salt is a percentage of the total flour. King Arthur gives 1.8% to 2.2% as the usual range for salt.
4. Build the starter in time
The feed section makes enough ripe starter for the dough plus some to keep, at the ratio you pick. King Arthur tested feeding ratios at 78°F: 1:1:1 doubled in about 4 hours, 1:2:2 peaked at about 8 and 1:4:4 at about 12. Feed 1:1:1 in the morning for an afternoon mix, or 1:4:4 at night for a morning mix. A cooler kitchen takes longer.
Worked example
Two loaves of 900 g each at 75% hydration, 20% starter at 100% hydration and 2% salt. Total dough is 1,800 g, so the total flour is 1,800 g ÷ 1.77 = 1,017 g.
| Ingredient | Per loaf | Two loaves |
|---|---|---|
| Flour to add | 458 g | 915 g |
| Water to add | 331 g | 661 g |
| Ripe starter | 102 g | 203 g |
| Salt | 10 g | 20 g |
| Dough | 900 g | 1,800 g |
The 203 g of starter holds 102 g of flour and 102 g of water. That is why the water to add is 661 g, not 763 g. To have it ready by morning, feed 28 g of ripe starter with 113 g flour and 113 g water at 1:4:4 the night before. That makes 253 g: enough for the dough and 50 g to keep going, the amount King Arthur's maintenance feed keeps. For a same-day bake, 1:1:1 needs 84 g of starter instead.
Checking a recipe you already have
A recipe calls for 500 g flour, 350 g water, 100 g starter and 10 g salt. It looks like 70% hydration. Counting the starter, the flour is 550 g and the water 400 g, so the true hydration is 72.7% and the salt is 1.8%.
Where the sources differ
Recipes don't all write starter the same way. This calculator, like the template's Hydration Calc tab, gives starter as a percentage of total flour. Some recipes give it as a percentage of the flour you add, which makes the same starter look bigger: in the example above, 18.2% of total flour is 20% of the flour added. King Arthur describes a preferment by the share of total flour inside it (prefermented flour), and gives 10% to 50% as the common range. The calculator shows that number too.
Some bakers also quote hydration without the starter. Neither way is wrong as long as you know which one a recipe uses. When you copy a recipe from somewhere else, put it into Check my recipe first and compare like with like.