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Baker’s percentage calculator

A formula in baker’s percentages turned into weights for the batch you are making, with the starter’s own flour and water counted — which is the arithmetic almost everybody gets wrong once.

The question this answers: “The formula says 75% hydration and 20% starter. How much water actually goes in the bowl, and does the starter count?”

Including the flour inside the starter, which is how formulas are written.

Water as a percentage of total flour. Your formula's figure, not ours.

Starter as a percentage of total flour.

A starter fed equal weights of flour and water is 100%. A stiff one is lower.

As a percentage of total flour.

Result

Flour to add
450 g
Total flour is 500 g, of which 50 g arrives inside the starter.
Water to add
310 g
Total water is 360 g, of which 50 g arrives inside the starter.
Starter
100 g
Salt
10 g
Total dough
870 g
Before any loss to the bowl and the bench, which is a few per cent.
The mistake this avoids
Adding 500 g flour and 360 g water on top of the starter gives 74.5% rather than 72%
A starter is flour and water. Counting it twice is why a formula that behaves for its author can arrive slack in somebody else’s kitchen, and it is the commonest arithmetic error in the hobby.

Worth knowing about this answer

  • Every percentage here is the formula’s, not this tool’s. It converts a formula into weights and has no view on what any of them should be.
  • The ratios are the easy half. What decides a sourdough loaf is fermentation time at your kitchen temperature, and no arithmetic reaches it — a correctly weighed dough taken out of bulk too early is still a dense loaf.
Show the working
  1. Starter500 g × 20%100 g
  2. Flour inside it100 ÷ (1 + 1)50 g
  3. Flour to add500 − 50450 g
  4. Water to add360 − 50310 g

Flour to add: 450 g. Water to add: 310 g. Starter: 100 g. Salt: 10 g. Total dough: 870 g. The mistake this avoids: Adding 500 g flour and 360 g water on top of the starter gives 74.5% rather than 72%

How it works

Baker’s percentages are all expressed against total flour, so the weights follow directly: water is the hydration percentage of the flour, starter and salt likewise. The only step that is not obvious is the starter, which is itself flour and water — its flour is its mass divided by one plus its own hydration, and the remainder is water. Both are subtracted from the totals to give what actually goes in the bowl.

What it assumes

  • That the formula quotes hydration against total flour including the starter’s, which is the standard convention. A few sources quote it against added flour only, and where they do the two readings differ by several per cent.
  • That the starter is at the hydration you state. A neglected starter that has been fed by eye is not, and the discrepancy goes into the dough.
  • That nothing is lost. A few per cent stays on the bowl and the bench, which matters when a dough is being divided to a target loaf weight.

Where it stops being reliable

  • It has no view on what any percentage should be. Hydration, starter proportion and salt are the formula’s choices and they are what distinguishes one loaf from another.
  • It cannot tell you anything about timing, and timing is what decides a sourdough loaf. A correctly weighed dough taken out of bulk fermentation too early is still dense.
  • It knows nothing about your flour. Different flours absorb different amounts of water at the same stated hydration, which is why a formula moved between two bags behaves differently.

Runs entirely in your browser — nothing you type is sent anywhere. Last reviewed 2026-08-19. More on Sourdough.