Split the starter first
Let S be starter weight and H its hydration percentage. Starter flour = S ÷ (1 + H ÷ 100). Starter water = S − starter flour. A 150 g starter at 50% hydration therefore contains 100 g flour and 50 g water. This formula assumes the starter consists of flour and water.
Calculate overall hydration
Add the starter flour to your added flour; add the starter water to your added water. Divide total water by total flour and multiply by 100. Salt is excluded from this ratio. The default 500 g flour + 350 g water + 100 g starter at 100% + 10 g salt weighs 960 g and has hydration 400 ÷ 550 × 100 = 72.727…%.
Solve for added water
Added water = total flour × target hydration ÷ 100 − starter water. If that result is negative, the starter alone exceeds the target water allowance. We show an explanation instead of suggesting a negative ingredient weight.
Scale a complete recipe
Scaling factor = desired mixed dough weight ÷ original mixed dough weight. Multiply added flour, water, starter and salt by this factor. Do not add the starter’s flour and water to the ingredient list again: they are already inside the starter weight.
Scope and limits
This is a mass-ratio calculator for flour, water, a flour-and-water starter and salt. It does not model evaporation, flour moisture, fermentation loss, milk, eggs, inclusions or baked yield. A mathematically valid percentage is not a complete bread recipe or a forecast of how the dough will behave.