Sourdough hydration calculator
A hundred grams of 100% hydration starter is fifty grams of flour and fifty of water. Most hydration calculators ignore this, so a recipe stated at 75% with 20% starter is actually running at 75% only if the starter is counted — and about 73% if it is not. At low starter percentages the difference is small; at 30% or more it is enough to change how the dough handles.
Baker's percentages state every ingredient against the flour weight. A 500 g flour dough at 75% hydration needs 375 g of total water — but 20% starter at 100% hydration already contributes 50 g of that, so only 325 g is added directly.
How to build a sourdough
Hydration is the main lever on both handling and crumb. Below about 65% the dough is stiff and the crumb tight; around 75% it is workable and open; past 80% it becomes slack, needs coil folds rather than kneading, and rewards a banneton and a hot Dutch oven. None of these are better or worse — a bagel wants 55% and a ciabatta wants 85 — but the number tells you what technique the dough will demand before you start.
Questions
Every ingredient stated as a percentage of the flour weight. Flour is always 100%.