Hydration is the single most useful number in sourdough. It describes how wet your dough is, and once you understand it you can compare any two recipes and predict how a dough will handle before you ever touch it.
Open the free hydration calculator →Hydration is simply the weight of water as a percentage of the weight of flour — not of the total dough. The formula is: hydration % = (water ÷ flour) × 100. So 700g water with 1000g flour is 70% hydration. It is always expressed against flour because flour is what the water has to wet, and that ratio is what governs the dough's behaviour.
Your starter contains flour and water too, and it counts. A typical 100%-hydration starter is half flour and half water by weight. If a recipe uses 200g of that starter, that is 100g flour and 100g water you must add to your totals before dividing. Skipping this is the most common reason a "70%" dough feels wetter than expected. Add up all flour, add up all water (including the starter's share), then divide.
If you are new, work at 65–70% until shaping feels natural, then climb a few points at a time. Most everyday country loaves live happily around 72–75%. Push past 80% only once your gluten development and shaping are solid, or the dough will spread into a frisbee.
SourdoughLog's built-in calculator works both directions: enter flour and water to get the percentage, or set a target hydration and flour weight to get the exact grams of water to add — no floury-handed arithmetic mid-bake.
Calculate hydration & log your bake — free →