Guide
Starter Discount Math
Your levain is both starter and ingredient. It brings flour and water into the dough that count toward your final hydration target. Most beginners skip this math and end up with dough that's wetter than intended. The starter discount is the five-line calculation that fixes it.
What is the starter discount?
A sourdough levain is typically 100% hydration — equal weights of flour and water, held by a small amount of active starter. When you add 100g of 100% hydration levain to a dough, you're adding 50g of flour and 50g of water. These count toward the dough's total flour and water. If you don't subtract them from the 'new' flour and water you add, your final dough is over-hydrated relative to the recipe target.
The calculation
Five lines of math, done once per bake. Start with the recipe's total flour weight. Levain weight = recipe flour × levain percentage / 100. Levain flour = levain weight / (1 + levain_hydration / 100). Levain water = levain weight - levain flour. Final flour to add = recipe flour - levain flour. Final water to add = recipe total water - levain water. Our calculator does this continuously as you type.
Worked example 1: 20% levain at 100% hydration
500g recipe flour, 75% hydration (375g total water), 20% levain at 100% hydration. Levain weight = 500 × 0.2 = 100g. Levain flour = 100 / 2 = 50g. Levain water = 100 - 50 = 50g. Final flour to add = 500 - 50 = 450g. Final water to add = 375 - 50 = 325g. So you weigh out 450g of flour (not 500g) and 325g of water (not 375g), plus the 100g levain and 10g salt.
Worked example 2: 15% stiff levain at 50% hydration
500g recipe flour, 75% hydration target, 15% stiff (50% hydration) levain. Levain weight = 500 × 0.15 = 75g. Levain flour = 75 / 1.5 = 50g. Levain water = 75 - 50 = 25g. Final flour = 450g. Final water = 375 - 25 = 350g. Notice: at the same levain flour percentage, a stiff levain contributes less water, so your added-water weight is higher than with a liquid levain.
Worked example 3: Why it matters for high-hydration doughs
Ciabatta at 85% hydration with 20% levain. Without discount: you add 500g flour + 425g water + 100g levain. Effective hydration is actually 475g water / 550g flour = 86.4% — over target, and at this hydration, 1-2% matters. With discount: add 450g flour + 375g water. Effective is 425g / 500g = 85.0%. On point.
Common mistakes
Three failure modes. First, skipping the discount entirely — dough is 2-4% over-hydrated and you can't figure out why it won't hold shape. Second, double-counting the levain — treating it as ingredient plus assuming it's outside the flour percentage, so you add both 500g flour AND a 100g levain. Third, wrong levain hydration — bakers who maintain a stiff starter but run the math for a 100% levain. Our calculator has an explicit levain-hydration field to eliminate this.
FAQ
Do I have to do starter discount math for every bake?
Yes — unless you don't care about 2-3% hydration precision. For low-hydration doughs (55-65%), the error is forgiving. For open-crumb hearth loaves (75-85%), the starter discount is the difference between on-target and over-hydrated.
Why does my dough feel wetter than the recipe's stated hydration?
Almost always the missing starter discount. You're adding the recipe's full water plus the levain's water, which overshoots by 2-4%. The calculator on this site applies the discount automatically — use it as a sanity check.
Does adding more levain change final hydration?
Slightly. More levain means more levain water. But the main effect is fermentation speed and flavor intensity. Going from 20% to 30% levain speeds bulk by ~30% and deepens flavor; hydration nudges by about 1% with a 100% levain, which is usually within your handling tolerance.