Starter hydration maths: splitting a starter into flour and water
Hydration is the weight of water divided by the weight of flour. A 100 % starter holds equal weights of both; an 80 % starter holds 80 g of water for every 100 g of flour.
From the starter's weight M and its hydration h (as a fraction, so 80 % is 0.8): flour = M ÷ (1 + h), and water = M − flour. For 180 g of starter at 80 %, flour = 180 ÷ 1.8 = 100 g and water = 80 g.
To raise the hydration, keep the flour and add water: water to add = flour × (target − current). Taking that 180 g starter from 80 % to 125 % needs 100 × (1.25 − 0.80) = 45 g of water, which gives 225 g of starter. To lower the hydration, keep the water and add flour: flour to add = water ÷ target − flour.
The same split is what makes discard usable in any recipe: 100 g of discard at 100 % brings 50 g of flour and 50 g of water, so that much flour and water comes out of the recipe. The starter hydration converter and the discard recipe converter do this arithmetic and show each step.
Limits: the maths treats the starter as flour and water only and ignores what fermentation has consumed, which is a small share of the weight.
Used by
- Sourdough starter hydration converterEnter your starter's weight, its current hydration and the hydration you want. You see how much flour or water to add, step by step.
- Sourdough discard recipe converterEnter the flour and liquid of any recipe and the discard you want to use. You see how much flour and liquid to leave out so the dough stays the same.