Convert a target weight into parts
For a 1:2:2 feeding, add the ratio parts: 1 + 2 + 2 = 5. Divide the desired final starter weight by 5 to find the weight of one part.
A 100 g final target therefore uses 20 g seed starter, 40 g flour, and 40 g water. If you need 100 g for a recipe and want to keep 10 g, calculate against a 110 g final target instead.
Hydration follows the flour and water parts
Equal flour and water parts make a 100% hydration feeding. A 1:2:1 feeding uses half as much water as flour and produces a 50% hydration feed, regardless of the seed part.
The seed starter already contains flour and water, so the exact hydration of the entire refreshed starter can differ slightly when the seed hydration differs from the feed ratio.
The calculator does not predict peak time
A larger feed ratio generally supplies more fresh food per gram of seed, but temperature, flour type, inoculation strength, container shape, and starter health all affect timing.
Use the weight result to mix the feed, then observe rise, bubbles, aroma, and your normal maturity markers. Do not convert the ratio output into a fixed food-safety or fermentation-time guarantee.
Related calculators
DoughMath guides are educational explanations for formula math. Use the linked calculators for exact weights before baking.