Sourdough Starter Feeding Calculator

Why the seed's hydration matters

A 20 g seed at 100% hydration already contains 10 g flour and 10 g water. Adding flour and water in an 80% ratio does not produce exactly 80% hydration unless you account for that existing water. The calculator solves the final balance instead of assuming all seed starters contain equal flour and water.

Hydration is a baker's percentage based on flour mass. At 100% hydration, water and flour masses are equal; water is 50% of total mass. At 80% hydration, 100 g flour is paired with 80 g water. Weigh ingredients: volume measures depend on flour packing and cannot be substituted into these gram inputs.

Method and equations

Let S be seed mass, h its hydration as a decimal, r the added-flour ratio, and t the target hydration as a decimal. Seed flour = S / (1 + h); seed water = S - seed flour. Added flour = S × r. Added water = (seed flour + added flour) × t - seed water. Total refreshed starter = S + added flour + added water.

A negative added-water result means the target cannot be achieved by this additive feed: the seed already contains too much water for the selected total flour. The tool displays an error, not instructions to remove water from a mixed starter. Increase the flour ratio or choose a higher target hydration.

Worked example: change a 100% seed to 80%

Keep 20 g seed at 100% hydration and choose a flour ratio of 3. Add 60 g flour. Total flour becomes 70 g, requiring 56 g total water at 80% hydration. Since 10 g water is already present, add 46 g water. The refreshed batch weighs 126 g, with a feed ratio of 1 : 3 : 2.3 by seed, added flour, and added water. A recipe taking 100 g leaves 26 g to retain.

Interpreting the remainder and shortfall

The retained amount is final mass minus the recipe requirement, never less than zero. If the recipe needs more than the batch, the shortfall is reported separately. To double the batch while keeping the same feed ratio and hydration, double seed mass; flour and water both double. Check that the remaining starter is enough for your next feed rather than assuming any positive remainder fits your routine.

This is a mixing calculation, not a prediction of starter maturity. Flour, temperature, seed activity, and feeding schedule affect rise and timing. Hydration alone cannot tell you when starter is ready or whether it is healthy. Default inputs illustrate arithmetic, not a recommended feeding schedule.

Sources, assumptions, and limits

The ingredient balance is self-contained arithmetic. For practical feeding context, see the baker-authored The Perfect Loaf starter maintenance routine (undated live guide, consulted 2026-10-09). Its routine is not built into this calculator and is not a universal fermentation timetable.

The model treats the seed as flour and water only and ignores fermentation mass loss, evaporation, inclusions, and scale error. Supported seed mass is 1–10,000 g, hydrations 10–300%, flour ratios 0.1–20, and recipe requirement 0–1,000,000 g. Dry flour-only seeds and zero seed are outside this tool. Results round to two decimals; ratios show up to four. It does not provide food-safety assessment or rescue guidance for spoiled starter.

Related kitchen planning

Use grams to ounces for weight conversion and the weekly grocery budget calculator for grocery spending. Neither balances starter hydration.

Last updated: 2026-10-09. Ingredient estimates, not fermentation timing advice.