The formula
each part = final starter ÷ (1 + parts flour + parts water)
Where it comes from
A feeding ratio is written starter : flour : water, and what decides the outcome is the proportion, not the amounts. A 1:1:1 has a lot of culture for little food and rises in a few hours; a 1:5:5 takes twice as long or more but arrives stronger and less sour, because the lactic bacteria have had less time to build up acid. It is the lever you use to fit the starter around your day, not a matter of taste.
How to work it out by hand
- Add up the parts: one of starter plus the flour and water parts
- Divide the total you want by that number of parts
- That is the seed; multiply it by the flour and water parts
- Whatever is left in the jar is discarded or used in something else
What is worth knowing
Temperature matters more than the ratio: every ten degrees colder doubles the time, so the same starter that doubles in six hours at 22 °C takes twelve at 12 °C and three at 32. In the fridge at 4 °C it slows down enough to be fed once a week. The discard need not be thrown away: it makes pancakes and crackers, or starts a second jar, and it is the only way not to spend a kilo of flour a week on a culture you do not use.