The best starter feeding ratio is commonly either 1:5:5 or 1:10:10. In the case of 1:5:5 that's 1 part old starter, 5 parts flour and 5 parts water. If you are using a stiff starter, use half the amount of flour. So that's 1:5:2.5. Depending on when you last fed your starter 1:10:10 might make more sense.

If the starter is old and hasn’t been fed recently the 1:10:10 ratio is a better choice. By reducing the starter inoculation ratio, you provide the microorganisms with a cleaner environment. This way they can reproduce and regrow into a more desirable balance to begin your dough fermentation.

Generally, think of your sourdough starter as a dough. Use the same ratios you use for your bread dough for your starter. Your starter should be trained in the same environment that you later use for your dough. This way your starter is perfectly suited to ferment the dough into which it is later inoculated.

The only exception to the 1:5:5 and 1:10:10 rule is the initial starter set-up stage. For the first days during the starter-making process there aren’t enough microbes yet. So using a 1:1:1 ratio can speed up the process.

You can read more on the topic in my free book. Head to the chapter covering sourdough starter basics.