Sourdough Starter Feeding Ratio & Schedule Calculator

Every sourdough feeding comes down to a ratio between three things: the mature starter you keep, the fresh flour you add, and the water you add. Get the ratio right for your schedule — fast and bubbly the same day, or slow and mild overnight — and the rest of the loaf tends to fall into place. This calculator takes the amount of starter you want to feed, a ratio, and your kitchen temperature, and gives you the flour and water in grams along with a rough window for when it'll be ready.

How this works

The calculator treats your "starter to keep" amount as one part, then multiplies it by the ratio to get flour and water. A 1:2:2 ratio with 20g of starter means 40g flour and 40g water — because each of those numbers is twice the starter amount, not twice each other.

1. Discard down to the amount you're keeping (the field above), so you're not feeding more starter than the ratio calls for.
2. Add the calculated flour and water, and mix until there's no dry flour left.
3. Mark the level on the jar with a band or a line — this is how you'll see it rise.
4. Leave it at room temperature until it's roughly doubled and domed or slightly fallen, not flat.
5. Use it at peak, or feed again if you're not baking yet.

The timing estimate scales the ratio's flour-to-starter multiple against a 75°F (24°C) baseline, then nudges the window earlier for warmer rooms and later for cooler ones. A bigger ratio means more food relative to yeast and bacteria, which takes longer to work through — that's why 1:10:10 is so much slower than 1:1:1.

What this estimate doesn't account for

Tips

• Weigh in grams rather than measuring in cups — flour volume varies enough between scoops to throw off a ratio that's supposed to be precise.
• If your starter is sluggish, try a smaller ratio like 1:1:1 for a feed or two to build up yeast activity before stretching it out again.
• A wide, straight-sided jar makes the rise easier to read than a narrow or tapered one — the level actually reflects the volume.
• Keep a rubber band or marker line at the starting level every feed; it's the simplest way to judge "doubled" without guessing.
• If you won't bake for a few days, feed at a bigger ratio (1:5:5 or larger) and refrigerate — it slows fermentation without starving the starter.
• Discard doesn't have to go to waste — unfed starter works in pancakes, crackers, and other recipes that don't need it to rise much.

How this calculator works

A feeding ratio has three numbers, read left to right: starter kept, flour added, water added, all by weight in grams. The amount you choose to keep is your "1 part." Flour to add equals the amount kept multiplied by the second number, and water to add equals the amount kept multiplied by the third number. So a 1:2:2 feed doubles the kept amount for both flour and water. The total after feeding is the kept starter plus the flour plus the water. If you entered the mature starter you have, discard equals that amount minus the amount kept.

The timing estimate is the only part that is not exact arithmetic. It scales the ratio's flour-to-starter multiple against a 75 degrees Fahrenheit (24 degrees Celsius) baseline, then moves the window earlier for warmer rooms and later for cooler ones. A bigger ratio means more food relative to yeast and bacteria, so it takes longer to work through, which is why 1:10:10 is much slower than 1:1:1. Treat the hours as a starting point for when to check the jar, not a timer: the reliable sign of readiness is a rise against your mark, domed or just starting to dip, and full of bubbles.

Worked example: the defaults, with 100 g on hand

Keep 20 g, ratio 1:2:2 (once-a-day maintenance), and mature starter on hand 100 g.

1. Flour to add: 20 g x 2 = 40 g.
2. Water to add: 20 g x 2 = 40 g.
3. Total after feeding: 20 g + 40 g + 40 g = 100 g.
4. Discard: 100 g - 20 g = 80 g, so you keep only 20 g and remove the other 80 g before feeding.
5. Timing: at a typical 75 degrees Fahrenheit (24 degrees Celsius) kitchen, the tool says roughly ready in 6 to 8 hours.

That matches the post's own quick answer: keeping 20 g at 1:2:2 needs 40 g flour and 40 g water, for 100 g total.

Quick reference: feeds at 1:2:2

Flour and water are each twice the amount kept, so the total is always five times the amount kept.

Pick the row that gives you enough starter for your bake, with a little left over to keep. If you will not bake for a few days, use a bigger ratio such as 1:5:5 and refrigerate, which slows fermentation without starving the starter.

Frequently asked questions