Starter math
Sourdough Starter Feeding Calculator
Feeding by feel is how a starter drifts. Type in what is in the jar, choose a ratio, and the calculator gives you the flour and water to add, what to discard, and what the jar should weigh when you are done — in grams, with cups and spoons alongside if you are not using a scale yet.
Your numbers
Your flour, water, discard and final weight will appear here once you add your numbers and press Calculate feeding.
- Flour to add
- – g
- Water to add
- – g
- Starter to keep
- – g
- Discard
- – g
The part you feed
Taken out before feeding
Total starter after feeding:– g
How the feeding calculator works
Ratios here are written starter : flour : water, by weight — the same order the starter guide uses when it says to adjust a feed to 2:1:1. The starter you keep is the base, and the other two numbers are parts of it:
- Flour to add = starter kept × (flour parts ÷ starter parts)
- Water to add = starter kept × (water parts ÷ starter parts)
- Total after feeding = starter kept + flour + water
- Discard = starter on hand − starter kept
Worked example. You have 200 g of starter and you keep 100 g at 1:1:1. You discard 100 g, add 100 g flour and 100 g water, and the jar ends up at 300 g. Those are the numbers the starter guide feeds — the calculator is just doing that arithmetic for whatever amount is actually in your jar.
Which ratio should you pick?
| Ratio | For every 1 part starter | Usually used for |
|---|---|---|
| 1:1:1 | 1 part flour, 1 part water | Feeding on the counter every day. Peaks soonest of the four. |
| 1:2:2 | 2 parts flour, 2 parts water | A lighter feed when you want a little more time before it peaks. |
| 2:1:1 | 1/2 part flour, 1/2 part water | The stiffer adjustment from the starter guide, for a culture that needs tightening. |
| 1:5:5 | 5 parts flour, 5 parts water | A big feed before or after cold storage, or when you feed less often. |
If you are not sure, 1:1:1 with equal weights of flour and water is the everyday feed. It keeps the hydration at 100% and gives you a starter that peaks within the same afternoon.
What the numbers do not tell you
- Cups and spoons are approximate. The gram weights are the exact answer; the cup and spoon line assumes 120 g of flour per US cup and 236 g of water per US cup. A scale beats a scoop every time.
- The next-check window is guidance, not a measurement. It comes from the starter guide's timeline: a fed starter is ready when it has doubled, usually 4 to 8 hours later. Your kitchen temperature moves that window.
- Arithmetic is not diagnosis. If your starter bubbles without rising, smells sharp, or has gone watery, the numbers will be right and the jar will still need a fix. Those pages are linked at the end.
Sourdough starter feeding questions
- How much starter should I keep when I feed it?
- Keep a weighed amount you will actually use. The starter guide feeds about 100 g at a time, which is a good default for a home jar: enough to bake with, small enough that a day's feed is not wasteful. Keep less and you discard less but build up less active starter; keep more and you can go straight into a bigger recipe. Set the keep amount and the calculator works out the rest.
- Should I feed 1:1:1, 1:2:2 or 2:1:1?
- Use 1:1:1 for an everyday counter feed, 1:2:2 when you want a longer gap before the starter peaks, and 2:1:1 for the stiffer adjustment the starter guide uses when a culture needs tightening. All three keep flour and water in equal weights, so the starter stays at 100% hydration — only the amount of food relative to starter changes.
- How long after feeding is my starter ready?
- In the guide's timeline a fed starter is ready when it has doubled, usually 4 to 8 hours later. Warm kitchens are faster and cool ones slower, so go by the rise rather than the clock: mark the level of the jar when you feed it, then watch for it to double.
- Do I have to discard, and can I use what I take out?
- You do not have to discard. Set the starter to keep to the same number as the starter on hand and the calculator shows 0 g of discard, feeding the whole jar — it just grows with every feed, which is why most bakers discard to keep the jar a workable size. What you take out is unfed starter, so it can go into discard recipes instead of the bin.
- Why feed by weight instead of cups?
- A cup of flour can weigh anywhere from about 100 g to 150 g depending on how it is scooped, so a starter fed by volume slowly drifts thicker or thinner. Weighing keeps the flour-to-water ratio identical at every feed. The cup and spoon lines in the results are there for the days when there is no scale on the counter, and they are approximate.