Sourdough Starter Calculator
Feeding ratios, hydration and how much starter to build for a recipe - flour and water in grams, with discard and keep-back worked out.
All weights in grams, by weight - never by volume. Feeding ratios are meaningless in cups, because a cup of flour and a cup of water are nowhere near the same mass.
How it works
Sourdough feeding is baker's-percentage arithmetic, and there are only two definitions you need.
The feeding ratio is written starter : flour : water, and every number is a weight. At 1:2:2 you multiply your seed by 2 to get the flour, by 2 again to get the water, and end with 1 + 2 + 2 = 5 times the seed you started with. That multiplier is the whole trick behind planning mode: to finish with a known total, divide by it.
Hydration is water divided by flour, as a percentage - not water divided by the total. This is the part that trips people up, and it is why the standard starter fed equal weights of flour and water is called a 100% hydration starter rather than a 50% one.
The one wrinkle is that your existing starter is not a neutral ingredient: it is already part flour and part water. A 100 g starter at 100% hydration is 50 g of flour and 50 g of water. This calculator splits it out and counts it, which is why the hydration you end on is not simply the hydration of the feed you added.
hydration % = water ÷ flour × 100
flour to add = seed × flour parts · water to add = seed × water parts
total after feeding = seed × (1 + flour parts + water parts)
seed needed = (recipe amount + keep-back) ÷ (1 + flour parts + water parts)
Worked example
Your recipe wants 200 g of starter and you want 50 g left in the jar afterwards, feeding at 1:4:4. The multiplier is 1 + 4 + 4 = 9, so the seed is 250 ÷ 9 = 27.78 g. Feed that with 27.78 × 4 ≈ 111.1 g of flour and the same again of water, and you land on exactly 250 g - 200 g for the dough, 50 g to carry on with. If your jar holds 200 g, you discard 172.2 g first (or bake it into something).
What this calculator will not tell you
How long the feed takes to peak, and what temperature to do it at. Both are real and both matter, but neither has a settled published formula we could implement honestly, so we ship weights only rather than invent a schedule. Watch the jar: a starter is ready when it has risen and domed and is just starting to flatten, whenever that happens to be in your kitchen.
Sources: hydration is defined per King Arthur Baking's “Bread hydration, explained” - "dividing the total weight of water … by the total weight of flour", with their worked example of 750 g water to 1,000 g flour = 75%. Feeding-ratio notation follows King Arthur's “Testing different sourdough feeding ratios” - "a 1:1:1 feeding ratio means the sourdough starter, flour, and water have been added in equal amounts by weight", and "feeding ratios are always based on weight, not volume". The maintenance routine (keep 50 g, add 50 g water and 50 g flour) is King Arthur's “Feeding and Maintaining Your Sourdough Starter”. The stiff-starter figure (about 50% hydration, “twice as much flour as water by weight”) is King Arthur's “Artisan sourdough bread tips, part 1”. Every one of those worked examples is a test case in our suite.
The arithmetic here is exact - it is weights in, weights out. What it cannot know is your culture: how fast it ferments, how thirsty your flour is, or how warm your kitchen runs. Use the weights, then trust your eyes on timing.
Frequently asked questions
- What does a 1:1:1 feeding ratio mean?
- The three numbers are starter : flour : water, always by weight. A 1:1:1 feed means equal weights of all three - King Arthur Baking put it plainly: "if you use 100 grams of seed, you will then add 100 grams of flour and 100 grams of water." A 1:2:2 feed means 100 g of starter gets 200 g of flour and 200 g of water. Bigger ratios mean more fresh flour per gram of existing starter, so the culture takes longer to peak - King Arthur's feeding-ratio trial says it directly: the more flour and water in proportion to the starter, "the longer it will take to reach peak ripeness" - which is why bakers reach for a higher ratio like 1:5:5 before an overnight rise.
- How do I calculate sourdough starter hydration?
- Hydration % = water ÷ flour × 100. It is the weight of water divided by the weight of flour, not by the total. So 750 g of water to 1,000 g of flour is 75% hydration; equal weights of each is 100%. Note the number can exceed 100 - a 125% starter simply has more water than flour, which is normal for a slack, liquid starter.
- Why is a normal starter called 100% hydration?
- Because it is fed equal weights of flour and water, and water ÷ flour = 1 = 100%. It catches people out because the starter is obviously not "all water" - but hydration in baker's percentages is always relative to the flour weight, never to the total weight of the dough. Every ingredient in a baker's formula is expressed as a percentage of flour, which is why the numbers add up to well over 100%.
- How much starter should I make for a recipe?
- Use the planning mode above. Tell it how much starter the recipe calls for and how much you want to keep back to carry the culture forward, and it works backwards to the seed you should start from: seed = (needed + kept back) ÷ (1 + flour parts + water parts). Feeding that seed at your chosen ratio lands you on exactly the total you asked for, with the keep-back already accounted for - so you are not left scraping the jar clean on bake day.
- How much starter do I discard?
- Whatever is left over above the seed the feed needs. The classic King Arthur maintenance routine keeps 50 g and discards the rest, then adds 50 g of water and 50 g of flour - a 1:1:1 feed ending at 150 g. Enter what is in your jar and the calculator tells you the discard weight. If your jar has less than the seed the plan needs, it shows a shortfall instead, so you know to do an extra build first.
- What is a stiff starter and how does the math change?
- A stiff starter is simply one at lower hydration - typically around 50%, meaning twice as much flour as water by weight. Nothing about the arithmetic changes; you just set the hydration field lower, and set your feed ratio so the water parts are half the flour parts. The calculator tracks the flour and water already inside your existing starter, so it can tell you the hydration you will actually land on after the feed, which is rarely the same as the hydration of the feed itself.
- Does the feed change my starter's hydration?
- Yes, unless you feed at exactly the hydration your starter already has. The result always lands somewhere between the two: feed a 100% starter with a stiff 2-parts-flour-to-1-part-water mix and the whole jar drifts drier. The "hydration after feeding" output shows where you actually end up, because the existing starter's own flour and water are counted, not ignored.
- How long will my starter take to peak after this feed?
- We deliberately do not answer that with a number. Peak time depends on temperature, flour type, the health and age of the culture and the feed ratio, and we found no single published formula worth implementing - so this calculator ships weights only, not schedules. As a rule of thumb a bigger feed ratio takes longer, and a warm kitchen is faster than a cold one; watch the jar, not the clock.
- Why is a sourdough calculator on a homebrewing site?
- Because it is the same problem. A sourdough starter is a yeast-and-bacteria culture that you build up before pitching into a larger batch, exactly like a brewer's yeast starter - both are propagation math. If you came here for beer, the yeast starter calculator and pitch rate calculator are the brewing side of the same idea.