Food & Fermentation · Sourdough
Sourdough starter establishment timeline calculator
Building a sourdough starter from scratch just means feeding flour and water on a schedule until wild yeast and bacteria in the air and flour take hold and start reliably rising and falling. How long that takes varies more than most recipes let on, mostly because of two things: what flour you start with and how warm your kitchen is. This calculator uses both to give you a realistic date range instead of the single fixed number most recipes quote.
Likely ready to bake with
7 days
- Don't watch the calendar. Watch the jar. Your starter is ready when it reliably doubles in size within 6 to 8 hours of a feeding, not on a fixed day.
- A cold kitchen slows everything down. If your normal room temperature is below 68°F, move the starter somewhere warmer, on top of the fridge or near (not on) a running oven, to keep it on this timeline.
What to actually buy
WECK Tulip Jar, 1 Liter
A wide-mouth glass jar makes it easy to see the rise and fall of your starter between feedings and to scrape it down without losing half of it on the sides. It's a common choice for exactly this job.
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How this is calculated
The number you get is a starting point built from two variables King Arthur Baking's own starter guide calls out as the biggest drivers of speed: flour type and room temperature. There's no single accepted formula for this, since every kitchen, flour bag, and water source is a little different, so this tool builds an honest estimate rather than pretending to false precision.
Whole rye and whole wheat flour still carry the bran and germ, where wild yeast and the bacteria that acidify a starter are most concentrated, so starters built on them tend to take hold fastest, around 6 to 7 days at a 70°F room. All-purpose and bread flour have that bran and germ milled out, which is exactly why King Arthur's own recipe opens with whole grain flour and only switches to all-purpose once the starter is running. Their guide notes plainly that an all-purpose starter "simply takes a little longer to get going," so this tool gives it a longer baseline, around 10 days.
Temperature then scales that baseline up or down. Bakers commonly use a rule of thumb that fermentation speed roughly doubles or halves for every 15°F swing, the same relationship used to predict bread dough rise times at different room temperatures. This tool applies that same ratio to the total days-to- establishment, anchored to the 68 to 70°F range King Arthur names as ideal, and caps the adjustment at the extremes so an unusually hot or cold room doesn't produce an unrealistic number.
The spread between the earliest and latest date reflects real variance, not error. King Arthur's own guidance says a starter can be ready in "7 days or less" or take "up to a couple of weeks" under the same basic method. The real signal to watch for is a starter that reliably doubles in size within 6 to 8 hours of a feeding. Treat the date range here as a planning estimate, not a fixed deadline.
Sources
Frequently asked questions
How do I know when my starter is actually ready to bake with?
The date range this calculator gives you is a planning estimate, not the actual test. A starter is ready when it reliably doubles in size within 6 to 8 hours of a feeding, on a consistent, repeatable basis, not just once. If it's not doing that yet on the date the calculator suggests, keep feeding on schedule, it just needs more time.
Why did my starter rise fast at first, then stall?
That's a normal, well-documented phase, not a failure. Early activity is often a different, less stable set of microorganisms taking hold before the actual lactobacilli and wild yeast that make a mature starter establish and take over. King Arthur Baking's guide specifically warns not to be discouraged by a lull between days 3 and 5, it usually resolves with continued regular feeding.
Does the type of water I use matter?
Chlorinated tap water can slow things down since chlorine is added specifically to suppress microorganisms, including the ones you're trying to grow. Letting tap water sit out uncovered for an hour or using filtered or bottled water removes most of that risk if you suspect your tap water is heavily treated.
Can I use this calculator for a starter I'm reviving, not building from scratch?
Not really, this tool is built around establishing a brand-new starter from flour and water. A dormant or refrigerated starter you're reviving usually bounces back in a few feedings over a couple of days, much faster than building one from nothing, since the microorganism population is already established and just needs to wake back up.