How this instrument works
A sourdough starter is a living culture of wild yeast and bacteria that you keep alive by periodically "feeding" it — discarding some and mixing in fresh flour and water so the culture has new food to consume. The feeding ratio (starter : flour : water) controls two things at once: how much the culture grows and how long it takes to become active and bubbly enough to bake with.
A 1:1:1 ratio — equal parts starter, flour, and water by weight — is the standard maintenance feed for keeping a starter alive between bakes, and it's the ratio King Arthur Baking's own sourdough guide uses as its default. It rises fastest of the three ratios here because there's proportionally more established culture relative to the fresh food, but it also means you're maintaining a smaller total amount.
Looser ratios like 1:2:2 or 1:5:5 add proportionally more flour and water relative to the starter, which does two things: it grows a bigger total volume (useful the night before a big bake) and it slows the rise, since the culture has more new food to work through relative to its size. A 1:5:5 levain build is a common way to grow a large, slow-rising pre-ferment for baking day without disturbing your main starter's maintenance schedule.
- Weigh your starter on a kitchen scale and enter that weight in grams.
- Pick a feed ratio: 1:1:1 for a quick maintenance feed, 1:2:2 for a moderate feed, or 1:5:5 to build a larger levain.
- Read Flour to add and Water to add for the exact grams of each to mix in.
- Read Total weight after feeding to know how much starter you'll have once it's mixed and ready to grow.
- Let the fed starter sit at room temperature until it's roughly doubled and domed or bubbly before using or refrigerating it.
Worked example — feeding a 50 g starter three different ways
A standard 1:1:1 maintenance feed on a 50 g starter: flourToAddG = 50 × 1 = 50 g, waterToAddG = 50 × 1 = 50 g, and totalAfterFeedG = 50 × (1 + 2×1) = 150 g total. This is the everyday feed for keeping a starter alive between bakes, whether it lives on the counter or in the fridge.
Building a 1:5:5 levain from that same 50 g of starter instead — say, the night before a big baking day — gives flourToAddG = 50 × 5 = 250 g, waterToAddG = 250 g, and totalAfterFeedG = 50 × (1 + 2×5) = 550 g, more than three times the total weight of the 1:1:1 feed, and it will rise more slowly since there's proportionally more new food relative to the amount of active culture.
A smaller starter of 20 g at a moderate 1:2:2 ratio lands in between: flourToAddG = 20 × 2 = 40 g, waterToAddG = 40 g, totalAfterFeedG = 20 × (1 + 2×2) = 100 g — a useful middle ground when you want more starter than a 1:1:1 feed gives without the multi-hour wait of a full levain build.
Questions
What's the difference between 1:1:1, 1:2:2, and 1:5:5 feeding ratios?
The ratio is starter : flour : water by weight, and it controls both growth and speed. 1:1:1 is a quick maintenance feed that rises fastest since there's proportionally more active culture relative to new food. 1:2:2 is a moderate feed that grows more total starter and rises a bit slower. 1:5:5 builds the largest volume — commonly used to grow a levain the night before baking — and rises slowest of the three, since the culture has the most new food to work through.
Why is the ratio measured by weight instead of volume?
Starter consistency varies with hydration and how much it's been stirred down, so a cup of starter can weigh quite differently from one baker's kitchen to another's. Weight is consistent regardless of how airy or deflated the starter looks, which is why virtually every serious sourdough guide, including King Arthur Baking's, recommends a kitchen scale over measuring cups for both the starter and its feed.
How do I know when my fed starter is ready to use?
Look for it to roughly double in volume and develop a domed or bubbly surface, usually within a few hours for a 1:1:1 feed at room temperature, or longer for looser ratios like 1:5:5. A starter that's already started to collapse or sink in the middle has passed its peak and will give a weaker rise if used at that point — feed again if you've missed the window.
Can I use this calculator for discard-based recipes too?
This calculator is built for feeding your live starter, not for discard recipes, which typically use unfed starter directly in a batter or dough rather than growing it further. If a recipe calls for a specific weight of "fed" or "active" starter, though, you can use this tool to work out exactly how much flour and water to mix into your starter to reach that weight before using it.
Do I need to discard starter before feeding it?
Not strictly for the math here — this calculator only tells you how much flour and water to add to whatever starter weight you enter, regardless of whether you discarded first. Many home bakers do discard down to a smaller amount before feeding, mainly to keep the total starter volume manageable and to avoid endlessly increasing container size; discarding isn't required for the culture's health.