You have a sourdough starter bubbling on your counter — fed, active, and smelling pleasantly sour. Then you open a recipe. It calls for 7 g of instant yeast. No mention of starter anywhere. Do you abandon the recipe, or do you know how to translate it?
The reverse situation is equally common. You have a jar of instant yeast and a sourdough recipe that wants 200 g of active starter. The starter you made three months ago is sitting in the fridge, ignored. You need dinner rolls in two hours. The conversion matters.
The problem is not conceptual — most bakers understand that starter and yeast do the same job. The problem is quantity. Sourdough starter is not concentrated yeast. It is a living culture of wild yeast and bacteria suspended in a paste of flour and water. That hydration — the ratio of water to flour in the starter — changes how much leavening power sits in every gram. A stiff Italian lievito madre at 75% hydration and a loose, pourable starter at 125% hydration contain very different amounts of active culture per gram, even if both look healthy and bubbly.
This calculator handles the conversion accurately. Enter your starter amount, select its hydration, choose the yeast type you are converting to, and get an exact gram equivalent — along with the flour and water adjustments your recipe needs to stay in balance. No guesswork. No flat loaves.
A sourdough starter — also called a levain, chef, mother dough, or lievito madre — is a fermented mixture of flour and water that has been colonised by wild yeast and lactic acid bacteria (LAB). Unlike commercial yeast, which is a single purified strain of Saccharomyces cerevisiae, a healthy starter is a community: typically several wild yeast strains alongside bacteria like Lactobacillus sanfranciscensis (now reclassified as Fructilactobacillus sanfranciscensis).
The wild yeast produces carbon dioxide for leavening. The bacteria produce lactic and acetic acids, which give sourdough its characteristic tang and also inhibit mould growth — which is partly why properly fermented sourdough loaves stay fresh longer than commercial yeast breads.
Starter hydration is expressed as a percentage — the weight of water relative to the weight of flour. A 100% hydration starter contains equal parts flour and water by weight: thick, sticky, and the most common type for home bakers. A 75% hydration starter (stiff, dough-like) is used in Italian baking traditions. A 125% hydration starter is pourable, almost pancake-batter consistency.
Why does this matter for conversion? Because the higher the hydration, the more of each gram of starter is water rather than active flour-fed culture. A stiff starter delivers more leavening power per gram than a wet one. This calculator adjusts for hydration automatically — you just need to know which one you have.
Creating a starter from scratch takes 5–10 days. The process is straightforward: mix flour and water, wait for wild yeast and bacteria from the flour and environment to colonise it, and feed it daily until it is reliably active. Here is what actually happens at each stage.
Mix 50 g of whole wheat or rye flour with 50 g of room-temperature water (100% hydration). Cover loosely — do not seal. Wild yeast and bacteria from the flour and air begin to colonise. You may see very little activity; this is normal.
Discard all but 50 g of the mixture. Add 50 g flour + 50 g water. You should start to see bubbles forming and the mixture rising and falling. It may smell strongly acidic or even unpleasant — that is normal at this stage; bacterial succession is underway.
Continue twice-daily feedings (discard + 50 g flour + 50 g water). The starter becomes predictably active: rising to a peak 4–6 hours after feeding and falling back. When it reliably doubles in 4–8 hours and smells pleasantly sour and yeasty, it is ready to bake with.
Once active, a starter kept at room temperature needs feeding once or twice daily. If you bake less than weekly, store it in the fridge after feeding and feed once a week. Before baking, remove from the fridge and feed 1–2 times at room temperature until it doubles reliably again — usually 12–24 hours.
Wild yeast in a starter is more temperature-sensitive than commercial yeast strains. Getting the environment right is what separates a predictable, powerful starter from an unreliable one.
Walk into a supermarket and you will find at least two or three different formats of commercial yeast on the shelf, often with different brand names for the same thing. Here is what actually matters when you are converting from starter.
Finely milled and dried to 4–6% moisture. The most concentrated commercial yeast available. Added directly to dry ingredients — no rehydration needed. Common packages: 7 g sachets (one standard packet), 100–500 g tins. Brands include Allinson Easy Bake, Doves Farm, SAF Instant, Fleischmann's RapidRise.
Add directly to flourCoarser granules with a coating of dead cells that requires dissolving in warm water (38–43°C) for 5–10 minutes before use. About 25% less active per gram than instant. Standard in North American recipes. Common packages: 7 g sachets (sold in strips of 3), 113 g jars. Brands include Fleischmann's Active Dry, Red Star.
Proof in water firstSoft, moist blocks containing ~70% water. Highly perishable (2–3 weeks refrigerated) and not always available in supermarkets — more common in delis, European food stores, and from bakery suppliers. The heaviest format per unit of leavening power. Common packages: 25 g cubes, 42 g blocks, 500 g catering blocks.
Crumble into flour or waterThe difficulty with converting sourdough starter to commercial yeast is that starter is not pure yeast. It is a mixture of wild yeast, bacteria, flour, and water. The leavening power is diluted across all of that mass. To convert accurately you need to know two things: how much of the starter's weight is active culture, and how that maps to the activity of commercial yeast.
Based on accepted baking convention and the work of institutions including King Arthur Baking and the San Francisco Baking Institute, the working ratio is:
150 g of 100% hydration active starter ≈ 1 g of instant yeast
This is a conservative ratio that assumes a reliably active starter. An under-fed or sluggish starter will perform below this figure; an extremely vigorous young starter on its peak day might exceed it slightly. For baking purposes, the 150:1 ratio gives consistent, predictable results.
The ratio above assumes 100% hydration. A stiffer starter contains more flour per gram — more substrate for the culture — so it delivers more leavening power per gram. A wetter starter is diluted with more water. The correction factor adjusts for this:
When you remove starter from a recipe and replace it with commercial yeast, you also remove the flour and water that the starter contained. Those need to be added back to keep the dough hydration correct. The calculation is straightforward:
For 200 g of 100% hydration starter: flour = 200 ÷ 2 = 100 g, water = 200 × 1 ÷ 2 = 100 g. Add both back to your recipe's dry and wet ingredients respectively.
All figures assume a 100% hydration active starter at peak. For other hydrations, use the calculator above for adjusted values.
| Starter Amount (100% hydration) | Instant Yeast | Active Dry Yeast | Fresh Yeast | Flour to Add Back | Water to Add Back |
|---|---|---|---|---|---|
| 50 g starter | 0.33 g (≈ ⅛ tsp) | 0.42 g | 1.0 g | 25 g | 25 g |
| 100 g starter | 0.67 g (≈ ¼ tsp) | 0.83 g | 2.0 g | 50 g | 50 g |
| 150 g starter | 1.0 g (≈ ⅓ tsp) | 1.25 g | 3.0 g | 75 g | 75 g |
| 200 g starter | 1.33 g (≈ ½ tsp) | 1.67 g | 4.0 g | 100 g | 100 g |
| 300 g starter | 2.0 g (≈ ⅔ tsp) | 2.5 g | 6.0 g | 150 g | 150 g |
| 400 g starter | 2.67 g (≈ ¾ tsp) | 3.33 g | 8.0 g | 200 g | 200 g |
| 500 g starter | 3.33 g (≈ 1 tsp) | 4.17 g | 10.0 g | 250 g | 250 g |
* Teaspoon equivalents are approximate (1 tsp instant yeast ≈ 3.1 g). Always weigh for accuracy. Hydration adjustments not shown — use the calculator for 75% or 125% hydration starters.
Both work. Neither is categorically superior. The choice depends on your schedule, flavour goals, and what is in your kitchen right now.
When you convert a sourdough recipe to commercial yeast, you lose the acidity and complexity that come from long wild fermentation — the characteristic tang, the slightly chewy crust, and some of the shelf life. You gain speed, predictability, and simplicity. The loaf will taste milder and more neutral.
Going the other direction — converting a commercial yeast recipe to starter — adds flavour at the cost of time. A sandwich loaf that takes 2 hours with instant yeast will take 8–18 hours with starter, depending on your kitchen temperature. The trade-off is almost always worth it for lean breads (baguettes, country loaves, pizza), and less critical for enriched doughs where butter and eggs dominate the flavour profile anyway.
Whether you are using a sourdough starter or commercial yeast, the underlying biology is the same: yeast cells consume fermentable sugars and produce carbon dioxide, which inflates the gluten network. What differs between starter and commercial yeast is the speed of that process and the byproducts produced alongside it.
Every leavened dough goes through two rise phases. Bulk fermentation (also called the first rise) happens before shaping — the dough develops flavour, extensibility, and initial volume. Final proof (the second rise) happens after shaping — the shaped loaf or roll expands to its baking size. With commercial yeast, both phases are faster and more predictable. With starter, both are slower and more temperature-dependent, but the flavour developed during that longer window is why sourdough tastes the way it does.
Temperature is the primary control variable for both starter and commercial yeast. Wild yeast in starter is generally less aggressive than S. cerevisiae in commercial yeast, which is why starter doughs take longer to proof at the same temperature. The Q10 rule applies to both: fermentation activity roughly doubles for every 10°C increase in temperature, above a dormancy threshold of around 4°C.
A standard white dough with 1% instant yeast at 24°C might bulk ferment in 60–90 minutes. The same dough formulation using an equivalent starter would typically need 4–8 hours at the same temperature. This is not a defect — it is the mechanism that produces sourdough's complex flavour.
Both starter doughs and commercial yeast doughs can be cold-proofed (retarded) in a refrigerator at 4–6°C. Cold proofing pauses activity to near-dormancy, allowing the baker to delay baking by 8–72 hours with no loss of structure. For starter doughs, cold proofing also develops additional acetic acid — the sharper, more vinegary note in the sourness profile — compared to room-temperature fermentation, which favours lactic acid and a milder, more yoghurt-like sourness.
When converting a starter recipe to commercial yeast and planning a cold proof, use a lower yeast percentage than the standard conversion — typically 30–50% less instant yeast — to avoid over-fermentation during the extended cold period.
The conversion uses a three-step model that accounts for starter hydration before translating to any commercial yeast type.
The starter amount is converted to grams. The base ratio — 150 g of 100% hydration starter ≈ 1 g instant yeast — establishes the starting instant-equivalent value.
The flour fraction of the starter is calculated as 1 ÷ (1 + hydration). At 75% hydration this is higher than at 100%, meaning more culture per gram. The instant-equivalent figure is scaled up or down accordingly — stiffer starters need less volume, wetter starters need more.
The instant-equivalent grams are multiplied by the type conversion factor: ×1.25 for active dry yeast, ×3.0 for fresh yeast. Results are shown in grams, teaspoons, tablespoons, and ounces.
The flour and water contributed by the removed starter are calculated and displayed as additions to keep the recipe hydration unchanged. Flour = starter_g ÷ (1 + hydration). Water = starter_g × hydration ÷ (1 + hydration).
Click any example to load it into the calculator above.
The widely accepted conversion used by professional bakers is approximately 150 g of 100% hydration active starter per 1 g of instant yeast. This is the ratio used by this calculator. It assumes a healthy, well-fed starter at or near peak activity. Starter hydration affects the figure — a 75% hydration (stiff) starter needs slightly less volume per gram of yeast equivalent than a 125% hydration (wet) starter.
Yes, in any recipe that calls for commercial yeast. The main trade-offs are time (starter doughs ferment 4–10× slower at room temperature) and flavour (starter produces a tangier, more complex result). Use the conversion table above to find the starter equivalent for any commercial yeast amount, then add the starter's flour and water content back into the recipe to keep the dough hydration balanced.
One standard 7 g sachet of instant yeast is equivalent to approximately 1,050 g (just over 1 kg) of 100% hydration active starter. In practice, most home bakers do not have that much starter, which is why sourdough recipes are designed around much smaller starter percentages (typically 15–30% of flour weight) and proportionally longer fermentation times rather than high starter quantities.
¼ cup of 100% hydration starter weighs approximately 60 g. Using the 150:1 ratio, that equals roughly 0.4 g of instant yeast, or 0.5 g of active dry yeast. That is a very small amount — about ⅛ teaspoon of active dry yeast — which is typical for slow, long-ferment doughs rather than quick-rise recipes. If your recipe calls for ¼ cup of starter for a same-day bake, it is likely assuming the recipe will take several hours rather than 90 minutes.
Multiply the instant yeast amount by 150 to get the equivalent weight of 100% hydration starter. Then reduce the recipe flour by the flour content of the starter (starter weight ÷ 2 for 100% hydration) and the water by the water content (same calculation). Expect the bulk ferment to take 6–12 hours at room temperature rather than 1–2 hours. If the recipe uses active dry yeast, convert it to an instant equivalent first by multiplying by 0.8, then apply the 150× factor.
300 g of 100% hydration starter ÷ 150 = 2 g of instant yeast, or 2.5 g of active dry yeast, or 6 g of fresh yeast. You would also add back 150 g of flour and 150 g of water to the recipe to replace what the starter was contributing. Use this calculator for other hydrations.
Somewhat. Starters fed on whole wheat or rye flour are generally more active than those fed on white bread flour, because bran contains more minerals and nutrients that support bacterial and yeast activity. The conversion ratio assumes a standard bread-flour-fed starter at peak. If your starter is very vigorous (rye-fed, fed daily, highly active), you may need slightly less commercial yeast than the calculator suggests — reduce by 10–15% as a starting point.
For a quick mental estimate: multiply the instant yeast amount in grams by 150 to get the starter weight in grams at 100% hydration. Then subtract half that starter weight from both your flour and water totals in the recipe. So 5 g instant yeast → 750 g starter → reduce flour by 375 g and water by 375 g. That is the rough calculation. For anything more precise — particularly with non-standard hydration starters — use the calculator.
The conversion assumes a healthy, peak-activity starter. If yours is sluggish — rises slowly, does not pass the float test, smells flat or unpleasant — the ratio will underestimate how much you need. Either feed your starter 1–2 times and wait until it is active before converting, or add 20–30% more starter than the calculator suggests to compensate for reduced activity. Using an under-active starter and not adjusting is the most common cause of dense, under-risen loaves when converting recipes.
Sourdough starter and commercial yeast are not competing philosophies — they are the same biology at different speeds. Wild yeast ferments more slowly, produces more complex acids alongside CO₂, and creates bread with a depth of flavour that fast-acting commercial yeast simply cannot replicate in a two-hour window. Commercial yeast ferments quickly, predictably, and consistently, giving bakers reliable control over timing in a way that a living culture in a jar never quite can.
The conversion between the two is precise enough to work well in practice, but it is not a magic swap. You will not get sourdough flavour by swapping starter into a commercial yeast recipe and baking it at the same speed — the flavour comes from time, not from the culture itself. And you will not get the convenience of commercial yeast by simply using more starter. Each form has strengths that emerge from how it is used, not just what it is.
What this calculator gives you is the correct quantity for the conversion — the one number that most recipes and guides get wrong or leave vague. From there, the fermentation time, the room temperature, the dough's feel under your hands, and the poke test tell you the rest. No formula replaces those.
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