A bread recipe calls for 7 g of instant yeast. You want to use the active starter sitting on your counter instead. How much do you use?
This is one of the most common questions in home baking — and one of the most misanswered. The instinct is to substitute roughly equal weights, swap the timing, and hope for the best. It rarely works. Sourdough starter is not concentrated yeast. It is a living ferment of wild yeast and lactic acid bacteria suspended in a paste of flour and water, and only a fraction of its weight is active leavening power. A gram-for-gram swap will leave you with a profoundly under-leavened loaf.
The second mistake is ignoring what the starter contributes to the recipe's flour and water balance. Remove 200 g of starter and drop it into a recipe designed around commercial yeast, and you have just added 100 g of flour and 100 g of water that were not in the original formula — which changes the dough's texture, hydration, and bake entirely.
This calculator handles both problems. Enter your yeast amount and type, choose the hydration of your starter, and get the correct starter weight — plus the exact flour and water you need to subtract from the recipe to keep everything in balance. Precise conversion. No flat loaves.
There are three main reasons bakers make this swap — and understanding which one applies to you changes how you approach it.
Wild fermentation produces lactic and acetic acids alongside CO₂. These acids give sourdough its characteristic tang, add depth to the crumb flavour, and create a slightly chewy texture that commercial yeast alone cannot replicate — even in a long-rise recipe.
Your starter is active and ready. The recipe calls for instant yeast you do not have. Rather than abandoning the bake, a precise conversion lets you proceed — with longer fermentation times accounted for.
Long-fermented sourdough breaks down more phytic acid (an anti-nutrient found in flour) than short commercial yeast bakes. The organic acids also inhibit mould growth, giving sourdough loaves a longer shelf life without preservatives.
Before converting, identify exactly what the recipe uses. The three commercial yeast formats are not interchangeable by weight — they have different moisture levels and activity rates, and the conversion to starter differs accordingly.
The most concentrated and most active commercial yeast. Finely milled to 4–6% moisture content. Added directly to dry ingredients without proofing. Standard 7 g sachet = one packet. Brands: SAF Instant, Allinson Fast-Action, Doves Farm, Fleischmann's RapidRise.
Reference unit for conversionsCoarser granules coated in dormant cells — must be dissolved in warm water (38–43°C) for 5–10 minutes before use. About 25% less active per gram than instant yeast. Common in North American recipes. Standard sachet: 7 g. Brands: Fleischmann's Active Dry, Red Star.
Multiply by 0.8 to get instant equivalentSoft, moist blocks containing ~70% water. The heaviest format per unit of leavening. Perishable (refrigerator life: 2–3 weeks). Less common in supermarkets; more often found in delis, European stores, and bakery suppliers. Common formats: 25 g cubes, 42 g blocks.
Divide by 3 to get instant equivalentThe accepted baking convention, used by institutions including King Arthur Baking and the San Francisco Baking Institute, is:
So if a recipe calls for 7 g of instant yeast, you need 1,050 g of starter to match it — which is why sourdough recipes are rarely direct one-to-one conversions of commercial yeast recipes. Instead, bakers reduce the total leavening and extend the fermentation time, allowing the slower wild yeast to do the same work over more hours.
If your recipe uses active dry or fresh yeast, convert to an instant equivalent before applying the 150× factor:
Then multiply the instant-equivalent grams by 150 to get the starter weight.
When you add starter to a recipe that was designed for commercial yeast, the starter brings its own flour and water — and those need to come out of the recipe's totals. For a 100% hydration starter, exactly half the weight is flour and half is water. For other hydrations, the split shifts accordingly:
For example: 300 g of 100% hydration starter = 150 g flour + 150 g water. Reduce the recipe's flour by 150 g and water by 150 g, then add the 300 g of starter. The dough will have the same hydration as originally intended.
Starter hydration is the ratio of water to flour in the culture, expressed as a percentage. A 100% hydration starter contains equal parts flour and water by weight. A 75% hydration starter is stiffer — less water per gram of flour. A 125% hydration starter is wetter and more pourable.
Hydration matters for two reasons in this conversion:
The hydration also changes how much flour and water the starter contributes to your recipe — a stiff starter brings more flour and less water per gram than a wet one. The calculator adjusts for both the leavening correction and the recipe deductions automatically when you select a non-standard hydration.
All figures use 100% hydration active starter at peak. For other hydrations or yeast types, use the calculator above.
| Instant Yeast | Active Dry Yeast | Fresh Yeast | Starter Required (100% hydration) | Reduce Flour By | Reduce Water By |
|---|---|---|---|---|---|
| 1 g (≈ ¼ tsp) | 1.25 g | 3 g | 150 g | 75 g | 75 g |
| 2 g (≈ ½ tsp) | 2.5 g | 6 g | 300 g | 150 g | 150 g |
| 3 g (≈ 1 tsp) | 3.75 g | 9 g | 450 g | 225 g | 225 g |
| 5 g | 6.25 g | 15 g | 750 g | 375 g | 375 g |
| 7 g (1 sachet) | 8.75 g | 21 g | 1,050 g | 525 g | 525 g |
| 10 g | 12.5 g | 30 g | 1,500 g | 750 g | 750 g |
* Teaspoon equivalents assume 1 tsp instant yeast ≈ 3.1 g. The large starter quantities for higher yeast amounts illustrate why sourdough recipes use lower leavening percentages and longer times rather than direct swaps. Use the calculator for non-100% hydration starters.
Converting the quantity is only half the job. Wild yeast ferments more slowly than commercial Saccharomyces cerevisiae — typically 4–10× slower at the same temperature. A dough that takes 90 minutes to bulk ferment with 1% instant yeast will take 6–12 hours with an equivalent starter at room temperature. This is not a flaw in the conversion; it is the mechanism that produces sourdough's complex flavour.
The conversion runs in four steps, applied in sequence.
If you enter teaspoons or ounces, the input is converted to grams first. If the yeast type is active dry, the gram value is multiplied by 0.8 to get the instant equivalent. If it is fresh yeast, it is divided by 3. Instant yeast requires no adjustment.
The instant-equivalent gram value is multiplied by 150 to give the starter weight in grams, assuming 100% hydration as the base case.
The starter weight is scaled by the ratio of the flour fraction at 100% hydration vs the flour fraction at the selected hydration: correction = 0.5 ÷ (1 ÷ (1 + hydration)). Stiff starters (75%) require slightly less volume; wet starters (125%) require slightly more.
The flour and water the starter contributes are calculated and displayed as deductions from the recipe. Flour to remove = starter weight ÷ (1 + hydration). Water to remove = starter weight × hydration ÷ (1 + hydration). These keep the dough's total flour, water, and hydration unchanged.
Click any example to load it into the calculator above.
7 g of instant yeast × 150 = 1,050 g of 100% hydration active starter. This is a large quantity — more than most home bakers keep on hand at once. In practice, converting a recipe that calls for 7 g of instant yeast to sourdough is usually done by redesigning the recipe around a lower starter percentage (10–20% of flour weight) and a much longer ferment, rather than attempting a direct weight-for-weight substitution.
Yes. First convert the active dry yeast to an instant equivalent: multiply the active dry amount by 0.8. Then multiply that by 150 to get the starter weight. For example, 5 g active dry yeast × 0.8 = 4 g instant equivalent × 150 = 600 g starter. Remember to reduce the recipe flour and water to account for what the starter brings in.
One standard packet of instant yeast is 7 g, which equals approximately 1,050 g of 100% hydration active starter. For active dry yeast (also 7 g per packet but 25% less active), the starter equivalent is around 840 g. These amounts are impractical for most home recipes — which is why sourdough recipes are built around the slower activity of wild yeast rather than matching commercial yeast at 1:1 leavening power.
Yes — this is the step most guides skip, and it is why direct conversions often produce sticky or dense dough. Starter is not pure yeast; it contains significant amounts of flour and water. A 100% hydration starter is exactly half flour and half water by weight. If you add 200 g of starter to a recipe without adjusting, you have added 100 g of extra flour and 100 g of extra water — which increases total dough weight and changes the hydration. The calculator shows you exactly how much to subtract from each.
Select the correct hydration in the calculator — it adjusts both the quantity and the flour/water deductions automatically. A 75% hydration (stiff) starter has more flour per gram and delivers more leavening per gram, so you need slightly less of it. A 125% hydration (wet) starter is more dilute, so you need slightly more. The deductions also shift: a stiff starter removes more flour and less water from the recipe; a wet starter does the opposite.
Expect bulk fermentation to take 6–12 hours at room temperature (22–26°C) rather than 1–2 hours. The final proof after shaping typically takes 1–3 hours at room temperature, or 8–16 hours if you cold-proof in the fridge. These times vary significantly with kitchen temperature — a cooler kitchen means longer fermentation. Use the poke test to judge readiness rather than relying purely on elapsed time.
Only if you give it time. The sour, complex flavour associated with sourdough comes from lactic and acetic acids produced by the bacteria in the starter during a long, slow ferment — not from the starter itself. If you substitute starter and bake at the same speed as a commercial yeast recipe, the bread will taste mild and yeasty, not sour. The acids responsible for sourdough flavour take many hours to develop. A 10–18 hour bulk ferment at room temperature, or a cold retard overnight in the fridge, is what produces the characteristic tang.
Not directly. A dormant, unfed starter is significantly less active than a peaked starter, and the conversion ratio assumes full activity. Feed it at room temperature — discard all but 50–100 g, then add equal weights of flour and water — and wait for it to peak (dome, bubbles throughout, doubles in height) before using it in the conversion. This typically takes 4–12 hours after one feeding. For a long-neglected starter, two feedings 12 hours apart may be needed to restore full activity.
Converting commercial yeast to sourdough starter is one of the most satisfying swaps in bread baking — not because it is particularly difficult, but because the result is so different from what you started with. A standard white loaf recipe, run through this conversion and given a long overnight ferment, will produce bread with more flavour, better crust, and longer shelf life than the original formula ever could.
The key insight is that this conversion has two parts: the quantity (how much starter matches how much yeast) and the balance (removing the flour and water the starter brings with it). Get both right and the dough will handle exactly as intended. Skip the second step and you will have stickier dough, altered texture, and a result that does not match the recipe.
What the calculator cannot give you is patience. Wild yeast does the same job as commercial yeast — it just does it on a longer schedule. The fermentation time is not waiting: it is working. Every hour the dough sits and slowly expands is an hour acids are building, gluten is maturing, and flavour is developing. The bread at the end of that process is worth the timeline.
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