Can I substitute sourdough starter for commercial yeast?

Yes, but the conversion is not straightforward because sourdough starter activity varies greatly. As a rough guide, 100g of active 100%-hydration sourdough starter (equal weights flour and water) can replace approximately 7-10g of fresh yeast in a recipe. This introduces 50g of flour and 50g of water from the starter, which must be deducted from the recipe. The timing will be significantly longer -- typically 3-5x longer than commercial yeast.

What does it mean to proof yeast and why is it important?

Proofing yeast means testing whether it is still alive by dissolving it in warm water with sugar and observing whether it becomes foamy and bubbly within 10 minutes. This is essential for active dry yeast (which needs rehydration before it activates) and useful for any yeast if you are unsure of its freshness. Instant yeast does not require proofing but can be tested the same way. Skipping the proof and using dead yeast means wasting all the other ingredients in the recipe.

Does the type of yeast affect the flavour of the bread?

Yes. Fresh yeast tends to produce a cleaner, lighter flavour. Active dry yeast can sometimes add a slight 'yeasty' taste if over-proofed or used in excess. Instant yeast contains additional enzymes and emulsifiers that slightly accelerate fermentation but can reduce the complexity of flavour compared to fresh yeast or a long slow ferment. For the most flavourful bread, use less yeast and a longer, cooler fermentation regardless of which type you use.

Yeast Conversion Calculator — Free Hospitality Calculator

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A recipe from a French boulangerie uses 12g of fresh yeast. You only have instant dry yeast in your pantry. Before mixing, you need the correct weight of instant yeast to use as a substitute without over or under-leavening the dough.

Yeast Conversion Calculator

Baking

Amount (g)

From yeast type

Fresh yeast has the highest moisture content and shortest shelf life. Active dry yeast needs to be proofed in warm water before use. Instant yeast can be added directly to dry ingredients and acts faster. General ratio: 1g instant ≈ 1.2g active dry ≈ 3g fresh yeast.

Reference: King Arthur Baking — Yeast Guide

ℹ️ Results are estimates for planning purposes. Verify with current standards and a qualified professional.

1 What this calculator does

Converts between fresh yeast, active dry yeast and instant (fast-action) yeast. Enter the amount of any one type to see the equivalent weight for the other two. Shows notes on proofing requirements and timing differences.

2 Formula & professional reasoning

Fresh to active dry: multiply by 0.40 (40% of fresh weight)
Fresh to instant: multiply by 0.33 (33% of fresh weight)
Active dry to fresh: divide by 0.40 (multiply by 2.5)
Active dry to instant: multiply by 0.82
Instant to fresh: divide by 0.33 (multiply by 3.0)
Instant to active dry: divide by 0.82

Fresh yeast contains approximately 70% water and is less concentrated than dried yeast. Active dry yeast is partially dried and granular, with some dormant cells surrounding a live core -- it needs rehydrating in warm water (proofing) before use. Instant yeast is fully dried to a fine powder with added enzymes and emulsifiers, making it more potent per gram and usable directly without proofing. The conversion ratios reflect the different concentrations of live yeast cells in each product.

3 Worked examples

⚠️ Illustrative example only — not clinical or professional instruction.

Basic

Convert 12g fresh yeast to instant dry

Given: From: fresh yeast | Amount: 12g

Working: Instant: 12 x 0.33 = 3.96g | Active dry: 12 x 0.40 = 4.8g

Answer: 12g fresh yeast = 4g instant yeast OR 4.8g active dry yeast

💡 Round 3.96g to 4g for practical use. 4g of instant yeast for a typical 500g flour bread recipe is 0.8% baker's percentage -- a standard amount for a 3-4 hour bulk ferment at room temperature.

Standard

Convert 7g instant yeast sachet to fresh and active dry

Given: From: instant yeast | Amount: 7g (one standard sachet)

Working: Fresh: 7 / 0.33 = 21.2g | Active dry: 7 / 0.82 = 8.5g

Answer: 7g instant = 21g fresh yeast OR 8.5g active dry yeast

💡 A standard 7g sachet of instant yeast is the most common package size globally -- equivalent to approximately 2.25 teaspoons. This amount is typically correct for one standard loaf recipe (500-600g flour).

Advanced

Scaling down yeast for a slow cold ferment

Given: Standard recipe: 10g instant yeast (for same-day bake) | Reduce for 12-hour cold retard

Working:

Cold retard: yeast activity slows but does not stop in the fridge | Typical reduction for cold ferment: use 25-40% of standard amount | Reduced amount: 10 x 0.30 = 3g instant yeast

Answer: Reduce to 3g instant yeast for a 12-hour cold retard vs 10g for a standard 2-hour bulk

💡 For cold retard baking (bulk in fridge overnight), use significantly less yeast as fermentation continues slowly at 4-8C. Too much yeast causes over-fermentation and collapse.

4 Sanity check

Key conversion ratios

Fresh to instant: divide by 3 (or multiply by 0.33) | Fresh to active dry: multiply by 0.4 | Active dry to instant: multiply by 0.82

Proofing requirements

Fresh yeast: crumble directly into warm water or flour | Active dry yeast: MUST proof in 35-40C water for 5-10 minutes | Instant yeast: add directly to dry ingredients without proofing

Storage and shelf life

Fresh yeast: refrigerate, use within 2-3 weeks | Active dry: cool dry place, 1-2 years | Instant: cool dry place, 1-2 years (refrigerate after opening for best activity)

Temperature sensitivity

Yeast is killed above 60C | Optimal activity: 27-32C | Below 10C: very slow | Fridge (4-8C): very slow but still active

5 Common errors

Error	Cause	Consequence	Fix
Not proofing active dry yeast before adding to the recipe	Treating active dry yeast like instant yeast	Dough fails to rise -- the granules did not fully rehydrate and many cells remain dormant	Active dry yeast must be proofed: dissolve in 35-40C water with a pinch of sugar and wait 5-10 minutes until foamy before adding to the dough. If the yeast does not foam, it is dead and must be replaced. Instant yeast can be added directly to dry ingredients.
Using the same weight of dried yeast as fresh yeast	Not applying the conversion ratio	Dough over-leavened with a strong yeasty flavour and possible collapse	Fresh yeast is approximately 3x heavier than the equivalent amount of instant yeast. Divide the fresh yeast weight by 3 to get the instant yeast equivalent, or use the conversion table in the calculator.
Using expired yeast without testing it	Assuming dried yeast stays active indefinitely	Dough fails to rise with no indication until bulk fermentation is complete	Test yeast activity before use: dissolve 1 tsp dried yeast in 60mL warm water (35-40C) with 1 tsp sugar. After 10 minutes, the mixture should be foamy and have doubled. If not foamy, the yeast is not active enough -- discard and use fresh yeast.
Adding yeast to water that is too hot	Using water that feels 'warm' without checking the temperature	Yeast cells killed -- dough fails to rise	Yeast is killed above 60C. For proofing active dry yeast or mixing doughs, water should be 35-40C (feel warm but comfortable on the inside of your wrist). Use a thermometer for reliability.

6 Reference & regulatory links

🇦🇺 Australia

🇺🇸 United States

7 Professional workflow

Common tools used alongside this one:

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Baker's Percentage Calculator

Baker's Percentage Calculator shows yeast as a percentage of flour -- useful to check the converted amount is in the expected range (0.2-1.5%)

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Bread Proofing Time Estimator

Bread Proofing Time Estimator adjusts timing based on yeast percentage -- confirm the converted amount gives the desired fermentation window

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Sourdough Starter Ratio

Sourdough Starter Ratio -- sourdough uses no commercial yeast; use this calculator when augmenting a weak starter with a small amount of commercial yeast

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Input	Result
12g fresh yeast	4g instant \| 4.8g active dry
7g instant sachet	21g fresh \| 8.5g active dry
Standard bread loaf (500g flour)	~4-7g instant yeast