Why Yeast Types Are Not Interchangeable at 1:1
All three commercial yeast forms — instant, active dry, and fresh — contain the same organism (Saccharomyces cerevisiae), but their moisture content and processing differ, which affects leavening potency by weight or volume.
Standard Conversion Ratios
| Yeast Type | Relative to Instant (by weight) | Moisture | Notes |
|---|---|---|---|
| Instant yeast | 1× | ~5% | Add directly to dry ingredients; no proofing needed |
| Active dry yeast | 1.25× | ~8% | Traditionally proofed in warm water (110°F / 43°C) |
| Fresh (cake) yeast | 3× | ~70% | Highly perishable; used by professional bakers |
Source: King Arthur Baking yeast guide and standard baker's reference tables.
Worked Example
A recipe calls for 2¼ tsp active dry yeast. To use instant instead: 2.25 tsp ÷ 1.25 = 1.8 tsp instant, or about 1¾ tsp. In grams: 2.25 × 4 g/tsp ÷ 1.25 = 7.2 g instant.
The Standard US Yeast Packet
One standard US yeast packet (sold by Fleischmann's, Red Star, SAF) contains 7 grams = 2¼ teaspoons. This is the amount needed to leaven approximately 3–4 cups of flour in a typical bread recipe. When recipes say "1 packet," they mean this exact amount regardless of yeast type — but the weight is specified for instant yeast equivalence.
Sourdough Starter: A Different Framework
Sourdough starter is a live fermented culture, not concentrated yeast, so the conversion to commercial yeast is approximate. A common starting point for converting a yeasted recipe to sourdough is to use 20% starter based on flour weight (baker's percentage). If your recipe has 500 g flour, that's 100 g of 100% hydration starter. Adjust the water and flour in the dough by the amounts contributed by the starter (starter at 100% hydration: 50 g flour + 50 g water per 100 g starter).
For more details see our articles on instant vs. active dry yeast and converting a yeast recipe to sourdough. Use our baker's percentage calculator to manage hydration when converting.