The conversion ratio
Fresh yeast contains more water, so it takes more weight to get the same activity. A widely used baking rule is 1 g fresh = 0.4 g active dry = 0.33 g instant.
Instant yeast can be mixed straight into flour. Active dry should be rehydrated in warm water first.
Frequently Asked Questions
Can I use less yeast and rise longer?
Yes. Halving the yeast roughly adds 1–2 hours to a room-temperature rise. Slow, cold rises build flavour.