Fresh yeast can easily replace dried yeast; the general rule is to use roughly twice the amount of fresh yeast as the dried yeast a recipe calls for. If a recipe asks for 7 grams of dried yeast, for example, use about 15-20 grams of fresh yeast instead.
Why does fresh yeast need to be doubled?
Dried yeast has had its moisture removed during processing, which concentrates the living cells, so it has a stronger rising power gram for gram than fresh yeast. Because fresh yeast contains much more water, you need a larger weight of it to reach the same number of active cells.
This ratio is not an exact science — some bakers use 2.5 times the amount, others use 3. A 1:2 ratio works well enough for most home recipes, but if the dough rises noticeably faster or slower than the recipe describes, the ratio can be adjusted slightly next time.
How does the method change?
Dried yeast is usually mixed straight into the flour, while fresh yeast should be crumbled into warm water (35-40°C) and left for a few minutes before being added to the dough; this lets the yeast activate and start to foam. If the water is too hot the yeast cells die and the dough will not rise, so water that feels comfortably warm to the touch is enough.
Fresh yeast is a more delicate product than dried yeast and only stays good in the fridge for a couple of weeks; do not use fresh yeast that has gone mouldy or turned dark. Dried yeast, on the other hand, can sit unopened in the pantry for months without spoiling.
Where is the difference most noticeable?
In recipes that need to rise quickly, like pizza dough, fresh yeast can give slightly faster results because its cells are already active, whereas dried yeast needs a few extra minutes in water to wake up. In slow-rising recipes like bread, the difference between the two barely shows once the ratio is right.
The same logic works in reverse where fresh yeast is hard to find: if a recipe calls for fresh yeast, use roughly half that weight in dried yeast, following the same activation step of soaking it in warm water first.
Points worth keeping in mind
Whichever type of yeast you use, salt and yeast should never be added to the dough touching each other directly, since salt slows down yeast activity. Mixing the yeast with the water first and the salt with the flour avoids this problem.
When dough rises more slowly than expected, the cause is usually the water temperature or how fresh the yeast is rather than the ratio itself, so checking the yeast's expiry date and the water's warmth first is a more reliable first step than changing the amount. The same rule applies when switching between different yeasted doughs, such as pizza dough, poğaça or açma; even though the recipe changes, the ratio and activation step stay the same.