A recipe calls for fresh yeast, your cupboard holds instant, and the bake stalls before it starts. This is one of the most common bread problems, and it has a simple fix: the three yeast types are the same organism in different forms, and they convert by fixed ratios. Fresh yeast is the baseline, active dry yeast is more concentrated, and instant yeast is the most concentrated of all. This calculator converts any amount of one type into the matching amount of another, so no recipe is ever blocked by the yeast you happen to own.
The ratios are easy to remember. Fresh yeast counts as 100 percent. Active dry yeast counts as 40 percent of fresh, because drying removes water and concentrates the yeast. Instant yeast counts as 33 percent of fresh, or one third. So 30 grams of fresh yeast becomes 12 grams of active dry yeast, or about 10 grams of instant yeast. Going the other way, 10 grams of instant yeast equals about 30 grams of fresh. The calculator uses these exact factors, and rounds to one decimal place, which is more precision than dough ever needs.
Here is a worked example. Your recipe asks for 25 grams of fresh yeast, but you only have instant. Divide by the fresh factor and multiply by the instant factor: 25 divided by 1, times 0.33, is 8.25 grams, shown as 8.3 grams. If instead you had active dry yeast, the same recipe needs 25 times 0.4, which is exactly 10 grams. Weigh yeast on a small scale when you can, because a teaspoon of instant yeast weighs about 3 grams, and spoon measures vary enough to shift a small dough.
How you use each type matters as much as the amount. Fresh yeast crumbles into dough and dissolves quickly, but it keeps only a week or two in the fridge and a few months in the freezer. Active dry yeast has a protective coating of dead cells, so it should be dissolved in warm water, around 40 degrees, for a few minutes before mixing, or it can stay grainy in the dough. Instant yeast has finer granules and no coating to dissolve, so it mixes straight into the flour and starts working at once.
Rising time also changes with the type and the amount. More yeast means a faster rise, but not a better loaf: too much yeast can make bread taste sour or yeasty and collapse in the oven. Most recipes use about 1 to 2 percent yeast by flour weight for a normal rise, and slow overnight doughs use far less, sometimes 0.2 percent. When you convert types, keep the recipe rising time as a guide, not a rule: watch the dough, and bake when it has roughly doubled and springs back slowly when poked.
Storage decides which type is worth buying. If you bake weekly, fresh yeast from a bakery or supermarket fridge is lovely and cheap, but check the date, because old fresh yeast loses strength fast. If you bake monthly, instant yeast in a sealed jar in the freezer lasts a year and is ready in seconds. Active dry sits between the two: long shelf life, but needs the warm-water step. Whichever you keep, convert with this tool, and the yeast in your cupboard will never again be the reason a recipe waits.
Temperature is the quiet partner of yeast amount, and it explains many failed rises. Dough rises fastest around 25 to 28 degrees, slows in a cool kitchen, and nearly stops in the fridge, which is why the same recipe can take one hour in summer and three in winter. If your kitchen is cold, give the dough more time rather than more yeast, because extra yeast changes flavour while extra time improves it. A simple test is the poke test: press a floured finger into the dough, and if the dent fills slowly, the dough is ready, while a dent that springs back fast needs more time.
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