Substitution Formula
To substitute active dry for instant (or rapid rise) yeast: Use 25 percent more active dry. For example, if the recipe calls for 1 teaspoon of instant yeast, use 1 1/4 teaspoons of active dry. And don't forget to "prove" the yeast, i.e. dissolving it in a portion of the water from the recipe, heated to 105 degrees.
To substitute instant (or rapid rise) yeast for active dry: Use about 25 percent less. For example if the recipe calls for 1 packet or 2 1/4 teaspoons of active dry yeast, use 1 3/4 teaspoons of instant yeast. And you do not need to prove the yeast, just add it to the dry ingredients.
To substitute fresh yeast for active dry yeast, use a ratio of roughly 2:1, i.e. use one small cake (0.6 ounce) of compressed fresh yeast in lieu of 1 packet (.25 ounces) of active dry yeast.
Note a packet of active dry or instant yeast contains about 2 1/4 teaspoons (.25 ounces) of yeast.
(America’s Test Kitchen, September 1, 2004)