To convert cake yeast to instant yeast:
For 1 packed tablespoon cake yeast use 2 teaspoons instant yeast or
2-1/2 teaspoons active dry

OR
To convert recipes calling for instant yeast to active dry yeast: Use 1.25 times the weight; or, for 1 teaspoon instant yeast, use about 1 1/4 teaspoons active dry yeast

To convert recipes calling for active dry yeast to instant yeast:

Use 0.67 times the weight; or, for 1 teaspoons active dry yeast, use 3/4 teaspoon instant yeast

To convert recipes calling for fresh compressed yeast to instant yeast: Use 0.32 times the weight; or, for 1 packed Tablespoon (21 grams) fresh yeast, use 2 teaspoons instant yeast

from the Bread Bible, from Rose Levy Beranbaum

http://www.bigoven.com/172559-Best-Bread-recipe.html#

[edited June-28-2009]