One envelope of active dry yeast is equal to 2 1/4 teaspoons. An envelope of yeast is also 1/4 ounce. One packet of yeast is typically the amount required for one bread recipe. One-third of a 2-ounce cake of dry yeast is equal to one envelope.
To check if the yeast is still good, the cook can stir 1 teaspoon into a 1/4 cup of warm water, and let it stand. The water should not be too hot because it kills the yeast. If the yeast foams after 10 minutes, it is still good. This process is called proofing the yeast.