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If a password of k bits is chosen at random there are 2 to the k exponential possible values and the password is said to have k bits of entropy. If a password of length l characters is chosen at random from an alphabet of b characters (e.g., the 94 printable characters on a typical keyboard) then the entropy of the password is b to the l exponential. See the following InCommon Assurance link for page for Password Entropy Calculators https://spaces.at.internet2.edu/display/InCAssurance/Password+Entropy+ Calculators. Most Institutions who are pursuing InCommon Silver are using the The University of Wisconsin calculator.
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