What are the most useful shortcuts for performing back-of-the-envelope calculations of computing problems?
With a [back-of-the-envelope calculation][1] you can mentally obtain an answer to a question, within an order of magnitude of its actual value. Some [shortcuts][2] can simplify these calculations.  In computing I've found the rule 

10^(3*n) is-close-to 2^(10*n)

invaluable.  For instance, through this I can say that I can binary search through a billion records with about 30 operations.  What other shortcuts do you use and recommend?  Let's put them all here and vote your favorites.


  [1]: http://en.wikipedia.org/wiki/Back-of-the-envelope_calculation
  [2]: http://www.vendian.org/envelope/
What are the most useful shortcuts for performing back-of-the-envelope calculations of computing problems?
===

With a [back-of-the-envelope calculation][1] you can mentally obtain an answer to a question, within an order of magnitude of its actual value. Some [shortcuts][2] can simplify these calculations.  In computing I've found the rule 

10^(3*n) is-close-to 2^(10*n)

invaluable.  For instance, through this I can say that I can binary search through a billion records with about 30 operations.  What other shortcuts do you use and recommend?  Let's put them all here and vote your favorites.


  [1]: http://en.wikipedia.org/wiki/Back-of-the-envelope_calculation
  [2]: http://www.vendian.org/envelope/