The author is the winner of the 2007 Melba Newell Phillips Award given by the American Association of Physics Teachers. Previously, he was awarded their Oersted Medal. Physicists use back-of-the-envelope estimates to check whether or not an idea could possibly be right. In many cases, the approximate solution is all that is needed. This compilation of 101 examples of back-of-the-envelope calculations celebrates a quantitative approach to...