With a back-of-the-envelope calculation you can mentally obtain an answer to a question, within an order of magnitude of its actual value. Some shortcuts can simplify these calculations. In computing I've found the rule
103n is-close-to 210n
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.