On many occasions, physicists, other scientists, and engineers need to make estimates of a particular quantity. These are sometimes referred to as guesstimates, order-of-magnitude approximations, back-of-the-envelope calculations, or Fermi calculations. The physicist Enrico Fermi was famous for his ability to estimate various kinds of data with surprising precision. Estimating does not mean guessing a number or a formula at random. Instead, estimation means using prior experience and sound physical reasoning to give a rough idea of a quantity's value. As determining a reliable approximation usually involves the identification of correct physical principles and a good guess about the relevant variables, estimating is very useful in developing physical intuition. Estimates also allow us to perform "sanity checks" on calculations or policy proposals by helping to rule out certain scenarios or unrealistic numbers.
Many estimates are based on formulas in which the input quantities are known only to a limited level of precision. To make some progress in estimating, one needs to have some definite ideas about how the variables may be related. The following strategies could help practice the art of estimation:
