Fermi Estimation

Order-of-Magnitude Estimation, Back-of-the-Envelope Calculation, Fermi Problem, Guesstimation

Break an unknown quantity into a chain of smaller sub-quantities you can each estimate with some confidence

Reason in powers of ten; aim to land the right power of ten rather than an exact figure

For each sub-quantity, pick a plausible lower and upper bound, then take the geometric mean of the bounds as the estimate

Independent over- and under-estimates tend to cancel, so the product of many rough guesses is often within a factor of 2-3 of the true value

Use the estimate to test whether a claim, design, or measured number is even plausible before investing in precision

Fermi’s classic teaching example — "How many piano tuners are there in Chicago?" — solved by chaining population, pianos per household, tuning frequency, and tuner throughput

Enrico Fermi (estimated the Trinity-test yield by dropping paper scraps in the blast wave, landing within an order of magnitude); Lawrence Weinstein and John A. Adam ("Guesstimation: Solving the World’s Problems on the Back of a Cocktail Napkin", Princeton University Press, 2008)