Chapter 3 of 10All chapters
Chapter 3 of 10
Fermi estimates
Breaking the unguessable into the guessable.
The method
Split the question into factors you can each guess within a factor of three, then multiply. Errors in different directions partly cancel, so the product is better than any single guess.
- Population times rate times frequency covers most such questions.
- State your assumptions out loud so someone can challenge the weak one.
The classic
How many piano tuners are in a city? Population, households per piano, tunings per year, tunings per tuner per day. Four rough guesses give a surprisingly good answer.