What is a Fermi question?
Named after the physicist Enrico Fermi, a Fermi question asks for a number nobody can look up in their head, such as how many piano tuners work in a large city or how many litres of water a person drinks in a lifetime.
The trick is not to know the answer but to reach a sensible estimate by combining a few smaller guesses you can make with some confidence.
Break the question into parts
Take the piano tuner example. Estimate the city's population, the share of households with a piano, how often a piano is tuned, and how many pianos one tuner can service in a year. Multiply and divide, and you reach a number that is usually within a factor of a few of reality.
- Write down the unknown you want to estimate.
- Split it into quantities you can guess more easily.
- Estimate each one with a round number.
- Combine them, then sanity-check the scale of the result.
Think in orders of magnitude
The most useful question is often whether an answer is in the hundreds, thousands or millions. Getting the scale right matters more than the first digit. Round numbers keep the arithmetic simple and errors in different parts tend to cancel out.
From an estimate to a range
A Fermi estimate gives you a centre point; your confidence in each step tells you how wide the range should be. In Approx that is exactly the decision that scores points: a range tight enough to be valuable, wide enough to contain the answer.