What is a Fermi Question?

Fermi Process The technique that allows you to formulate an answer to a problem based upon a series of logical assumptions. An order of magnitude solution will result. No exact answer (likely) exists. For example, here is a Fermi Question: how many piano tuners are there in Toronto?

One solution/process Estimate the population of Toronto. Estimate what fraction of this population plays/has access to a piano. Estimate how often a piano needs to be tuned. Estimate how long it takes to tune a piano.

Solution cont. 2.5 million people If 20% play a piano, then 500,000 pianos If a piano needs tuning once per year, then 500,000 piano tunings needed per year. If it takes 4 hours to tune a piano, then 2 million hours of work per year needed tuning pianos.

Solution cont. Assume a tuner works a 40 hour week, 50 weeks per year. Then 2,000,000 hours per year divided by (40 x 50) hours per year per tuner = 1,000 tuners needed.