The classic Fermi problem, generally attributed to Fermi,[2] is “How many piano tuners are there in Chicago?” A typical solution to this problem would involve multiplying together a series of estimates that would yield the correct answer if the estimates were correct. For example, we might make the following assumptions:
There are approximately 5,000,000 people living in Chicago.
On average, there are two persons in each household in Chicago.
Roughly one household in twenty has a piano that is tuned regularly.
Pianos that are tuned regularly are tuned on average about once per year.
It takes a piano tuner about two hours to tune a piano, including travel time.
Each piano tuner works eight hours in a day, five days in a week, and 50 weeks in a year.
From these assumptions we can compute that the number of piano tunings in a single year in Chicago is
(5,000,000 persons in Chicago) / (2 persons/household) × (1 piano/20 households) × (1 piano tuning per piano per year) = 125,000 piano tunings per year in Chicago.
We can similarly calculate that the average piano tuner performs
(50 weeks/year)×(5 days/week)×(8 hours/day)/(2 hours to tune a piano) = 1000 piano tunings per year per piano tuner.
Dividing gives
(125,000 piano tunings per year in Chicago) / (1000 piano tunings per year per piano tuner) = 125 piano tuners in Chicago.
-- Wikipedia on Fermi Problems
Fermi calculations are a quick way to come up with rough numerical estimates of various things, and some companies use Fermi problems as interview questions. Coming up with various Fermi problems, breaking them apart into various assumptions and then checking to see whether the calculations were correct can be an entertaining activity for a while. The skill of doing Fermi calculations may come useful, as does a knowledge of how accurate your Fermi calculations tend to be.
Fermi Problems
-- Wikipedia on Fermi Problems
Fermi calculations are a quick way to come up with rough numerical estimates of various things, and some companies use Fermi problems as interview questions. Coming up with various Fermi problems, breaking them apart into various assumptions and then checking to see whether the calculations were correct can be an entertaining activity for a while. The skill of doing Fermi calculations may come useful, as does a knowledge of how accurate your Fermi calculations tend to be.