Guide
Fermi estimation: how to work out (almost) any number
How many trees are there on Earth? Nobody knows, but you can get surprisingly close by breaking it into small guesses you can make.
What is Fermi estimation?
A Fermi question is a "how many?" you can't know, but can reason your way towards. It's named after physicist Enrico Fermi, famous for quick rough maths that landed near the real answer.
The aim isn't to be exact. It's to land in the right ballpark.
How to answer a Fermi question
- 1
Break it into pieces
Turn one big number into a few small ones.
- 2
Estimate each piece
Use round numbers from everyday life.
- 3
Multiply it out
Keep track of the zeros.
- 4
Sanity-check it
Does it feel plausible? If not, a piece is off.
Worked examples
Past Daily Ballpark questions, on a napkin.
Past hole
How many full years of your life will you spend looking at your phone?
- About 4 hours a day on your phone.
- 4 hours is a sixth of a 24-hour day.
- A sixth of an 80-year life.
≈ 13 years
The classic
How many piano tuners are there in Chicago?
- About 1.1 million households.
- 1 in 20 has a piano: 55,000 pianos, tuned yearly.
- A tuner does 1,000 a year.
≈ 55 piano tuners
Practise one a day
One Fermi question a day, the same for everyone, scored like golf. Work out an impossible-sounding number, then see how you compare.