Source: University of Western Ontario
Published: November 2018

Fermi Questions

Circulated: May 11, 2020

A “Fermi question” is a question in physics or mathematics which seeks a fast, rough estimate which is either difficult or impossible to measure. It is designed to teach approximation, also known as back-of-the-envelope calculations.

Example question

What is the circumference of the Earth?

1. How many time zones do you pass through flying from New York to Los Angeles?

Answer: 3

2. How many miles is it, about, over that same distance?
Answer: About 3,000 (4,800 km).

3. How many miles per time zone, on average?
Answer: About 1,000 (1,600 km).

4. How many time zones must there be around the world?
Answer: 24 because there are 24 hours in a day.

5. How many miles around the world?
Answer: 24 time zones x 1,000 miles per time zone = 24,000 miles (38,000 km)

Yes, it is about 24,000 miles around the world. The actual distance is 24,901 miles (40,075 km).