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).

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