Estimate like Fermi

On 16 July 1945 the New Mexico sky lit up from the world’s first atomic bomb blast. Ten miles away, scientist Enrico Fermi tore up a sheet of paper. As the shock wave hit, he dropped the bits of paper which were carried away by the blast. He measured the distance the paper travelled, consulted a table, and estimated the bomb yield at 10 kilotons of TNT. Later the precise yield was calculated: 18.6 kilotons.

Fermi’s estimate was was off by a factor of two, good enough for the field.

What is a Fermi estimate?

A Fermi estimate or Fermi problem uses simple calculations to approximate a more complex equation. Fermi estimates are useful for sanity-checking a complicated calculation to make sure it’s not spewing gibberish. Or for getting a quick estimate instead of waiting for a more precise answer.

Learn to estimate

You can learn to make estimates like Fermi.

Why do Fermi estimates work?

A Fermi estimate works because it’s composed of several estimates of terms. Some of the terms are over-estimated, some are under-estimated so the errors cancel out.


Cooper, Lance. Making Estimates in Research and Elsewhere.

Fermi, Enrico. My Observations During the Explosion at Trinity on July 16, 1945.

Rhodes, Richard. The Making of the Atomic Bomb. Simon and Schuster, 1986.