Which formula did Feynman and Hans Bethe develop for fission-bomb yield?

Feynman and Hans Bethe developed the Bethe–Feynman formula for estimating the yield of a fission bomb.

The formula emerged from their work at Los Alamos during World War II. It provided a simplified way to estimate how much energy a rapidly expanding fission chain reaction could release. The calculation connected the growth of the chain reaction with the changing size and physical conditions of the bomb’s fissile core.

The Bethe–Feynman formula built on earlier work by Robert Serber and was first derived in 1943, following developments made in 1942. It was valuable because weapon designers needed practical estimates of explosive yield while working with incomplete experimental data and highly compressed timescales.

The name can cause confusion because Bethe also contributed to many other major areas of nuclear physics, while Feynman is often remembered primarily for quantum electrodynamics and his later lectures. The Bethe–Feynman formula was specifically associated with fission-bomb efficiency and yield, not with stellar physics, radioactive decay, or the Drake equation.