About 40 seconds after the explosion the air blast reached me. I tried to estimate its strength by dropping from about six feet small pieces of paper before, during, and after the passage of the blast wave. Since, at the time, there was no wind I could observe very distinctly and actually measure the displacement of the pieces of paper that were in the process of falling while the blast was passing. The shift was about 2 1/2 meters, which, at the time, I estimated to correspond to the blast that would be produced by ten thousand tons of T.N.T.
Emilio Segrè, who witnessed the event together with Fermi, gives a few more details. In his biography Enrico Fermi, Physicist, he writes that Fermi had done the necessary calculations in advance, "having prepared himself a table of numbers, so that he could tell immediately the energy liberated from this crude but simple measurement."
At Los Alamos, Enrico Fermi had the role of an "oracle": Because of his enormous knowledge and competence in all areas of physics, he was consulted for all kinds of physical problems. However, his mastery of physics could be intimidating to other physicists.
As a bonus, here is a story remembered by Subrahmanyan Chandrasekhar, who was a colleague of Fermi at the University of Chicago in the early 1950s (Bull. Amer. Math. Soc. Vol. 84, No. 3 (1978), p. 431):
Some twenty-five years ago, I met a colleague of mine emerging from the office of Enrico Fermi. He told me that he had been discussing physics with Fermi; and after a moment's pause asked, "Why am I doing physics? I should probably be a grocer".