Harold Agnew with the Plutonium Core

The plutonium core (the box) in the Fat Man weighed 6.2 kg or about 14 lb, the pit is 9 cm (4 inches) across. And only about one fifth of it, a bit over 1 kg (2 pounds) undergoes a fission reaction. And only a gram (1/30th of an ounce) of that gets converted into explosive energy equal to 21,000 tons of TNT.

---