The Nuclear and Industrial Safety Agency (NISA) says the estimate released today was requested by a parliamentary panel.
But it said a simple comparison between an instantaneous bomb blast and long-term accidental leak is impossible.
The report estimated for each of the 16 isotopes leaked from "Little Boy" and 31 of those detected at the Fukushima plant but didn''t provide the total.
NISA has said the radiation leaked from Fukushima was about one-sixth of what the Chernobyl disaster released in 1986.
"Little Boy" was dropped on the Japanese city at the end of World War II.