About 80,000 people were killed outright in the first blast. This was from a bomb rated at 18 kilotons of TNT.
By the end of 1945, injury and radiation brought total deaths to 120,000-140,000. These final figures accounted for about one-quarter of the greater-city population.
Two years later, deaths from the Hiroshima bomb, from all attributable causes, totalled over 200,000.
If the Nazis had perfected their bomb and dropped it on a city in Europe, it would have been classed as a war crime.
Leo Szillard, one of the development scientist on the Manhattan Project, wrote to President Truman after the war ended, "Suppose Germany had developed two bombs before we had any bombs. And suppose Germany had dropped one bomb, say, on Rochester and the other on Buffalo, and then having run out of bombs she would have lost the war. Can anyone doubt that we would then have defined the dropping of atomic bombs on cities as a war crime, and that we would have sentenced the Germans who were guilty of this crime to death at Nuremberg and hanged them?"
Neither Japanese city was a primary military target, no warning or demonstration was given, and the vast majority of deaths were civilians. Almost certainly, the International War Crimes Tribunal would have judged the bombs to be in breach of The Hague and the Geneva Conventions, and perhaps so would the International Criminal Court today. But the Tribunal didn't exist then and the winners made the rules.
Modern atomic weapons, hydrogen rather than plutonium bombs, are rated at 3 - 10 megatonnes. That's about 500 times the destructive force of the Hiroshima bomb.