(Source for image: US National Archives Heroes & Villians display.)
According to the Department of Energy's "Manhattan Project" website, the yield of the bomb was estimated to be 21 kT (kilotons) of TNT. One ton of TNT is equivalent to 4184 MJ (megajoules) of energy. Thus 21000 tons of TNT is equivalent to (21000)(4.184 x 109) = 8.786 x 1013 J. Using Einstein's famous equation E=mc2, with E = 8.786 x 1013 J, c = 299792458 m/s, then the mass = 9.78 x 10-4 kg. This is 0.978 g. This means that all of the energy of the bomb was created by converting 0.978 g of matter into energy.
In other words, all of the devastation you see in the image above was caused by less than a gram of
matter potential energy being converted into kinetic energy.
UPDATE: I tripped across a video by PBS Space Time in which they state that mass is not converted into energy ("that would be alchemy", they claim). Fine. Whatever. If I state that less than a gram of potential energy is converted into kinetic energy, is that okay? (NOTE: If you're from PBS Space Time, I think you're getting to the point of being pedantic here. Seriously.)