 The detonation created a blast equivalent to 16 kilotons of TNT ( 67 TJ ) . The U @-@ 235 weapon was considered very inefficient , with only 1 @.@ 7 % of its fissile material fissioning . The radius of total destruction was about one mile ( 1 @.@ 6 km ) , with resulting fires across 4 @.@ 4 square miles ( 11 km2 ) . Americans estimated that 4 @.@ 7 square miles ( 12 km2 ) of the city were destroyed . Japanese officials determined that 69 % of Hiroshima 's buildings were destroyed and another 6 – 7 % damaged . Some 70 @,@ 000 – 80 @,@ 000 people , or some 30 % of the city 's population , were killed by the blast and resultant firestorm , and another 70 @,@ 000 injured . Out of those killed , 20 @,@ 000 were soldiers . 
