The energy released by an atomic bomb comes from the transformation of mass into energy (E = mc^2; c = speed of light). But if you want to calculate the energy of a lorry crashing or a meteorite that brakes, a similar relationship takes part, but the energy now comes from the mass that is braking. The energy lost by braking is equal to m/2 (meteorite's mass) * v^2 (meteorite's velocity).

Therefore, in Hiroshima the amount of mass converted into energy is 0.7 gr. * speed of light, equal to around 16,000 tons of TNT. The meteorite entered the atmosphere at such high speed as 18 km/s. Enormous, but about 15,000 times smaller than the speed of light. Yet it was much heavier, around 7,000 tons. If we divide both numbers, we would get that the meteorite energy was 20 times larger than the Hiroshima bomb.

SOURCE: http://blogs.tn.com.ar/desmitificador/2013/02/18/el_meteorito_de_rusia_equivale_20_bombas_de_hiroshima_como_es_eso/