In a spin off RP of Jack of All Trades (which hasn't started yet), the opening of the game starts with Victorian London becoming a giant smoking crater.

Let's calcuate that

So, Victorian London is 2.41402 Kilometers in total length, and thereby, 1.20701 Kilometers in radius. So, using the formula for a hemisphere...

(2/3)×π×1.20701^3 is 3.68291095178609 Kilometers, or for our purposes, 3.682910951786E15 centimeters cubed.

It's entirely Vaporized, so times 25700 is....

9.465081146090E19 Joules, 22.6220868692405 Gigatons, Island level