[PvK]

E = mass x C^2 is valid to give the energy for total conversion from matter to energy. But ya, assuming that's what you can get from an anti-matter bomb is an assumption.

Nuclear weapons kilotons are in terms of equivalent explosive force compared to a certain mass of T.N.T., which of course has its own conversion factor involved. I bet it's not too hard to google up.

There is of course another web page with lots of discussion about what it takes to destroy a planet in various ways.