Direct energy equivalence. One tonne of TNT has the explosive energy (roughly) of 2.931 billion Joules according to internet references. Plug that into Einstein's famous equation and you get the answer. Then compare that to how much antimatter is being produced. You will find that we are perhaps a factor of 10 away from full-scale weapons production. Initially these would be special tactical weapons because of their high yield and small size (picture a 1 kiloton bomb that could be wrapped in a condom, swallowed or otherwise, and smuggled into a country) could only be produced in limited quantities.