According to my Langenschedit's Pocket Dictionary

Billion : trillion


Billions, trillions etc used to be easy to work out, a billion was a million to the power of two, a trillion was a million to the power of three, a quadrillion to the power of four. I believe it's possible to derive a formula for the short versions, but I can't be bothered working it out.

The outcome is that I've stopped using the words and just say ten to the twelve, ten to the eighteen etc.