Exa is not a measurement unit. It is a prefix which means a quintillion or 10^18. So, for example,
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1018
At least in Europe it would be Exa- (=1018) (http://en.wikipedia.org/wiki/SI_prefix)
After tera (10^12) comes peta, which is 10^15, followed by exa, which is 10^18.
well, if you think about it, it is infinite. the furthest I know is exa, tera, giga, mega, centa, deca, deci, centi, milli, micro, nano, pico, femto, atto. so, did that help?
Giga - 10 is to 9 Tera - 10 is to 12 Peta - 10 is to 15 Exa - 10 is to 18 Zetta - 10 is to 21 Yotta - 10 is to 24