There are a few calculators on the web, but most of them suck. All you can really do is look up the draw for your major components add them up, and add a bit for "misc".
CPU power specs can be gotten from AMD & Intel's web sites. Ditto for hard drives. For Optical drives this may work as well depending on who made it. I've also seen power specs printed on drives sometimes. Video cards take a bit more work, but you should be able to find the info at least if it is/was a high end one. Fans normally have thier power draw in amps printed on them.
Or you can just post your specs & we'll guess what you need.