ACTUAL reach of APS-C vs Full-frame

How do I determine the actual reach of APS-C vs Full-frame cameras? I hear the 20D has the same reach as a 1Ds Mark III, because the III has way more pixels and larger pixels to offset the crop factor of the 20D. Can anybody tell me how I can determine this formula myself? What do I need to know to calculate it?

I was given this and can't get them to equal:
Canon 20D: 1.6x crop, 8 mp, 6.4 pixel size
Mark III: 1.0x, 21 mp, 6.4 pixel size

I'm missing something! Please help!

Its 1.6 period. Its not more complicated than that. You might have come across somebody trying to make some rationalization about cropping after the fact that compares pixel counts or pixel size but its hocus pocus.

GIven your user name, I assume you are typically shooting birds. This is a photography subject that has photographers shooting with their longest lenses and cropping all shots. In such a case, you are correct that you can compare cameras based on their pixel density. Remember though that there is more to this than pure pixel density as later sensors have improved with things like gapless microlenses etc. In my own experience, once you have 'enough' pixels for the print size you want to make then having more doesn't help much. So don't assume if one camera cropped gives 6 MP and another gives 10 MP that the 10 MP image is going to be noticeably better in print.

With that caveat, here is what you asked for:

To find out how many pixels you would have left if you were to crop a FF sensor down to APS_C, multiply the FF pixel total by 0.39. So for the 5D Mark II or 1Ds Mark III you get 21,000,000 x 0.39 = 8,190,000.

To find out how many pixels you would have left if you were to crop APS-H down to APS-C, multiply the APS-H pixel total by 0.68. For the 1D Mark IV you get 16,000,000 x 0.68 = 10,880,000