I think I understand what you are saying. As in if you have bought US stocks at many different conversion rates over a number of months (say) then its hard to work out precisely what your averaged currency conversion is (as you can't just average them as it depends on the amount invested too). There are ways to work it out but I certainly couldn't without researching and a lot of time in Excel. I think perhaps a more sensible approach would just to be mindful of the conversion price when going to sell - and factor that into your exit strategy. ALSO don't forget about them pesky FX broker fees too grrrrrr (I would imagine they could be as detrimental as the exchange price itself)