Investment portfolios generally contain assets from several countries. It is necessary to both convert the returns of the various securities into referential currency and calculate the portfolio returns in that currency. Exchange rates allow the quotes of a security in one currency to be converted into its equivalent value in another currency. Therefore it is possible to express the value of foreign assets in the currency of the country that has been chosen as a reference. This problem becomes even more complicated when investment fund or investors operate in many countries. In this article exchange rates are briefly presented and calculation formulas are explained when returns are either hedged or not hedged against currency risk.