Subtract the pre-devaluation exchange rate (against the dollar or your currency of choice) from the devalued exchange rate. For instance, if the pre-devaluation rate is 24 dinars (or other currency), and the post devaluation rate is 37 dinars, the difference is 13 dinars on the dollar. 2 Divide the result by the pre-devaluation figure to get the percentage of the devaluation. Here, 13 divided by 24 is 0.54, indicating a 54 percent devaluation. 3 Figure the change in purchasing power of a given number of dollars. Divide the number of dollars that you would exchange by 54 percent to see what the value of the same number of dollars would be post-devaluation. For example, an exchange of $250 prior to the devaluation would have the purchasing power of about $463 (250 divided by 0.54) after the devaluation. The prices in the country of the devaluation are static, but what you could purchase increased dramatically in this case.