There is a myth that you can just "add 30" to your shoe size in the United States and arrive at nearly the correct shoe size in Europe. This method does not always work and might leave you with very uncomfortable feet! For example, a woman who wears a size 7.5 in the United States would certainly find a size 37 too small in European shoe sizes.
A reliable conversion chart is the best way to find out your true shoe size in European measurements. There are many conversion charts found online. You may also be able to obtain a portable chart from a salesperson at a reputable shoe store.
Even the best conversion chart can't measure up to actually trying on the shoe itself; the chart should serve only as a guide. Try the shoes on both feet, and walk a few steps to make sure you can walk comfortably without cramping or pinching. When you have found the perfect fit, you'll know your size for certain.