Outlet Birkenstock UK has a Dimension Transformation Graph that will help you discover your European dimensions.

For research, I typically use a sizing 8/8.5 large in my jogging shoes (Saucony) along with a dimensions 8 in boot styles. Should I adopted the Birkenstock conversion chart, I would use a dimensions 39. Even so, once you have installed, I found that my foot fits the best in a size 38 in Birkenstock sandals and a dimension 39 in Birkenstock boot styles.