Shoe sizing, especially online, is not a perfect process. That's why we have an exchange policy.
For EU/UK/JP sizing, there's a conversion chart at the bottom of the product description.
The TerraFlex now runs fairly true to size. We recommend going with your usual athletic shoe size.
The TerraFlex is wider than conventional trail running shoes.
Sizing for a shoe is different than a sandal, so if you own one of our sandals, that doesn't mean you'll pick the same size in the TerraFlex, so use the measuring info if you can.
Keep in mind, though, that because the foot is three-dimensional, any two-dimensional measuring tool, such as a ruler or Brannock device, can only approximate your true shoe size. Manufacturers model their shoes on different foot shapes, which is why you might be an 38 in one brand and a 39 in another. And then, there's personal preference: three people with identical feet might think the same shoe fits differently.
If your foot is really wide and larger than a women's 38.5, you may want to check out the "men's" TerraFlex. The conversion between men's and women's is 1, so a women's 38.5 is the same as a men's 39.5.