Shoe sizing, especially online, is not a perfect process. That's why we have a free exchange policy.
For EU/UK sizing, there's a conversion chart at the bottom of the Prio product description.
The Prio fit true to size. We recommend going with your usual athletic shoe size.
Also, the Prio is wider than conventional 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 Prio, 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 8 in one brand and a 9 in another.
If your foot is really narrow and smaller than a men's 10, you may want to check out the "women's" Prio. The conversion between men's and women's is 1.5, so a men's 8 is the same as a women's 9.5.