They are a pretty dense rubber (like an outsole) and 5 mm. They warranty them to 5000 miles and I wear them a lot around town and haven’t noticed significant wear. So they seem to be pretty good, but I have only had this first pair
Discussion
I will say that most of the reviews seem to indicate that the cord is more likely to tear out of the rubber than the rubber wearing through if you are using them for activities
I own two pairs of Xeros, but full shoes, not sandals like these. One low rise, the other semi-boot height for the winter. Adequately wide for the toes, zero
rise. which is the most important thing. I personally remove the insoles, so I'm basically walking on a 5mm sole just like the sandals. I walk on asphalt in the city, and a lot on sand, gravel and pebbles. They're indestructible.