Be sure to size for whatever socks you will be wearing. I have worn size 6 in just about everything since adulthood. The 38 fits like a glove with a pair of thin Defeet cycling socks without much room for swelling which my feet do a lot in hot weather. When I road bike this is OK, but when I mtbike I just have to have a tad more space because my feet are moving around a lot more. I use a 38.5 for mtbiking in hot weather or if I want to wear multiple socks. Hey, another reason for multiple pairs of shoes (not to mention I hate putting on wet shoes when I'm riding consecutive days).