I don't know which measurement is the primary one relied on. I provided both and the UFlex Athletics Knee Brace fit perfectly. This brace stabilized my knee as well as a custom brace my insurance paid for costing hundreds of dollars. That brace chafed my leg when adjusted snug enough to stabilize my knee. I had to cut off the long sleeves of a long sleeve t-shirt to put on under it to prevent the chafing. That combo contributed to sweating. The UFlex Athletics Knee Brace does not contribute to sweating. I guess it breathes better. I'm 165 pounds of medium build and use a large.