i just went through this. the local lbs said to go off of seatpost diameter. when i called back to find out if they had one another guy who actually knows what he is doing told me he needed the seattube diameter, this of course makes more sense but the sizes i was seeing on the internet led me to believe that they may sell them based off of seatpost dia. seatpost collars are sold off of the seattube outside diameter. you'll have to measure it with some calipers as it can vary based on how thick the seattube is.