Seatpost diameter-How to DIY your bike with good carbon frame

Seatpost diameter

Make sure your post matches your frame. There are shims available to make smaller posts fit larger-diameter seat tubes, but that can make your build a little more complicated.

Road bikes: Typically use 27.2mm, but some will have larger diameter seatposts. MAny Cannondales have 25.4mm seatposts. Some vintage and fixed-gear frames use 26.8mm seatposts. 
Mountain bikes: Usually use 30.9mm or 31.6mm, but there are exceptions. Some XC models use 27.2mm posts. A few bikes use oversized 34.9mm seatposts.