Different bicycles require different diameter seatposts. To find out the diameter of your seatpost, you must remove the seatpost from the bicycle frame and look for the size, which is stamped just below the “Minimum Insert” line toward the bottom. The number will be a three-digit number (such as 26.8 or 27.2 or 31.6). If you are unable to read the size, you may need to take the seatpost to your local bike shop and have them measure it using calipers.