Using Calipers
The picture below shows the process of measuring a seatpost. The caliper here clearly indicates that the seatpost is 26mm. However, it is possible to get an even more accurate reading of the diameter. The numbers on the lower part of the caliper going from 0 through 9, then another 0 indicated tenths of a millimeter. The line near one of those numbers should match up with the line above it on the body of the caliper. In this case, it is the 0 that matches up, and the seatpost is 26.0mm.
For measuring seatposts, it helps to know the basics of how to use calipers. Calipers can be capable of very accurate measurements, but you will not need to make very accurate measurements with them at the co-op. Incidentally, road bicycle brakes are also called calipers. Measuring a seatpost using calipers is straightforward: clamp the post between the jaws lightly and read the number indicated at the 0 mark in mm (millimeters). Once you know the size of the post to within a millimeter, you can file it in the appropriate drawer. It is a good idea to look at the seatpost carefully before you measure it, because many nicer seatposts have the size etched into the side, which will save you the effort.