Every cyclist should be familiar with the basic mechanics of his or her bicycle and also have a record of bike setup or measurements. Why? Seatposts, handlebars and other parts can slip or rotate, maybe you will need to travel with your bike and need to take it apart and rebuild it, if you get a new bike you will need to know how to set it up to be similar to your old bike, if you are uncomfortable and want to refine your position having accurate measurements will allow you to make precise adjustments and have record of the changes made. These are just a few of many reasons why a cyclist should have a record of their current bike measurements.
Use the diagram below to fill in the measurements. Keep your measurements from bike to bike, year to year. Determining optimum fit can be a lengthy process and any previous fit data can be helpful in determining changes necessary.
In addition to the measurements below, I always measure my saddle height and mark the seat post height with electrical tape. I measure seat height along the down tube, measuring from the center of the seat to the center of the bottom bracket. This way I can quickly see by the tape if my seat height has changed and also reset it to the proper height.