In this video Rob Kozler from www.MotoLabDirtBikes.com provides an example of how exactly to measure your dirt bike sag with a standard tape measure.
This is very critical and one of the very first steps in tuning your suspension. Measuring and setting your sag affects many aspects, such as; geometry, handling, chassis balance, steering…etc. measure this first to determine whether or not you have the correct spring in there for your weight. ALWAYS consult your owners manual for specific model information. This is provided as an example, not a direct replacement for your individual make and models specifications.