First turn the chair up-side-down so that the and in the center of the base bottom underside can be seen.
Spread newspapers on floor to prevent greasy falling parts from causing stains
Now measure and select a replacement
An often asked question-"Whats the difference between a COM and only the cylinder?"
A COM includes the gas cylinder, also includes a bearing kit and new column plastic liner, all preassembled.
Precise measurement is unnecessary
Instead, you can select a replacement according to your height