When measuring closures or bottles you use the letters T, E, H and I to give you the most basic dimensions of the bottle or closure. When measuring it is recommended you use a vernier calliper to provide an accurate measurement.
When describing the size of a cap or bottleneck you refer to the T value which for a bottle is the outside of the thread. On a closure it is the wall to wall measurement and often measures just over the final size, EG a 28mm cap will measure 28.6mm.
The remaining measurements are there to assist in providing a good fitment which will not jump the thread.
If you want further information or require help please contact our team at Caps and Closures.