"T" Dimension
The outside diameter of the thread. The tolerance range of the "T" dimension will determine the mate between bottle and closure.

"E" Dimension
The outside diameter of the neck. The difference between the "E" and "T" dimensions divided by two determines the thread depth.

"I" Dimension
The inner diameter of the bottle neck. Specifications require a minimum "I" to allow sufficient clearance for filling tubes. Linerless closures, with a plug or land seal, and dispensing plugs and fitments require a controlled "I" dimension for a proper fit.

"S" Dimension
Measured from the top of the finish to the top edge of the first thread. The "S" dimension is the key factor which determines the orientation of the closure to the bottle and the amount of thread engagement between the bottle and cap.

"H" Dimension
The height of the neck finish. Measured from the top of the neck to the point where the diameter "T", extended down, intersects the shoulder.

Screw thread cap sizes are generally expressed as two numbers. The first number refers to the cap diameter (in millimeters) and the corresponding Glass Packaging Institute (GPI)/Society of the Plastics Industry (SPI) finish diameter.

The second number refers to the height and thread configuration of the closure or finish. Below is a selection of commonly used caps and finishes.