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Essential Resources : Closure Sizing 101

Cap and Neck FinishesCap size calculations

  • "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 that 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.

Common GPI/SPI Neck Finishes

Cap sizes and images for lotion bottles

Measuring a Neck Finish
Soap making bottle cap measurements

Find a cap's diameter, measuring from one side of the inner wall to the opposite side. Calculate a bottle's neck finish by measuring the diameter of the outermost threads. The resulting millimeter measurement will be the "T" dimension.

Then, see how many times the threads pass one another to determine the finish. (ex. 24 mm "T" dimension with 1 thread turn = 24/400 neck finish)