Cap and Neck Finishes

The neck finish refers to the outer threads on the container that holds the caps, stoppers, dispensing caps, and other closures in place. If you are looking to pair a cap with a container, the neck finishes must match. For example, if a container has a 24/400 neck finish, 24/400 caps are compatible, not 20/400 caps.

Screw threads, also called continuous thread closure sizes, are shown on the website with two numbers with a hyphen or slash (example: 18-400 or 18/400). When referring to caps, the first number is the millimeter diameter measured across the inside of the cap's openings. For containers, the first number is the millimeter measurement of the outside of the container's threads.

The second number refers to the thread style, "GPI" or "SPI" finish. The Glass Packaging Institute (GPI) and Society of the Plastics Industry (SPI) are responsible for establishing uniform standards for glass and plastic container neck finishes. The packaging industry does not necessarily adhere to the same standards, so it is often best practice to purchase containers and caps from the same manufacturer when possible. SKS also offers samples to ensure compatibility.

For information about proper torque when applying closures to containers, visit our Torque Guide page.

"T" Dimension

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

"E" Dimension

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

"I" Dimension

Inner diameter of bottle neck. Specifications require a minimum "I" to allow sufficient clearance for filling tubes. Linerless closures w/ plug or land seal & dispensing plugs/fitments require controlled "I" dimension for 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 orientation of the closure to 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

  • 400: 1 thread turn
  • 410: 1.5 thread turns
  • 415: 2 narrow thread turns
  • 425: Buttress finish. Includes thick threads and a top bead (better seal, more application torque)
  • 2030: Lug Finish - Non-continuous threads
  • 2035: Lug Finish - Non-continuous threads, tall "H" dimension

How to Measure a Neck Finish

To find a cap's diameter, measure 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 1.5 thread turns = 24/410 neck finish)

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