Cap and Neck Finishes
Measurement and Compatibility
The neck finish on a container holds the cap, or closure, with protruding threads. A container and its corresponding cap must have matching finishes. For example, a 24/400 bottle will only accept a 24/400 closure.
Screw thread or continuous thread closure sizes are expressed with two numbers separated by a slash. The first number refers to the diameter (in millimeters) measured across the inside of the cap’s opening or the outside of the bottle’s threads. The second number refers to the thread style. The closure 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.
Common GPI/SPI Neck Finishes
Measuring a Neck Finish
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)