Caps and bottles must fit together perfectly so as not to allow any of the contents of the bottle to escape and not to allow any outside substances to enter the bottle. It makes sense that the terms that are used to designate the size of a cap are the same terms that are used to indicate the neck finish of a bottle.

Cap sizes and neck finishes are expressed as two numbers. The first number indicates the diameter. For caps, this number represents the inside diameter of the cap. For the neck finish, this number represents the outside diameter of the threads on the neck of the bottle. As can be seen in the example to the right, the dashed red line indicates the inside diameter of the cap as well as the outside diameter of the threads on the neck of the bottle. In this example, the number for both is 28.
Inside diameter of cap

The second number refers to the thread style. The most common thread styles are shown in the image below. Each style is designated by a number. The differences among threads styles are the number of thread turns and height of the neck finish.

Going back to our example cap and bottle neck shown to the right, the threads make one and a half thread turns around the bottle. Comparing this with the common thread styles image below indicates that the thread style is a 410. So the second number for our example is 410. We now have all we need to determine both the cap size and the neck finish for our example. The diameter is 28mm and the thread style is a 410, so the cap size and neck finish for our example is 28-410.

When choosing caps and bottles, the cap size must match the neck finish of the bottle. For example, if your bottle has a 28-410 neck finish, be sure to match it with a 28-410 cap.

Common Thread Styles

common thread styles

We have learned that a cap actually gets its name from the measurement of its inside diameter. However, when choosing a bottle capping chuck to fit your cap, it is the outside diameter of the cap that is the critical dimension.

The image to the right once again shows our example cap and bottle neck. We have now indicated the outside diameter of the cap, 30mm. For small to medium cap sizes, up to about 53mm, the outside diameter of a cap is usually about 2-3mm greater than what the name of the cap indicates. For larger caps, the difference between the inside diameter and the outside diameter of the cap can be even greater. In some cases, such as child resistant caps, the outside diameter of the cap can be far greater than what the name of the cap indicates. So when choosing a chuck to fit your cap, do not rely on the name of the cap, be sure to measure its outside diameter.
Outside Diameter of a cap