Neck Sizes Explained
How do I choose the correct closure size?
Closure sizes are expressed using two different numbers.
The first number you see refers to the diameter of the closure or bottle opening.
The second number is the Glass Packaging Institutes (GPI) thread finish and refers to the height of the cap and the style of the threads on the closure or bottle.
For example, a closure size of (20/400) has a diameter of 20 mm and fits a bottle with a 400 GPI thread finish.
Some neck sizes are interchangeable. For instance, a 20/410 closure will usually fit on a 20/400 bottle.
Also, 410 and 415 are usually interchangeable if the closure is not tamper evident.