Bottle neck finish: what it determines

A bottle's neck finish is the threaded part of its neck, the part a closure screws onto. It determines both whether a given closure screws onto it at all and the seal achieved once it is screwed on. Two bottles can share the same overall shape and still not accept the same closure: it is the neck finish, not the bottle as a whole, that settles that compatibility. To designate it without ambiguity, the packaging industry uses a two-number notation, such as 28/410.

This convention is named after the GCMI, short for the Glass Containers Manufacturers Institute, the American body that originally established it for glass containers. It has since been renamed the Glass Packaging Institute, but the notation it set down survived that change and spread beyond glass: today it designates the neck finish of a plastic bottle just as readily as that of a glass one.

In 28/410, the first number — 28 — is the outer diameter of the thread, expressed in millimetres: a direct measurement, taken at the widest point of the thread.

The second number describes the shape of the thread, not its size. 400 corresponds to one full turn of thread; 410 and 415 denote taller threads with more turns, engaged over a greater length of the neck. Two bottles can therefore share the same first number and differ in the second: they then have the same outer diameter but a distinct screwing profile. The practical result is direct: a bottle in 28/400 and a bottle in 28/410 have the same diameter and do not accept the same closure, because the thread of one does not match what the other was shaped for.

That second number is not an approximate label: it points to a published technical specification that fixes the thread's pitch, its depth, and where it starts on the neck. It is these same shared parameters, common to everyone who refers to them, that let a closure maker and a bottle maker each produce, independently, parts that fit together.

This is something this catalogue lets you check directly: the product sheets that carry a neck-finish value show only the first number — the diameter — never the second. Two bottles showing the same neck-finish value on this site therefore share an identical outer thread diameter, but that alone does not mean they accept the same closure: the thread shape, the only information that settles that question, does not appear here. Anyone who needs to match a closure to a specific bottle needs that second number, and will have to ask for it separately.