What a bottle diameter does and does not define

A diameter is the easiest thing to measure and the easiest thing to over-read. It tells you the disc has to fit the cap and the neck. It does not tell you what the liner has to do.

Four inputs, not one.

• The container and closure. The neck finish, the cap material and the cap height decide which structures can seat and seal at all.
• The contents. Oil, alcohol, acid and aroma decide whether a foam, a foil or a barrier is even a candidate.
• The sealing method. Induction, pressure-sensitive or a physical foam seal — and the equipment the line already has.
• The opening and supply format. Peel or tab behaviour, and whether the line feeds a disc or a roll.

What to send instead of a diameter.

Send the bottle or a neck drawing, the cap, the contents and the current seal. A photo with a ruler is a useful start; it is not a substitute for the drawing or the sample when the die is made.

The diameter will still be in the specification. It is just not the whole specification.