How To Match A Closure To Your Bottle Neck Finish

Why the fit matters

A closure that almost fits is a leak waiting to happen. Caps that strip, back off in transit, or fail to seal cost more in returns and lost product than the closure ever saved. The good news is that matching a cap to a bottle follows a simple, standard code.

Reading the neck finish

Most bottle necks are described with two numbers, such as 38-400.

• The first number is the diameter of the neck in millimeters, measured across the outside of the threads, known as the T dimension. A 38 mm cap fits a 38 mm neck.
• The second number is the finish style, set by industry standards from the Glass Packaging Institute (GPI) and the Society of the Plastics Industry (SPI). It describes how far the thread wraps around the neck and how tall the neck is.

Common finish styles

• 400: a short neck with roughly one full turn of thread.
• 410: a slightly taller neck with about one and a half turns.
• 415: about two full turns, on a neck that is typically taller.

Both numbers must match. Mixing finishes, such as a 415 cap on a 400 bottle, can leave the cap seated wrong and lead to leaks, even though the diameters are the same.

Beyond the threads

Fit is only half of a good seal. The liner inside the cap does the sealing work, and the right choice depends on your product. Foam liners suit many general products, while induction seals add a tamper-evident foil barrier that is heat-bonded to the bottle rim. Dispensing caps, trigger sprayers, and child-resistant closures each come in standard finishes too, so the same code applies.

Bringing it together

• Confirm the neck finish on your bottle before ordering caps.
• Match both the diameter and the finish style.
• Choose a liner that is compatible with your product.

Not sure what finish your current bottle uses? Send the Inmark team a sample or a photo, and we will identify the finish and match it to the right closure.