How to measure bottle openings

Everyone from consumers to scientists know that the cap on your bottle has to fit correctly to prevent leakage, evaporation and potentially dangerous spills.

What many don’t know is how to make sure the cap is the right size for the bottle.

Round plastic containers can be measured and assessed in a variety of ways. Height, diameter, circumference.

One of the most common questions is how to determine the neck finish of round plastic containers and closures. It’s a valid and important question since each component must have matching finishes to be functional.

Screw thread closure sizes are expressed in two numbers:

- The first number refers to the diameter of the closure or bottle opening (in mm) and refers to the nominal diameter measured across the inside of the cap at the opening or the outer diameter of the bottle.

- The second number represents the GPI thread finish and refers to the height of the cap and the style of threads on the cap and the container. GPI refers to the Glass Packaging Institute which standardized neck finishes many years ago for both glass and plastic containers.

As a gift to the universe, a group of packaging experts specializing in glass (GPI, Glass Packaging Institute) and plastic (SPI, The Society of Plastics Industry) came together to craft the finish guidelines for round plastic containers and closures, along with glass containers to ensure uniformity and functionality between different types of packaging.

A measurement example:

A closure has a 28–410 neck finish.

28–410 may seem like an encrypted code, but the measurement system is far less complicated than it appears.

The 28 means that the diameter of the closure (measured from inside to inside for closures, outside to outside for containers) is 28millimeters (mm).

The second number, 410 refers to the closure’s height and thread count.