How to Select the Right Size Cap for your Bottle
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.
Before you select a cap, you need to know the correct size. Cap sizes are not interchangeable – a 28-400 cap will not fit properly on a bottle with a 28-410 neck finish. Cap sizes and the neck finish on a bottle have a unique system of measurements.
How are closures sizes defined?
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.
How do I measure a bottle and/or bottle cap accurately?
The Qorpak Cap and Bottle Measurer makes taking those crucial measurements easy. Call us at 800.922.7558 to get your FREE cap measurer.
Click here to download a printable copy of the Qorpak Cap Measurer. If printed on standard 8.5 x 11" paper, cap measurer will print to actual size.
You can also email us at firstname.lastname@example.org and let us know where you would like us to send this sturdy cap measurer.
Once you have your Qorpak Cap Measurer, follow these easy steps:
To measure a cap diameter, measure the cap from one side of the inner wall of the cap to the opposite inner wall. Simply place the cap on the lip of the left side of the Qorpak Cap Measurer with the notch flush against the cap inner wall. Read your measurement at the opposite cap inner wall. Measurements are shown in millimeters. This is also commonly called the "T" dimension.
To measure a bottle neck opening diameter, measure the diameter of the outermost threads by placing the opening of the bottle firmly against the notch on the right side of the cap measurer. Read to the outer edge of the opening on the opposite side of the neck opening.
To determine the thread finish, verify how many times the threads pass one another to determine the finish. For example, a bottle 1.5 thread turns has a 410 neck finish. This is also called the "H" dimension. Refer to the Thread Finish Guide on the back of the cap measurer or use the following chart to view common thread finishes.
Common Thread Finishes
See also our Cap Material & Liner Descriptions to ensure the integrity of your contents.