SankalpA cap size like 38-400 is a neck finish code. The first number, 38, is the neck diameter in millimetres. The second number, 400, is the GPI finish style, which describes the thread profile and the height of the finish, not a measurement in millimetres. Together they let a cap and bottle match exactly.
The two numbers in a neck finish code
Neck finish codes come from the GPI/SPI standard used across the packaging industry. They always appear as two numbers joined by a dash, such as 38-400 or 28-410.
- The first number is the outside diameter of the neck, measured in millimetres. In 38-400 that is 38 mm.
- The second number is a finish code, a three-digit reference to a standard thread style. It is not a millimetre value.
So 38-400 means a 38 mm neck with a “400” finish. A cap marked 38-400 will fit any bottle whose neck is made to the 38-400 standard, regardless of who moulded it. Plastic screw caps are ordered against these codes, one of the practical differences covered in ROPP cap vs screw cap.
What the finish number means
The finish number describes the thread turn and the general height of the finish. The most common codes sit close together and are easy to confuse, so it helps to see them side by side.
| Finish code | Thread turns | General profile |
|---|---|---|
| 400 | About one turn | Shortest finish |
| 410 | About one and a half turns | Taller than 400 |
| 415 | About two turns | Taller still |
The key point: the same diameter can come in different finishes. A 38-400 and a 38-410 are both 38 mm across, but their threads differ, so a cap made for one will not seat correctly on the other. Always match both numbers.
How this differs from ROPP notation
ROPP caps use a different notation. Instead of a diameter and a GPI finish code, a ROPP cap is described by diameter and height, both in millimetres, such as 28/15. What a ROPP cap is explains why: the thread is formed onto the neck at capping rather than moulded in advance. Here 28 is the neck diameter and 15 is the cap height. The next diameter up is covered in 30mm ROPP caps, the common spirits finish. Whichever finish you run, the capping head’s rollers have to be set to that thread profile, which the ROPP capping machine guide explains.
| Notation | Example | First number | Second number |
|---|---|---|---|
| GPI neck finish | 38-400 | Diameter in mm | Finish style code |
| ROPP cap size | 28/15 | Diameter in mm | Cap height in mm |
This is why ROPP sizing looks similar but reads differently. In 28/15 the second number is a real millimetre measurement of height, while in 38-400 the second number is a coded finish style. R Vision’s Aqua Cap and Bev Cap are 28/15 mm aluminium ROPP closures, so they follow the diameter-and-height convention, not the GPI code convention. That size has a page of its own, 28mm ROPP caps.
How to read and match a code
To match a closure to a bottle, work through both numbers in order. First measure or confirm the neck diameter in millimetres. Then identify the finish: count the thread turns or read the code from the bottle’s technical drawing. Only when both match is the fit correct.
If you are measuring a bottle by hand, how to measure bottle cap size walks through the diameter step. For a quick reference across ROPP diameters and heights, the ROPP cap size chart is the fastest way to pin down a match. When in doubt, ask the bottle supplier for the exact neck finish code rather than guessing from a cap.
Frequently asked questions
What does 38-400 cap size mean?
It means a cap for a neck that is 38 mm in diameter with a “400” GPI finish. The first number, 38, is the diameter in millimetres. The second number, 400, is a standard finish code describing the thread style and height, not a measurement in millimetres.
What does the second number in a cap size mean?
In a GPI neck finish code like 38-400, the second number is a finish style code that describes the thread profile and finish height. Common values are 400, 410, and 415, which differ mainly in how many turns the thread makes. It is a coded reference, not a millimetre measurement.
Are 400 and 410 caps interchangeable?
No. A 400 and a 410 finish of the same diameter have different thread profiles and heights, so a cap made for one will not seat correctly on the other. You must match both the diameter and the finish code. Always confirm both numbers before ordering.
How is ROPP cap sizing different from neck finish codes?
ROPP caps are sized by diameter and height in millimetres, written like 28/15, where 28 is the neck diameter and 15 is the cap height. GPI neck finish codes like 38-400 pair a diameter in millimetres with a coded finish style. The second number means a real height in ROPP but a coded finish in GPI.
More sizing guides are indexed on the learn page. Explore R Vision’s 28/15 mm range on the products page, or contact us to confirm the right size for your bottle.