Sankalp38-400 is a GPI neck finish code. The first number, 38, is the diameter across the neck threads in millimetres. The second, 400, is the finish style: a continuous thread making about one full turn, not a measurement. A cap and a bottle match only when both numbers match, so a 38-400 cap fits a 38-400 neck and nothing else.
The two numbers in a neck finish code
Neck finish codes come from the GPI and SPI standards, the numbering the Glass Packaging Institute set for glass and the plastics industry adopted for plastic bottles. They are written as two numbers joined by a dash or a slash, such as 38-400, 28/410 or 24-415.
- The first number is the T dimension: the outside diameter across the threads, in millimetres, rounded to a nominal size. In 38-400 that is 38 mm.
- The second number is the finish style, a three- or four-digit reference to a standard thread and finish height. It is not a millimetre value.
So 38-400 means a 38 mm neck with a 400 finish. A cap marked 38-400 fits any bottle whose neck is made to the 38-400 drawing, whoever moulded it. Plastic screw caps are ordered against these codes, one of the practical differences covered in ROPP cap vs screw cap.
The T dimension is not the neck body. The diameter of the neck at the root of the thread is the E dimension, a few millimetres smaller, and the height of the finish is H. That is why a caliper across a 28 mm finish reads a little under 28.
What the finish number means
The finish number describes how far the thread runs and, with it, how tall the finish is. The common codes sit close together and are easy to confuse.
| 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 |
| 2000 series | Lug segments, not a continuous thread | Lug finish for a twist-off lug cap, as in 38-2000 |
The same diameter comes in different finishes. A 38-400 and a 38-410 are both 38 mm across the threads, but the thread length and finish height differ, so a cap made for one will not seat correctly on the other. Always match both numbers.
Common codes decoded into millimetres
GPI tabulates the dimensions in inches. Converted to millimetres, the 410 finishes in the small sizes look like this.
| Code | Thread diameter, T | Neck at thread root, E | Finish height, H | Threads per inch |
|---|---|---|---|---|
| 18-410 | 17.37 to 17.88 mm | 15.24 to 15.75 mm | About 13.0 mm | 8 |
| 20-410 | 19.38 to 19.89 mm | 17.25 to 17.75 mm | About 13.8 mm | 8 |
| 22-410 | 21.39 to 21.90 mm | 19.25 to 19.76 mm | About 14.6 mm | On the drawing |
| 24-410 | 23.37 to 23.88 mm | 21.23 to 21.74 mm | About 16.2 mm | 8 |
| 28-410 | 27.00 to 27.64 mm | 24.61 to 25.25 mm | About 17.7 mm | 6 |
A 400 or 415 of the same diameter shares the T and E bands and differs in the height and the length of the thread. For larger codes such as 38-400, ask the bottle maker for the drawing with T, E and H on it: the code tells you the family, and the drawing gives the tolerances.
Is a GPI or SPI finish the same as a ROPP finish?
No. A GPI or SPI code describes a moulded thread for a cap that arrives already threaded, while a ROPP finish is drawn for an aluminium shell that is threaded by rollers at capping and carries a locking ring for the pilfer band, even where the two measure almost the same across the threads. The full comparison, with the dimensions at 28 mm, is in GPI and SPI finishes against ROPP finishes.
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 | Thread diameter in mm | Finish style code |
| ROPP cap size | 28/15 | Diameter in mm | Cap height in mm |
| BVS wine and spirits finish | 30 H 60 | Diameter in mm | Height of the closure it takes, in mm |
| PET finish in CETIE notation | 26/22 | Thread diameter in mm | A second diameter, followed by height, starts and pitch |
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.
Which closure fits which code
Each way of writing a finish belongs to a different closure, and the code on the bottle drawing tells you which kind of cap to buy before it tells you the size.
| Finish on the drawing | Closure it takes | Typical use |
|---|---|---|
| GPI or SPI 400, 410, 415 | Pre-threaded screw cap, plastic or metal | Cosmetic, household, pharma and food bottles |
| GPI 2000 series lug | Twist-off lug cap | Glass beverage and sauce bottles |
| PCO 1810, PCO 1881, 26/22 | Moulded plastic closure | PET soft drink and water bottles |
| MCA 1, MCA 3, MCA 7.5 R, 7.5 RF; IS 7511 (Part 4) | Short aluminium ROPP closure | Glass water, soft drink and liquor bottles |
| BVS 30 H 60, 28H44, 31,5H60 | Long skirt aluminium closure | Still wine, liqueurs and some spirits |
The roll-on glass finishes and their data sheet numbers are listed in glass bottle neck finishes for ROPP caps, and the Indian standards behind them in neck finish standards on Indian glass bottles.
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 thread diameter in millimetres. Then identify the finish: count the thread turns, look for a locking ring under the thread, 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 and depth readings and what size they give. 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 across the threads with a 400 GPI finish. The first number, 38, is the thread diameter in millimetres. The second number, 400, is a standard finish code for a continuous thread of about one turn, 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 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 share a thread diameter but differ in thread length and finish height, 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.
Is a 34/400 neck finish 38 mm?
No. The first number is the diameter, so a 34/400 finish would be 34 mm and a 38/400 finish is 38 mm. The 400 is the same thread style in both. 33 mm and 38 mm are the sizes closures are commonly stocked in, so confirm a 34 against the bottle drawing before ordering.
What are the dimensions of a 28-410 neck finish in mm?
GPI gives a 28-410 finish a thread diameter of about 27.00 to 27.64 mm, a neck diameter at the thread root of about 24.61 to 25.25 mm and a finish height of about 17.7 mm, cut at 6 threads per inch. Confirm the exact tolerances on the bottle maker’s drawing.
Sources
- Glass Packaging Institute, gpi.org, for the GPI finish code system.
- Evergreen Resources, glass neck finish dimensions, for the GPI 410 T, E and H values, converted here from inches.
- TricorBraun, long neck beverage bottle, 38-2000 lug neck finish, for 38-2000 as a lug finish.
- CETIE, Finish data sheets (GME), for PET finish notation and the MCA and two-start pilferproof sheets.
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.