A practical guide for packers and brands choosing plastic caps for jars and bottles — what neck finishes like 29/21 mean, how to measure your jar's cap size, and which closure fits which pack. Written by the team at Aditya Plastics, cap & closure manufacturers in Jaipur since 2014.
The neck finish is the standardised shape of a bottle or jar's threaded neck — its diameter, thread style and height. A cap and a container seal correctly only when both are made to the same finish. Finishes are written in millimetres: a "73 mm jar cap" fits a jar whose neck measures 73 mm across the outside of the threads.
In a two-number finish like 29/21, the first number is the neck's outer diameter in millimetres (measured across the threads) and the second is its inner diameter. The 29/21 CTC (continuous thread closure) finish is the Indian industry standard for edible oil PET bottles in 250 ml, 500 ml and 1-litre packs — if you pack oil in these bottles, this is almost certainly your cap.
Take the jar (not the cap) and measure the outer diameter of the neck across the threads with a ruler or vernier caliper — thread tip to thread tip, threading included. That measurement in millimetres is your size: an 83 mm jar means the neck, including threading, measures 83 mm. Match it to the nearest standard: 53, 63, 73, 83, 96 or 120 mm. If you only have a cap, measure its inner diameter across the threads instead — same result. When in doubt, courier us the jar or cap and we'll confirm the finish and send matching samples.
| Size | Typical use | Our cap |
|---|---|---|
| 29/21 | Edible oil PET bottles — 250 ml, 500 ml, 1 litre | 29/21 CTC cap |
| 38 mm | HDPE jars | 38 mm HDPE inner & outer caps |
| 46 mm | Pesticide & agrochemical bottles | 46 mm pesticide caps |
| 53 mm | Small jars — mouth fresheners, spices, balms | 53 mm knurling caps |
| 63 mm | Small-to-mid jars — toffees, dry spices | 63 mm knurling caps |
| 73 mm | Mid-size jars — confectionery, snacks, ghee | 73 mm plain & knurling caps |
| 83 mm | 1-5 kg jars — toffees, namkeen, dry fruits, edible oil | 83 mm plain & handle caps |
| 96 mm | Wide-mouth jars up to 5 kg | 96 mm plain & handle caps |
| 120 mm | Large 5-15 kg jars | 120 mm handle caps |
Once the size is fixed, the style depends on the pack: plain caps for a clean look and easy printing, knurling caps for grip, handle caps when the jar is heavy enough to be carried by its lid, and inner-plug-plus-outer-cap systems where leak resistance matters most.
29/21 is the neck finish of the bottle the cap fits: 29 is the neck's outer diameter in millimetres measured across the threads, and 21 is its inner diameter. In India, 29/21 CTC is the standard finish for edible oil PET bottles in 250 ml, 500 ml and 1-litre sizes.
Measure the outer diameter of your jar's neck across the threads, threading included, in millimetres — an 83 mm jar measures 83 mm across the neck including threads. Match it to the nearest standard size: 53, 63, 73, 83, 96 or 120 mm. Measuring an existing cap's inner diameter across its threads gives the same answer.
Jars from 1 to 5 kg commonly use 83 mm or 96 mm necks, and very large 5-15 kg jars use 120 mm — usually with a handle cap so the jar can be carried.
A plain cap has a smooth outer wall, giving a clean look and a good printing surface. A knurling cap has a ribbed, textured outer wall that improves grip for opening and closing. Both seal the same way; the choice is about appearance and handling.
Yes. Send us your jar or bottle (or its neck measurement and a photo) and we will send sample caps for fitment testing before you commit to a production run.
WhatsApp us a photo of your jar or cap with a ruler beside it — we'll identify the finish and send samples.
