One of the most common and easily avoided mistakes in pharma packaging procurement is misspecifying the cap neck size. A cap that doesn't fit correctly will fail the leak test — or worse, it will pass the lab leak test and then leak in transit or in-use. This guide explains how neck sizes work, the most common pharmaceutical standards, and how to specify correctly before ordering samples.
Understanding Neck Finish vs Cap Size
When packaging professionals say "28mm cap" they are referring to the bottle's neck finish — the outer diameter of the bottle neck at the thread, measured in millimetres. The cap is sized to match. The three measurements that define a neck finish are:
- T (thread diameter): The outer diameter of the bottle thread — this is the number we call the "size" (e.g. 28mm).
- I (inner diameter): The inner opening diameter at the top of the bottle neck.
- E (external diameter): The outer diameter of the neck below the thread.
The thread style (400, 410, 415, SP, etc.) determines how many turns are required to fully close the cap. The most common pharma thread style is 400 (continuous thread, 1.5 turns to close), but some child resistant caps use multi-start threads.
The Most Common Pharma Neck Sizes
18mm and 20mm
Used for small-volume injectable vials, nasal sprays, and ophthalmic drops. Very narrow — typically not suitable for oral liquid products where the opening must be wide enough for pouring or syringe dosing.
24mm
Common for 30ml and 60ml syrup bottles. The narrower opening means this size is typically used with oral dosing syringes rather than direct pouring.
28mm (Most Common)
The most widely used neck size for oral liquid pharmaceutical products in India and export markets. Suitable for bottle volumes from 60ml to 200ml. Our 28mm CRC-PP caps, pilfer proof caps, and oral dosing syringe adapters are all compatible with standard 28-400 and 28-410 finishes.
33mm
Used for 100ml–300ml syrup and suspension bottles where a wider pour opening is needed. Less common than 28mm and 38mm, but important for specific formulations like dry syrups.
38mm (Second Most Common)
Used for 200ml–500ml bottles. Common for antacid suspensions, vitamin syrups, and high-volume OTC products. Our 38mm range includes child resistant, pilfer proof, and plain screw caps.
45mm and Larger
Used for very large format bottles (500ml+) and wide-mouth pharmaceutical packaging for solid dosage forms. Less common in liquid pharma but important for nutraceutical and supplement markets.
How to Specify Correctly
Always specify both the neck finish diameter and the thread style when ordering caps. Example: "28mm 400-finish CRC-PP cap, natural colour." If you only have a physical bottle sample rather than engineering drawings, send us the bottle and we will measure it and confirm the correct cap size before you order.
Common mistakes to avoid:
- Ordering by bottle volume alone ("I need caps for my 100ml bottle") — bottle volumes don't map 1:1 to neck sizes.
- Mixing thread styles — a 28-400 cap does not seal correctly on a 28-410 finish without risk of cross-threading.
- Not specifying the liner requirement — some applications require a LDPE or foam liner inside the cap for additional sealing.
Order Samples First
We strongly recommend ordering evaluation samples before your bulk order — cap samples can be dispatched within 3–5 working days for standard sizes. Test the sample against your actual bottles before sign-off. Our full product range covers 18mm through 45mm in child resistant, pilfer proof, and standard closure variants.