13mm vs 20mm Vial Cap: How the Two Sizes Differ

13mm and 20mm are the two most common vial cap sizes, and the difference is the neck finish they fit: a 13mm cap fits a 13mm vial neck and a 13mm stopper, while a 20mm cap fits a 20mm neck and stopper. The 13mm size suits smaller vials and the 20mm size is the broad workhorse across injectables and many vaccines. The figure is the nominal neck designation, not the vial’s volume, so the choice is driven by the vial neck a product uses, not directly by how much the vial holds. Caps, stoppers, and vials must all share the same size to assemble correctly.

This guide compares the 13mm and 20mm sizes, explains what each is typically used for, and outlines how to choose between them.

Key takeaways

13mm and 20mm refer to the nominal vial neck size, not the vial’s fill volume.
A cap, stopper, and vial must all be the same size: a 13mm cap needs a 13mm neck and stopper.
13mm is used for smaller injectable vials; 20mm is the most common general-purpose size across injectables and many vaccines.
The size is set by the vial’s neck finish, standardised under the ISO 8362 series.
Both sizes come in the same seal types (flip-off, tear-off, tear-down, pull-ring); the size and the type are separate choices.
The right size is whichever matches the vial neck the product is filled in, so the decision usually follows the vial, not the cap.

What the size actually refers to

The “13mm” or “20mm” of a vial cap is the nominal neck size it fits, the standardised finish on the vial opening, not the volume of the vial. A 20mm vial is one with a 20mm neck finish, and it takes a 20mm stopper and a 20mm cap.

This matters because the same neck size can appear on vials of different capacities. A 20mm neck might sit on a 2 mL vial or a 10 mL vial; the neck (and therefore the cap) is the same, while the body and volume differ. So when someone specifies a cap size, they are specifying the neck family, and the fill volume is a separate dimension. How neck finishes are standardised is covered in vial neck types under ISO 8362.

13mm vs 20mm: the comparison

The two sizes differ in the neck finish they fit and, broadly, in the size of vial they tend to be used on, but they work the same way and come in the same seal types. The table summarises the practical differences.

Aspect	13mm vial cap	20mm vial cap
Fits	13mm vial neck and 13mm stopper	20mm vial neck and 20mm stopper
Typical use	Smaller injectable vials	General-purpose; injectables and many vaccines
Relative prevalence	Common for small formats	The most widely used size
Seal types available	Flip-off, tear-off, tear-down	Flip-off, tear-off, tear-down, pull-ring
Sealing method	Crimped over the stopper	Crimped over the stopper

Both sizes crimp over the rubber stopper the same way and both provide tamper evidence through their seal design. The difference is purely the dimension of the neck they are built for, and the kind of vial that neck tends to be used on.

What each size is used for

13mm caps are generally used on smaller vials, while 20mm caps are the broad default across a wide range of injectables and vaccines. Neither is “better”; each matches a different vial neck.

13mm: chosen where the product is filled in a vial with a 13mm neck, often smaller-volume injectables. The smaller neck suits compact vial formats.
20mm: the most common size, used across many injectable and vaccine vials. Its prevalence means a wide selection of matching stoppers and seal types is available.

Because the cap size follows the vial neck, the decision is usually made when the vial is chosen, taking into account the fill volume, the filling line, and the product’s existing qualification. Larger vials may use bigger necks again (for example 32mm), each with its own matched caps and stoppers.

How to choose between 13mm and 20mm

Choose the cap size that matches the vial neck the product is filled in; the cap follows the vial and stopper, not the other way around. In practice:

Start from the vial. The vial’s neck finish (13mm, 20mm, or another size) determines the cap and stopper size.
Match all three. The vial, stopper, and cap must share the same nominal size to seat and crimp correctly, as explained in pairing the rubber stopper and aluminium seal.
Then choose the seal type. Within the size, select flip-off, tear-off, tear-down, or pull-ring for the opening action the product and market need.
Respect existing qualification. If a product is already qualified with a particular vial and closure, the size is effectively fixed, since changing it triggers re-validation.

So the size question is really a vial question: pick the neck the product uses, and the matching cap size follows.

How this works in practice at Autofits

Autofits manufactures vial seals in 13mm, 20mm, and 32mm sizes, so a customer can match the cap to the vial neck the product uses. The range includes FlipTop Optima flip-off seals, tear-off and tear-down aluminium seals, FlipTop Pull Ring seals, and aluminium pilfer-proof (ROPP) caps; the full set is on the products page. Each size is produced to its standard neck finish with tight dimensional control so it crimps cleanly over the matching stopper. Production runs under an ISO 15378:2017 quality system, alongside ISO 9001:2015 and ISO 14001:2015 certification and a Drug Master File, in a 75,000 sq ft Nashik facility with an ISO Class 8 cleanroom and high-speed visual inspection. The full certification set is on the quality page.

Frequently asked questions

What is the difference between a 13mm and a 20mm vial cap?

The difference is the neck size each fits. A 13mm cap fits a 13mm vial neck and a 13mm stopper; a 20mm cap fits a 20mm neck and stopper. The 13mm size is generally used on smaller vials, and the 20mm size is the most common general-purpose size across injectables and many vaccines. Both crimp over the stopper the same way.

Does the cap size mean the vial’s volume?

No. The 13mm or 20mm figure is the nominal neck size, not the vial’s fill volume. The same neck size can appear on vials of different capacities, so the cap size designates the neck finish, and the volume is specified separately. A 20mm neck might be on a small or a larger vial.

Can I use a 20mm cap on a 13mm vial?

No. The cap, stopper, and vial must all share the same nominal neck size. A 20mm cap will not fit or crimp correctly on a 13mm neck, and vice versa. To assemble a sound closure, match the vial neck, the stopper, and the cap to the same size.

Which is more common, 13mm or 20mm?

The 20mm size is the most widely used vial cap size, found across a broad range of injectable and vaccine vials, with a wide selection of matching stoppers and seal types. The 13mm size is common for smaller-format vials. Which one a product uses depends on the vial neck it is filled in.

How do I choose the right vial cap size?

Choose the size that matches the vial neck the product is filled in: the cap follows the vial and stopper. Match the vial, stopper, and cap to the same nominal size, then select the seal type (flip-off, tear-off, tear-down, or pull-ring). If the product is already qualified with a particular vial and closure, the size is effectively fixed.

Sources

ISO: ISO 8362-1:2018, Injection containers and accessories, Part 1: Injection vials made of glass tubing (https://www.iso.org/standard/74398.html)
ISO: ISO 8362-2:2024, Injection containers and accessories, Part 2: Closures for injection vials (https://www.iso.org/standard/87251.html)

*Last updated: 2026-06-18. This article is general technical information, not regulatory or compliance advice; confirm current standard editions and your own qualification requirements with the relevant authorities.*