How to Choose a Vial Cap Size: Matching 13, 20, 28, 32, and 34 mm Seals

To choose a vial cap size, match the seal to the vial’s neck finish and stopper, not to the vial’s volume. The nominal size (13, 20, 28, 32, or 34 mm) is the neck designation, and the vial, stopper, and cap must all share it to seat and crimp correctly. So the size decision follows the vial: once the product is filled in a vial with a given neck finish, the stopper and the seal are fixed to that same size. Volume, fill line, and the market’s opening preference influence which vial neck is chosen, but the cap size itself is simply whatever neck the vial has.

This guide explains what the size number means, how the standard sizes are used, and the steps to select the right one.

Key takeaways

The size number (13, 20, 28, 32, or 34 mm) is the nominal neck finish, not the vial’s fill volume.
The vial, stopper, and cap must all be the same size; a 20 mm cap will not crimp on a 13 mm neck.
13 mm suits small vials; 20 mm is the general-purpose workhorse; 28, 32, and 34 mm are for larger-mouth and larger-volume vials.
Neck finishes are standardised under the ISO 8362 series, so a size is a defined dimension, not a brand.
Size and seal type are separate choices: pick the neck first, then flip-off, tear-off, tear-down, or pull-ring.
The cap follows the vial; if a product is already qualified, the size is effectively fixed. Browse by size on the by-size hub.

What a vial cap size actually means

A vial cap size is the nominal neck finish it fits, the standardised dimension of the vial opening, not the amount the vial holds. A “20 mm seal” fits a vial with a 20 mm neck finish and the matching 20 mm stopper.

This distinction is the single most common source of confusion. The same neck size can appear on vials of very different capacities: a 20 mm neck might sit on a 2 mL vial or a 10 mL vial. The neck, stopper, and cap stay the same across those; only the body and volume change. So specifying a cap size specifies the neck family, and the fill volume is a separate dimension chosen with the vial. How neck finishes are defined is covered in vial neck types under ISO 8362, and the base standard is ISO 8362-1.

The standard sizes and where they are used

The common vial seal sizes are 13, 20, 28, 32, and 34 mm, running from small vials up to large-mouth and larger-volume vials. Each is a defined neck finish with its own matched stoppers and seals.

Size	Typical use
13 mm	Small-volume injectable vials; a dominant size in ophthalmic and low-fill formats
20 mm	The general-purpose workhorse across injectables and many vaccines
28 mm	Larger-mouth vials, including some lyophilised and multi-dose formats
32 mm	Large-mouth, larger-volume vials
34 mm	The largest common finish, for large-volume vials

Larger seals cover a bigger opening and generally need more force to crimp and to open; the opening force rises with the size and the number of bridges in the flip feature. The size and the opening behaviour are related, but the size is still set by the vial neck, not chosen for opening force.

Size and seal type are separate choices

Choosing the size and choosing the seal type are two independent decisions: the size matches the vial neck, and the type sets how the seal opens. A single size is available in several seal types.

Size answers “which neck finish?” (13, 20, 28, 32, or 34 mm), and must match the vial and stopper.
Type answers “how does it open?” (flip-off, tear-off, tear-down, or pull-ring), and is chosen for the product and market.

So a 20 mm vial can take a 20 mm flip-off seal or a 20 mm tear-off seal; the size is the same, the opening action differs. Get the size right first, from the vial, then select the type. What a flip-off seal is and how it differs from other types is covered in what is a flip-off seal.

How to choose the right size, step by step

Choose the size that matches the vial neck the product is filled in; the cap follows the vial and stopper, never the other way around. In practice, work in this order:

Start from the vial. Its neck finish (13, 20, 28, 32, or 34 mm) sets the cap and stopper size. If the vial is not yet chosen, the neck follows the fill volume, format, and filling line.
Match all three components. The vial, stopper, and cap must share the same nominal size to seat and crimp correctly; mixing sizes leaves a leak path. See pairing the rubber stopper and aluminium seal.
Confirm the neck finish against the standard. Neck dimensions are defined in the ISO 8362 series, so the size is a specific finish, not just a diameter.
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, because changing it triggers re-validation.

The whole exercise reduces to one question: which neck does the vial have? Answer that, and the cap size follows.

How this works in practice at Autofits

Autofits manufactures FlipTop Optima flip-off seals in all five common sizes, 13, 20, 28, 32, and 34 mm, so a customer can match the cap to the vial neck the product uses. Each size can be browsed directly: 13 mm, 20 mm, 28 mm, 32 mm, and 34 mm, or through the by-size hub. The flip feature is tuned per size (for example the 13 and 20 mm seals use six tapered bridges), and the opening force is set accordingly, rising from about 30 N at 13 mm to 35 N at 20 and 28 mm and 65 N at 32 mm. Every seal is made to drawing-controlled dimensions and checked by 100% high-speed camera inspection, under an ISO 15378:2017 quality system in a 75,000 sq ft Nashik facility with an ISO Class 8 cleanroom, so each size crimps cleanly over its matching stopper and vial neck.

Frequently asked questions

How do I choose the right vial cap size?

Choose the size that matches the vial neck the product is filled in. The nominal size (13, 20, 28, 32, or 34 mm) is the neck finish, and the vial, stopper, and cap must all share it. Start from the vial, match all three components to the same size, confirm the neck finish against the ISO 8362 standard, then choose the seal type. The cap follows the vial, not the reverse.

Does the vial cap size mean the vial’s volume?

No. The size number is the nominal neck finish, not the fill volume. The same neck size can appear on vials of different capacities, so a 20 mm neck might be on a small or a large vial. The cap size designates the neck, and the volume is a separate dimension chosen with the vial body.

What are the standard vial cap sizes?

The common vial seal sizes are 13, 20, 28, 32, and 34 mm. The 13 mm size suits small-volume vials, 20 mm is the general-purpose size used across injectables and many vaccines, and 28, 32, and 34 mm are used for larger-mouth and larger-volume vials. Each is a defined neck finish with its own matched stoppers and seals.

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

No. The cap, stopper, and vial must all share the same nominal neck size. A 20 mm cap will not seat or crimp correctly on a 13 mm neck, and a 13 mm cap will not fit a 20 mm neck. Mixing sizes leaves a leak path and fails to form a sound closure, so all three components must be matched.

Does a larger seal need more force to open?

Generally yes. Larger seals cover a bigger opening and their opening force tends to rise with the size and the number of bridges in the flip feature. That said, the size is still chosen to match the vial neck, not for opening force. The opening behaviour is tuned within each size rather than being the reason a size is selected.

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 8362-6:2010, Injection containers and accessories, Part 6: Caps made of aluminium-plastics combinations for injection vials (https://www.iso.org/standard/52806.html)

*Last updated: 2026-07-06. This article is general technical information, not regulatory or compliance advice; confirm current standard editions and your own qualification requirements with the relevant authorities.*