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, gives the controlled dimensions, opening force and pack quantity for every size in the range, and sets out 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 ISO 8362-1, so a size is a defined dimension, not a brand.
- Total seal height runs from 7.62–8.38 mm at 13 mm to 15.00–16.00 mm at 34 mm, and aluminium gauge from 0.16–0.20 mm to 0.20–0.25 mm.
- Maximum opening force is 30 N at 13 mm, 35 N at 20 and 28 mm, and 65 N at 32 mm, set by the number of bridges.
- Standard packing falls with size: 15,000 per box at 13 mm down to 1,400 at 32 mm, all in a 420 × 270 × 320 mm shipper.
- Size and seal type are separate choices: pick the neck first, then flip-off, tear-off, tear-down, or pull-ring. 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. The two commonest finishes are weighed against each other in 13 mm versus 20 mm vial caps. How neck finishes are defined is covered in vial neck types under ISO 8362, and the base standard is ISO 8362-1.
The selection logic in one line
The seal size matches the vial neck finish, the vial neck finish follows ISO 8362-1, and the stopper carries the same nominal diameter, so all three components are locked to one number. Everything else in this guide follows from that chain.
Read it in order. ISO 8362-1 dimensions the injection vial made of glass tubing, including the neck finish, so a 20 mm vial has a defined mouth diameter and flange profile rather than an approximate one. The stopper is made to sit in that mouth, which is why it carries the same nominal diameter. The seal is made to pass over the seated stopper and roll its aluminium skirt under that flange, which is why it carries the same number again. Break the chain at any point and the crimp either does not form or does not hold: an undersized skirt cannot reach the flange, an oversized one folds instead of rolling.
Two consequences matter commercially. First, the cap is never the component that sets the size; if a product is already qualified on a vial, the size is already decided. Second, comparing seals across suppliers is a comparison within a size, on gauge, tolerance and finish, not across sizes.
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, and the five together are the whole range.
| Size | Typical use | Product index |
|---|---|---|
| 13 mm | Small-volume injectable vials; a dominant size in ophthalmic and low-fill formats | 13 mm seals |
| 20 mm | The general-purpose workhorse across injectables and many vaccines | 20 mm seals |
| 28 mm | Larger-mouth vials, including some lyophilised and multi-dose formats | 28 mm seals |
| 32 mm | Large-mouth, larger-volume vials, including veterinary presentations | 32 mm seals |
| 34 mm | The largest common finish, for large-volume vials | 34 mm seals |
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. All five indexes sit under the by-size hub.
Full dimensions across all five sizes
Controlled dimensions for the FlipTop Optima flip-off range across 13, 20, 28, 32 and 34 mm, from the Autofits engineering drawings. These are the numbers a filling line needs to set a capping head and to write an incoming specification.
| Size | Inner Ø aluminium seal (D1) | Outer Ø disc (D2) | Disc height | Inner aluminium height | Total seal height | Aluminium thickness | Bridges |
|---|---|---|---|---|---|---|---|
| 13 mm | 13.36 mm (min) | 14.99 mm (max) | 2.90–3.30 mm | 6.15–6.40 mm | 7.62–8.38 mm | 0.16–0.20 mm | 6 taper |
| 20 mm | 20.22 mm (min) | 23.14 mm (max) | 3.20–3.60 mm | 7.37–7.62 mm | 9.02–9.91 mm | 0.16–0.20 mm | 6 taper |
| 28 mm | 27.60–28.20 mm | 30.10–31.10 mm | On request | 8.70–9.30 mm | 11.06–12.06 mm | 0.17–0.23 mm | 8 |
| 32 mm | 32.10–32.90 mm | 35.50–36.50 mm | On request | On request | 14.30–15.50 mm | 0.21–0.25 mm | 12 taper |
| 34 mm | 33.60–34.00 mm | 35.50–36.50 mm | On request | On request | 15.00–16.00 mm | 0.20–0.25 mm | 8 taper |
Three patterns are worth reading off the table. Total seal height roughly doubles from 13 mm to 34 mm, because the skirt has to reach a flange that sits further out. Aluminium gauge steps up with size, from 0.16 to 0.20 mm at 13 and 20 mm to 0.21 to 0.25 mm at 32 mm, because a larger disc needs more metal behind it to hold the stopper down. And bridge count rises from six at 13 and 20 mm to eight at 28 mm and twelve at 32 mm, which is what sets the opening force.
Aluminium-plastics combination caps are made to ISO 8362-6; the quality system is ISO 15378:2017.
Opening force and standard packing by size
Maximum opening force and pack quantity are both set by size, and both belong in a purchase specification. Opening force varies with the number of bridges in the design.
| Size | Max opening force | Standard packing per box |
|---|---|---|
| 13 mm | 30 N | 15,000 (5,000 in each of 3 bags) |
| 20 mm | 35 N | 6,000 (2,000 in each of 2 bags) |
| 28 mm | 35 N | 3,000 (1,500 in each of 2 bags) |
| 32 mm | 65 N | 1,400 (700 in each of 2 bags) |
The standard shipper box is 420 × 270 × 320 mm at every size, which is why the pack quantity falls as the seal grows: the same box holds 15,000 at 13 mm and 1,400 at 32 mm. Tyvek bagging is available as an alternative to the standard bags. Plan warehouse space and goods-in handling from the pack quantity rather than the piece count, because a 32 mm line consumes boxes roughly ten times faster than a 13 mm line at the same output.
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, and the format-by-dose-count question is worked through in single-dose vs multi-dose vials.
Which sizes each seal family is made in
Not every family is made in every size, so the type decision is constrained by the size the vial has already fixed. Check availability before specifying.
| Seal family | Sizes made |
|---|---|
| FlipTop Optima flip-off | 13, 20, 28, 32, 34 mm |
| Tear-off, all-aluminium | 13, 20, 32 mm |
| FlipTop Tear-Down, aluminium-plastic | 13, 20, 32 mm |
| Tear-down, all-aluminium | 20 mm, other sizes on request |
| FlipTop Pull Ring | 20 mm (bridge), 32 mm (flower) |
| Aluminium pilfer-proof (ROPP) caps | Common ROPP sizes, full range available on request |
Controlled dimensions for the full-removal families, for the same cross-size comparison:
| Family and size | Inner Ø aluminium (D1) | Second Ø (disc or perforation) | Total height | Aluminium thickness | Bridges or fins |
|---|---|---|---|---|---|
| Tear-off 13 mm | 13.20–13.60 mm | On request | 6.90–7.70 mm | On request | 2 |
| Tear-off 20 mm | 20.00–20.60 mm | 9.70–10.50 mm (perforation) | 7.40–8.00 mm | 0.16–0.20 mm | 2 |
| Tear-off 32 mm | 32.40–32.60 mm | 17.80–18.20 mm (perforation) | 13.00–13.60 mm | On request | On request |
| Tear-Down 13 mm (aluminium-plastic) | 13.30–13.50 mm | 14.80–15.40 mm (disc) | 6.80–8.30 mm | 0.18–0.22 mm | Bridge |
| Tear-Down 20 mm (aluminium-plastic) | 20.00–20.60 mm | 22.00–23.00 mm (disc) | 8.80–9.80 mm (assembled) | On request | Bridge |
| Tear-Down 20 mm (all-aluminium) | 20.50–20.70 mm | 12.90–13.20 mm (perforation) | 7.50–8.50 mm | 0.18–0.22 mm | 2 taper |
| Tear-Down 32 mm (aluminium-plastic, flower) | 32.40–33.15 mm | 35.70–36.70 mm (disc) | 14.30–15.30 mm | 0.20–0.26 mm | 8 fins |
| Pull Ring 20 mm | 20.00–20.60 mm | 22.00–23.00 mm (disc) | 9.00–10.00 mm | 0.18–0.22 mm | 6 taper bridges |
| Pull Ring 32 mm | 32.15–32.40 mm | 36.20–36.60 mm (disc) | 14.30–15.30 mm | 0.20–0.26 mm | 8 fins (flower) |
All-aluminium seals relate to ISO 8362-3; the aluminium-plastic combinations follow ISO 8362-6.
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.
- Check the family is made in that size. Flip-off covers all five sizes; tear-off, tear-down and pull-ring cover a narrower set, as the availability table above shows.
- 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.
- Write the numbers into the specification. Total seal height, aluminium gauge, bridge count, maximum opening force and pack quantity all change with size, and all belong in the purchase specification and the incoming-goods check.
- 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, and the crimp window has to be re-established against container closure integrity data.
The whole exercise reduces to one question: which neck does the vial have? Answer that, and the cap size follows.
Forms of supply, packing and shelf life
Every size ships in the same three forms. Regular is the non-sterile form, for lines that wash and sterilise incoming components in house. Ready-to-Use seals are washed to remove particles, assembled in an ISO Class 8 cleanroom, double-lacquered, gamma or ETO sterilized and supplied double-bagged, so they can be passed straight into an aseptic area. Ready-to-Sterilize seals arrive clean and bagged for the customer’s own validated cycle. Shelf life is 3.5 years before capping for non-irradiated seals, plus 5 years after capping, and 2 years before capping for Ready-to-Use.
Standard packing follows the table above, in a 420 × 270 × 320 mm shipper, with Tyvek bagging available. Colours are supplied in a wide range, matte or glossy, customised to requirement, which is how strengths are differentiated on a shared neck size; the seal colour coding guide covers the practice. To specify a size, send the vial neck finish, the stopper reference, the nominal size, the seal family and the form of supply required through the contact page, or browse the full product range, the by-size hub and the by-colour hub.
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, from six tapered bridges at 13 and 20 mm to eight at 28 mm and twelve at 32 mm, and the opening force is set accordingly, rising from 30 N at 13 mm to 35 N at 20 and 28 mm and 65 N at 32 mm. Top designs are Bridge, Button (intrinsic tamper evidence, with a 12-stem flower under the button disc) and Flower, in glossy, matte, embossed or printed finishes. The shell is AA8011 aluminium (EN 15088:2005, tested to BS EN 485-2:2008) with an epoxy lacquer to 21 CFR 175.300, and the disc is polypropylene to 21 CFR 177.1520, moulded in house on Japanese Toyo injection machines. Every seal is made to drawing-controlled dimensions and checked by 100% high-speed camera inspection, with AQL at critical nil, major not more than 2.5% and minor not more than 4%, 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 ISO 8362-1, check the seal family is made in that size, 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, and those five are the whole range.
What is the total seal height at each size?
For the flip-off range, total seal height is 7.62 to 8.38 mm at 13 mm, 9.02 to 9.91 mm at 20 mm, 11.06 to 12.06 mm at 28 mm, 14.30 to 15.50 mm at 32 mm, and 15.00 to 16.00 mm at 34 mm. Aluminium thickness runs from 0.16 to 0.20 mm at the two smallest sizes up to 0.20 to 0.25 mm at 34 mm.
How many seals are in a box at each size?
Standard packing is 15,000 per box at 13 mm, supplied as 5,000 in each of three bags; 6,000 at 20 mm as 2,000 in each of two bags; 3,000 at 28 mm as 1,500 in each of two bags; and 1,400 at 32 mm as 700 in each of two bags. The shipper box measures 420 by 270 by 320 mm at every size. Tyvek bagging is available as an alternative.
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. Maximum opening force is 30 N at 13 mm, 35 N at 20 mm and 28 mm, and 65 N at 32 mm, and it varies with 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.
Related reading
- Browse seals by size
- 13 mm seals and 20 mm seals
- 28 mm, 32 mm and 34 mm seals
- FlipTop Optima flip-off seals
- Vial neck types under ISO 8362
- ISO 8362-1: injection vials made of glass tubing
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)
- ISO: ISO 8362-3, Injection containers and accessories, Part 3: Aluminium caps for injection vials (https://www.iso.org/)
- Autofits FlipTop Seals dossier and engineering drawings (working/spec-sheets/)
*Last updated: 2026-08-18. This article is general technical information, not regulatory or compliance advice; confirm current standard editions and your own qualification requirements with the relevant authorities.*
