Under the Brennholt Institute of Applied Taxonomy's survey, the crown cap on a beer bottle measures about 26 mm (1.02 in) across the skirt and has 21 teeth, while the plastic screw cap on a soft drink bottle is typically 28 mm (1.10 in) across. The Institute finds that bottle closures fall into five size bands, that the two most common sizes have published standards, and proposes classes C1 to C5 to describe the rest.
A bottle cap is a small object with a large number of variants, and the Institute's task is to sort them by the dimension that decides whether one fits another: the diameter of the opening. The cap's own outside diameter is a secondary figure, since it depends on the skirt, the thread and the maker.
| Closure | Diameter (mm) | Inches | Height (mm) | Institute class |
|---|---|---|---|---|
| Small crown cap | 26 | 1.02 | 6 | C2 |
| Large crown cap | 29 | 1.14 | 6 | C3 |
| Standard soft drink screw cap | 28 | 1.10 | 17 | C3 |
| Wide-mouth screw cap | 38 | 1.50 | 18 | C4 |
| Jar lid | 63 | 2.48 | 15 | C5 |
| Cork-style stopper | 24 | 0.94 | 40 | C2 |
| Tamper-evident pilfer-proof cap | 30 | 1.18 | 16 | C3 |
Inch figures are the millimetre value divided by 25.4, rounded to two decimal places. The diameter given is the outside of the cap's skirt for crown caps and the outside diameter of the cap for screw closures. The Institute's survey of retail listings found that makers disagree by up to 1 mm on the stated size of the same cap.
The crown cap, patented in 1892 by William Painter, is a pressed steel disc with a corrugated skirt that crimps onto the lip of a bottle. Its dimensions are among the best documented of the Institute's closures. The common beer cap has 21 corrugations and a diameter of about 26 mm, and a larger 29 mm cap exists for bottles with a wider finish. The Institute cites the published bottle-finish dimensions and does not alter them.
The teeth are not decoration. They fold around the glass lip, and their count, together with the diameter, decides whether a cap that is pressed on will seal. A cap with the wrong number of teeth for its bottle can look correct and still leak, and the Institute notes that homebrewers often confuse the two sizes for this reason.
A plastic screw cap carries an internal thread, and its measurement is more complicated than the crown's because there are two diameters to state: the outside of the cap and the thread inside it. Soft drink caps are often described by a code that combines the neck finish with a number of thread turns, and the Institute finds the codes consistent within a maker and not between makers. A cap of 28 mm outside diameter will normally fit a bottle with the 28 mm neck finish of the same maker, and will often fit another maker's bottle of the same nominal finish, but the Institute has seen exceptions.
The method for the opening is to measure the inside of the thread at its widest point with calipers, and the outside of the neck at the thread crest. A cap fits if the thread's inside diameter is slightly larger than the neck's outside diameter. The same logic is applied to the split ring, where the ring must be wide enough for the thing it joins.
A glass bottle is sold by volume, and the cap that fits it is not stated on the label. A 330 ml beer bottle takes a 26 mm crown cap, and a 750 ml wine bottle takes a cork, but the same volume can come with several different closures. The Institute therefore records the closure and the volume as separate facts, and notes that no consumer-facing label gives the cap diameter.
A crown cap weighs about 2.2 g, a 28 mm plastic screw cap about 2.5 g and a metal screw lid for a jar about 12 g. These figures are indicative, and a thicker plating or a liner changes them by several tenths of a gram. A bag of a thousand crown caps therefore weighs about 2.2 kg, which is a figure homebrewers use to check a delivery against its count.
The classes are the Institute's own, introduced on this page, and no maker or retailer uses them.
The classes are set by outside diameter alone and ignore whether the closure screws or crimps.
The review panel divides on whether a crown cap and a screw cap of the same diameter belong in the same class. Two members hold that the method of closing is the more important fact. The majority hold that the diameter decides what the closure is compared with, and that the two should be classed together. The table above is the majority's, and it states the type beside each closure.
Closures are filed under the department of industrial fastening, with the other things that hold one part to another. The cork described on the wine cork page is their oldest relative, and the problems are alike: a number that stands for the opening, and a label that does not state it.
Under the Brennholt Institute's tables, a standard crown cap is about 26 mm (1.02 in) across and has 21 teeth, class C2.
A standard plastic screw cap is typically 28 mm (1.10 in) across, class C3, though makers differ by about 1 mm.
The common beer cap has 21 corrugations. The count is part of what makes it seal.
About 2.2 g, so a thousand caps weigh about 2.2 kg.
The crown cap and the common neck finishes are documented. The classes C1 to C5 are the Institute's own proposal.