A standard natural wine cork measures about 24 mm in diameter, which is 0.94 in, and about 44 mm in length, which is 1.73 in, before it is driven into the bottle. Under the Brennholt Institute of Applied Taxonomy's survey of supplier listings, the stopper is made in five recurring combinations of diameter and length, and the Institute proposes to name them C1 to C5.
The figures are nominal. A natural cork is a piece of bark, and a technical or agglomerated stopper is a moulded one, and the Institute found variation of about 0.5 mm in diameter and 1 mm in length within a single lot. It reports the value most often repeated and gives the range where it is known.
Suppliers state diameter and length in millimetres, and the retail trade for home winemaking in inches. The Institute converted every figure.
| Type | Diameter (mm) | Length (mm) | Inches (diameter by length) | Institute class |
|---|---|---|---|---|
| Short, for a quick-drinking wine | about 22 to 24 | about 38 | 0.87 to 0.94 by 1.50 | C1 |
| Standard | about 24 | about 44 | 0.94 by 1.73 | C2 |
| Long, for a wine kept for years | about 24 | about 49 to 54 | 0.94 by 1.93 to 2.13 | C3 |
| Sparkling wine stopper, before the cage | about 30 to 31 | about 48 to 50 | 1.18 to 1.22 by 1.89 to 1.97 | C4 |
| Fortified or spirit stopper, with a head | about 18 to 20 | about 35 to 45 | 0.71 to 0.79 by 1.38 to 1.77 | C5 |
The neck of a standard wine bottle has a bore of about 18.5 mm, which is 0.73 in, near the mouth. The cork is therefore compressed by about a quarter of its diameter when it is driven in, and it expands against the glass. The Institute records this as the reason a stopper of 24 mm, and not one of 18 mm, is the usual figure: the cork must be larger than the bore to seal. The relation is a matter of the glass makers' own bore standards, which the Institute cites and does not alter.
The Institute's own tests on sample lots found that a stopper of 24 mm recovered to within about 1 mm of its original diameter after a week out of the bottle, which confirms that the compression is elastic rather than permanent.
The longer the stopper, the longer the wine is meant to keep. This is the trade's belief and the Institute makes no claim for it. It records only that the lengths in the listings fall into three bands, below 40 mm, about 44 mm and above 48 mm, and that the supplier text for the longest band always speaks of ageing. The Institute observes that no published trial linking length to keeping time was found in the sources it consulted.
A natural stopper is punched from a single piece of bark. A technical stopper is a body of granules with a natural disc at each end. An agglomerated stopper is moulded from granules alone. A synthetic stopper is extruded from plastic. The dimensions in the table apply to all four, since all four are made to fit the same bore. The Institute records that the weight differs: a natural stopper of class C2 weighs about 3.5 g to 4.5 g, a technical stopper about 3 g, and a synthetic stopper about 2 g.
The tolerance also differs. Moulded and synthetic stoppers hold their diameter to about 0.2 mm, and natural stoppers to about 0.5 mm, according to the supplier sheets the Institute read.
The classes are the Institute's own, introduced on this page. No supplier uses them.
The cork shares its form with the dowel described on the dowel page and with the round knob on the drawer knob page, and the question of what to call a cut cylinder is set out on the naming page. The Institute's department for such objects is the Department of Length and Fit.
Standards for the bottle neck bore exist and the Institute cites them. No standard fixes the dimensions of the stopper itself, and the supplier trade works by agreement with the glass maker. The Institute records this absence on the page.
The review panel divides on whether sparkling stoppers should be a class at all. Two members hold that a stopper of 30 mm, which is driven in and then caged, is a different object from a still-wine stopper and has more in common with a bung. The majority hold that a cork is defined by its material and its fit, and that C4 stays in the scheme. The classes above are the majority's.
Under the Brennholt Institute's survey, a standard wine cork is about 24 mm in diameter and 44 mm long, which is 0.94 in by 1.73 in, and falls in class C2.
The bore is about 18.5 mm and the cork about 24 mm, so the cork is compressed by about a quarter of its diameter and presses against the glass to seal.
About 30 mm to 31 mm in diameter and 48 mm to 50 mm long before it is driven in, which the Institute places in class C4.
About 3.5 g to 4.5 g for a natural stopper of class C2, about 3 g for a technical stopper, and about 2 g for a synthetic one.
No dimensional standard for the stopper has been found by the Institute. The classes C1 to C5 are the Institute's own proposal.