Demijohn vs Carboy

The terminology

Two words, mostly one thing

These terms get argued about more than they deserve. In practice they describe the same category of vessel: a narrow-necked container for bulk fermentation and aging, sealed with a bung and airlock.

• Demijohn is the older British and European term. It frequently implies glass, and historically implied a wicker or plastic protective cradle. European sizes are often smaller -- 5 liter and 10 liter demijohns are common where American carboys start at 3 gallons.
• Carboy is the standard American term, and is used for both glass and PET. A “wide-mouth carboy” is a distinctly modern variant with a 4 inch opening, which arguably stops being a carboy in the traditional sense.

So if a supplier offers you a “5 gallon demijohn” and a “5 gallon glass carboy,” you are almost certainly looking at the same vessel. Buy on material, size and neck diameter rather than on the word.

The real decision

Glass or PET

Everything here reduces to timescale. Over two or three weeks -- a beer fermentation -- PET’s oxygen permeability is immaterial and its practical advantages are decisive. Over six to twelve months -- a wine or a mead -- permeability starts to matter and glass is the correct choice.

The glass warning

Do not use glass for beer

Glass carboys are still sold as the premium beer fermenter and they should not be. The reasons are practical and serious:

• Weight. A full 6 gallon glass carboy weighs around 60 pounds, with no handle and a wet, slippery surface.
• Breakage. Glass fails without warning under thermal shock or impact, and a full carboy breaking is a serious laceration risk. Milk-crate carriers exist precisely because this happens.
• Cleaning. A narrow neck means a brush and optimism. You cannot reach a krausen ring with a cloth or properly inspect the interior.

For beer, a wide-mouth PET carboy gives you the same shape at a third of the weight with no breakage risk, and the oxygen advantage of glass is irrelevant on a two-week timescale. See conical fermenter vs carboy.

The problem neither solves

Headspace

Both vessel types share the same limitation: the lid or bung is fixed at the top, so a part-full vessel has air above the liquid. For a wine or mead aging for months that is the central oxidation risk, and the standard advice -- top up, or flush with CO2 -- is a mitigation rather than a solution.

The vessel that actually solves it is a variable-capacity stainless tank, whose floating lid drops to the liquid line so headspace is zero at every volume. It is expensive, and it is the only thing that removes the problem rather than managing it.

The cheap version of the same idea: size the aging vessel to the liquid you actually have. Two smaller carboys both full beat one large one that is not.