A Firkin is an old English unit of volume. The name is derived from the Middle Dutch word vierdekijn, which means fourth, i.e. a fourth of a full-size barrel.

Nor need you mind the serial ordeal
Of being watched from forty cellar holes
As if by eye pairs out of forty firkins.
— Robert Frost, "Directive"

For beer and ale a firkin is equal to 9 Imperial gallons (or about 40,915 l), and by origin the small wooden keg or other container that holds such an amount. It is traditionally used in beer measurement to represent one quarter of a barrel. Firkin is currently an atmospheric name for an English-style pub, especially outside Great Britain, e.g. "The Fox & Firkin".

For wine the firkin had a larger size, namely a third of a tun. A tun being 210 gallons in the UK and 252 fluid gallons in the US, thus a wine firkin is about 318 l (318,226 or 317,975). It is also called tertian or, preferably, puncheon (in the US also shortened to pon).

Butter and soap used to be sold by the firkin, too. In these cases it was rather a measure of weight, 56 lb. (25.4 kg) and 64 lb. (29 kg) respectively.

See also: Firkin Brewery

Reference

• Web WordNet
• Sizes.com