A barrel or cask is a hollow cylindrical container, traditionally made of wood staves and bound with iron hoops. The term "barrel" typically refers to wooden vessels that are small enough to be moved by hand, up to puncheon size (see below). Someone who makes such (wooden, iron-bound) barrels is known as a cooper. Contemporary barrels are also made of aluminium and plastic.
For nearly 2,000 years barrels were the most convenient form of shipping or storage container for those who could afford the superior price. All kinds of bulk goods, from nails to gold coins, were stored in them. Bags and most crates were cheaper, but they were not as sturdy and they were more difficult to manhandle for the same weight. Barrels slowly lost their importance in the 20th century, with the introduction of pallet-based logistics and containerization.
In the mid 20th century, 55-gallon steel drums began to be used for the storage and transport of fluids such as water, oils and hazardous waste. Empty drums occasionally became musical instruments in a steel pan band.
Water barrels are often used to collect the rainwater from dwellings (so that it may be used for irrigation or other purposes). This usage, known as rainwater harvesting requires (besides a large rainwater barrel), an adequate (water-proof) roof-covering and an adequate rain pipe.
The standard barrel of crude oil or other petroleum product (abbreviated bbl) is 42 US gallons (34.972 Imperial gallons or 158.987 L). This measurement originated in the early Pennsylvania oil fields, and permitted both British and American merchants to refer to the same unit, based on the old English wine measure, the tierce.
Earlier, another size of whiskey barrel was the most common size; this was the barrel for proof spirits, which was of the same volume as 5 US bushels. However, by 1866 the oil barrel was standardized at 42 US gallons.
Oil has not actually been shipped in barrels since the introduction of oil tankers, but the 42-US-gallon size is still used as a unit for measurement, pricing, and in tax and regulatory codes. Each barrel is refined into about 25 gallons of gasoline, the rest becoming other products such as jet fuel and heating oil, using fractional distillation.
The current standard volume for barrels for chemicals and food is .
Some wine is fermented "in barrel," as opposed to a neutral container such as a steel or concrete tank. Wine can also be fermented in large wooden tanks, often called "open-tops" because they are open to the atmosphere. Other wooden cooperage for storing wine or spirits are called "casks", and they are large (up to thousands of gallons) with either elliptical or round heads.
The English idiom over a barrel means to be in a predicament or helpless in a situation where others are in control: "I have no choice in the matter — my creditors have me over a barrel." The phrase is said to originate from two 19th century practices: rolling drowning victims over a barrel to clear their lungs of water, or flogging someone who is bent over a barrel.
The "chine hoop" is the iron hoop nearest the end of a wooden barrel, the "bilge hoops" those nearest the bulge, or centre.
The stopper used to seal the hole in a barrel is called the bung.
In the United States, the term "keg" commonly means a 'half barrel' size container.