A barrel is one of several units of volume, with dry barrels, fluid barrels (UK beer barrel, U.S. beer barrel), oil barrel, etc. The volume of some barrel units is double others, with various volumes in the range of about 100–200 litres (22–44 imp gal; 26–53 US gal).

Some products have a standard weight or volume that constitutes a barrel:

Fluid barrels vary depending on what is being measured and where. For barrels of oil, see the next section.

In the U.K. a beer barrel is 36 imperial gallons (43 US gal; 164 L). In the U.S. most fluid barrels (apart from oil) are 31.5 US gallons (26 imp gal; 119 L) (half a hogshead), but a beer barrel is 31 US gallons (26 imp gal; 117 L).

The size of beer kegs in the U.S. is based loosely on fractions of the U.S. beer barrel.

The standard oil barrel of 42 US gallons is used in the United States as a measure of crude oil and other petroleum products. Elsewhere, oil is commonly measured in cubic metres (m³) or in tonnes (t), with tonnes more often being used by European oil companies. International companies listed on American stock exchanges tend to express their oil-production volumes in barrels for global reporting purposes, and those listed on European exchanges tend to express their production in tonnes. There can be 6 to 8 barrels of oil in a ton, depending on density. For example: 256 U.S. gallons [6.1 bbl] of heavy distillate per ton, 272 gallons [6.5 bbl] of crude oil per ton, and 333 gallons [7.9 bbl] of gasoline per ton.

A barrel or cask is a hollow cylindrical container, traditionally made of vertical wooden staves and bound by wooden or metal hoops. Traditionally, the barrel was a standard size of measure referring to a set capacity or weight of a given commodity. For example, a beer barrel had originally a capacity of 36 US gallons (140 L) while an ale barrel a capacity of 32 US gallons (120 L). Wine was shipped in barrels of 31.5 US gallons (119 L). A small barrel is called a keg.

Modern wooden barrels for wine-making are either made of French common (Quercus robur) and white oak (Quercus petraea) or from American white oak (Quercus alba) and have typically these standard sizes: "Bordeaux type" 59.4 US gallons (225 L) and "Cognac type" 79.2 US gallons (300 L). Modern barrels and casks can also be made of aluminum, stainless steel, and different types of plastic, such as HDPE.

Someone who makes barrels is called a "barrel maker" or cooper. Barrels are only one type of cooperage. Other types include, but are not limited to: buckets, tubs, butter churns, hogsheads, firkins, kegs, kilderkins, tierces, rundlets, puncheons, pipes, tuns, butts, pins, and breakers.

Unit may refer to: