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.