A fluid ounce is a unit of volume typically used for measuring liquids. It is equivalent to approximately 30 millilitres. Whilst various definitions have been used throughout history, two remain in common use: the imperial and the United States customary fluid ounce. An imperial fluid ounce is 1⁄20 of an imperial pint, 1⁄160 of an imperial gallon or approximately 28.4 ml. A US fluid ounce is 1⁄16 of a US fluid pint, 1⁄128 of a US liquid gallon or approximately 29.6 ml. The fluid ounce is distinct from the ounce, a unit of weight or mass, although they do have a historical relationship, and it is sometimes referred to simply as an “ounce” where context makes the meaning clear.
The fluid ounce was originally the volume occupied by one ounce of some substance, such as wine (in England) or water (in Scotland). The ounce in question varied depending on the system of fluid measure, such as that used for wine versus ale. Various ounces were used over the centuries, including the Tower ounce, troy ounce, avoirdupois ounce, and various ounces used in international trade, such as Paris troy.
In 1824, the British Parliament defined the imperial gallon as the volume of ten pounds of water. The gallon was divided into four quarts, the quart into two pints, the pint into four gills and the gill into five ounces. Thus, there were 160 imperial fluid ounces to the gallon making the mass of a fluid ounce of water approximately one avoirdupois ounce (28.4 g). This relationship is still approximately valid even though the imperial gallon’s definition was later revised to be 4.54609 litres, making the imperial fluid ounce exactly 28.4130625 ml.
The US fluid ounce is based on the US gallon, which is based on the wine gallon of 231 cubic inches that was used in England prior to 1824. With the adoption of the international inch, the US fluid ounce became 29.5735295625 ml exactly, or about 4% larger than the imperial unit.
A litre (liter in the United States) is one of the metric units of volume. It is not an SI unit.
The metric system was first introduced in France in 1791. That system did not have its own unit of capacity or volume because volume can be measured in cubic metres. In 1793 work to make the metric system compulsory in France was started by the Temporary Commission of Republican Weights and Measures. Due to public demand, the commission said that the cubic metre was too big for everyday use. They said that a new unit based on the old cadil should be used instead. One cadil was to be 0.001 cubic metres. This was equivalent to a cube with sides 10 cm. The cadil was also known as the pinte or the litron. The pinte had been an old French unit of measure of capacity. In 1795 the definition was revised. The cadil was given the name litre.

One litre is the volume of one cubic decimetre, that is a cube of 10 x 10 x 10 centimetres. One litre of water has the mass of exactly one kilogram. This results from the definition given in 1795, where the gram was defined as the weight of one cubic centimetre of melting ice.
The symbol for litre is “L” or “l”.
For smaller volumes the decilitre is used: 10 dl = one litre.
For smaller volumes the centilitre is used: 100 cl = one litre.
For smaller volumes the millilitre is used: 1000 ml = one litre.
The capital letter “L” is preferred by some people as the small “l” can look like the number one “1”.
Most British people still use imperial units in everyday life for volume in some cases, especially pints of milk, beer, and rational fractions thereof, but rarely for canned or bottled soft drinks or petrol. Fuel consumption for vehicles is often discussed in miles per gallon, though official figures always include litres per 100 km equivalents. When sold “draught” in licensed premises, beer and cider must be measured out and sold in pints and half-pints. Cow’s milk is available in both litre- and pint-based containers in supermarkets and shops.