A gallon and litre (or US spelling liter) are units of measuring volume. There are two official symbols for litre: the Latin letter L in lower and upper case (l and L). The lower case L is often written as a cursive ℓ to avoid confusion with the number 1 in antiqua fonts. The litre appears in several versions of the metric system; although not an SI unit, it is accepted for use with the SI. The international unit of volume is the cubic metre (m3). One litre is equal to 0.001 cubic metre and is denoted as 1 cubic decimetre (dm3).
Historically, a gallon has had many different definitions, but there are three definitions in current use. These are the U.S. liquid gallon (≈ 3.8 litres) and the U.S. dry gallon (≈ 4.4 L) which are in use in the United States, and the Imperial (UK) gallon (≈ 4.5 L) which is in unofficial use within the United Kingdom.