Gallon.

Gallon is a measurement of volume. It is around 4litres(3.78541178 litres). However, there are different types of Gallon, the UK and US, so it may be a little

Its Origin:

Before and into the 19th century there were also several other gallons in use, with varying definitions. These are summarized in the table below. During some eras, the gallon was based on an exact conversion with a linear measure cubed. Other eras, the gallon was based on a rational approximation to the volume of a cylinder that could be used as a standard container, such as a basket, barrel, or jar. Other definitions were based on the density of a commodity, occasionally water, but more often a more marketable good such as wine or oats. Given these options and the variety of cultures that have used the gallon, it is not surprising that the exact value has drifted over the centuries.

Its relation to SI units:

Different cases.

In the case of US Gallons:3.78541178 litres

In the case of UK Gallons:4.545 liters.

Its usefullness:

Easier to calculate.

Its disadvantages:

May be confusing if only 'Gallons' is stated. People may be unsure whether it is referring to the UK or US unit.