© gothick_matt (Flickr) Gallons

© gothick_matt (Flickr)

History of a **gallon**

A **gallon** is a measure of volume, equating to either 3.79L or 4.55L depending on its usage.

The **imperial (UK) gallon** is equal to 4.546 L. It was made obsolete through The Units of Measurement Regulations 1995 (EU Directive 80/181/EEC). This effectively outlawed imperial measurements, replacing them with metric measurements.

A **US liquid gallon** equates to 3.785 L and is accepted as the definition of a **gallon** in the US and other countries such as Colombia, Ecuador, Haiti and Peru.

The **gallon** can be divided into eight pints or four quarts. It can also be further divided into gills. With the imperial measurements being non decimal it was rather difficult to convert between different units (eg. from pints to gallons). In the twentieth century the metric system was introduced with the aim of producing an internationally acceptable unit of measurement.

The significance of the imperial/metric system is that when posed with the question of ‘how **much** does a **gallon** of **water** **weigh**‘ it needs to be taken into consideration which type of **gallon** is being referred to; the US liquid **gallon** or the UK **gallon**.

Converting gallons using the US liquid **gallon**

Calculating the weight of **one** **gallon** of **water** first involves using the conversion factor of 8.33. This value means that **one** **gallon** of **water** will **weigh** 8.33 pounds. 10 gallons of **water** will **weigh** 83.30 pounds.

If you were to convert **one** US liquid **gallon** into metric units you would use the factor of 3.785. **One** US liquid **gallon** would equal to 3.785 L.

Converting gallons using the imperial (UK) **gallon**

The UK **gallon** has a factor of 10.022. Therefore **one** **gallon** would equal 10.022 pounds. Its metric counterpart has a factor of 4.546 and therefore **one** **gallon** would equal to 4.546 kg.

You could also convert the volume to metric units first (i.e. **gallon** to litre) and then convert to weight. Since 1 L of **water** = 1kg the factor of 4.546 would still hold true.

Weight of other liquids

**One** **gallon** of **water** will not **weigh** the same as **one** **gallon** of gas (gasoline). This is because gasoline is 0.72 times less dense compared to **water**.

Gasoline weighs 0.72 kg per L.

Using the value from above. 4.546 L (i.e. **one** imperial **gallon**) of gasoline would equal 3.27 kg The density of liquids also varies with a change in temperature.