The short answer is yes, UK and US gallons are different units of volume measurement. While they share the same name, there are distinctions between the imperial gallon used in the UK and the US customary gallon used in the United States.
What is a gallon?
A gallon is a unit of volume measurement used in both the imperial and US customary systems. The word gallon comes from the old French word galon, meaning a liquid container. While gallons in both systems share the same name, there are key differences between them in terms of their definitions and sizes.
Imperial gallon
The imperial gallon is a unit of volume in the imperial system, which was historically used in the British Empire and is still used in some Commonwealth countries today. It is defined as the volume occupied by 10 imperial pounds of water at a temperature of 62°F (17°C). One imperial gallon is equivalent to approximately 4.546 liters.
US gallon
The US gallon, also known as the US customary gallon, is a unit of liquid volume in the US customary system of measurement. It is defined as the volume occupied by 8 US customary pounds of water at a temperature of 62°F (17°C). One US gallon is equivalent to approximately 3.785 liters.
Key differences between imperial and US gallons
While the imperial and US gallon units share the same name, there are several key differences between them:
Size
The imperial gallon is approximately 20% larger than the US gallon. Specifically:
- 1 imperial gallon = 4.546 liters
- 1 US gallon = 3.785 liters
Definition
The imperial gallon is defined as the volume occupied by 10 imperial pounds of water. The US gallon is defined as the volume occupied by 8 US customary pounds of water.
Systems of measurement
The imperial gallon is part of the imperial system of measurement. The US gallon is part of the US customary system of measurement.
Countries of use
The imperial gallon continues to be used in the UK, Canada and some Caribbean nations. The US gallon is used in the United States and some Latin American and Caribbean countries.
Converting between imperial and US gallons
When converting between imperial and US gallons, it’s important to remember the difference in their sizes and use the appropriate conversion factor:
- 1 imperial gallon = 1.2 US gallons
- 1 US gallon = 0.832674 imperial gallons
To convert between the two units:
- Imperial gallons x 1.2 = US gallons
- US gallons x 0.832674 = Imperial gallons
For example:
- 10 imperial gallons x 1.2 = 12 US gallons
- 15 US gallons x 0.832674 = 12.5 imperial gallons
Comparing imperial and US gallons side-by-side
To summarize the key differences, here is a side-by-side comparison of imperial and US liquid gallons:
| Measure | Imperial Gallon | US Gallon |
|---|---|---|
| Volume in liters | 4.546 liters | 3.785 liters |
| Based on weight of | 10 imperial pounds of water | 8 US customary pounds of water |
| System of measurement | Imperial system | US customary system |
| Countries of use | UK, Canada, some Caribbean nations | US, some Latin American and Caribbean nations |
Uses of gallons in the UK and US
Both imperial and US gallons are commonly used for liquid measurement by consumers and industry in the countries where they are the standard units. Some specific examples of their uses include:
UK – Imperial gallons
- Fuel economy for vehicles is measured in miles per imperial gallon
- Milk and beer are often sold in imperial gallons or fractions of a gallon
- Car fuel economy is expressed in miles per imperial gallon (mpg)
- Large quantities of chemicals are measured in imperial gallons in industry
US – US gallons
- Gasoline is sold by the US gallon at filling stations
- Fuel economy for vehicles is measured in miles per US gallon
- Milk, soda, and other beverages are sold in US gallon containers
- Swimming pools and hot tubs have capacity labeled in US gallons
Why the imperial and US gallons differ
The imperial and US customary measurement systems evolved independently of each other from the early English system. So while their units share common names like gallon, inch, and pound, they diverged over time and are now defined in different ways.
There are a few key historical reasons why the imperial and US gallon units differ today:
- The imperial system standardized in 1824 while the US customary system took shape in the early 19th century. They were defined independently.
- The US customary system was heavily influenced by older English units like the wine gallon (231 cubic inches).
- The imperial gallon was based on a new 1824 British study for defining units by the mass of water.
- The US did not adopt the 1824 reforms, so its customary units retained older definitions.
Over time, the differences between the systems compounded as customary measurement evolved uniquely in the UK and US due to lack of standardization. The imperial and US gallon sizes diverged as a result of this independent evolution over 200 years ago.
Efforts to harmonize or convert units
Despite their long history of divergence, there have been some efforts to harmonize imperial and US customary units over the years:
- The British Imperial system and US customary system were both derived from the earlier English system.
- Theunta metrica proposed aligning US and imperial gallons in the early 1800s but wasn’t adopted.
- The common use of “Imperial” in both systems’ names reflects some attempts to harmonize them.
- Some legislation periodically aims to convert the US to the metric system, including imperial gallon conversion.
However, none of these efforts have led to full harmonization or conversion between imperial and US gallons. The differences persist today, though most measurements in science and trade use metric units like liters to simplify international communication.
Impact of the differences in gallons
The 20% difference between the imperial and US gallon has some notable effects in practice:
- Fuel economy figures for vehicles differ by about 20% between the UK (mpg) and US (mpg)
- Recipes may need to be adapted if using gallon sizes from different countries
- Goods like gasoline and chemicals need to be re-labeled if exported between the UK and US
- Consumers may unknowingly buy less fuel at the pump in countries with smaller gallons
However, the widespread use of metric units like liters on packaging internationally has helped reduce confusion from the differences between gallon sizes. Many consumers are also familiar with converting gallons when moving between countries.
Future outlook
Looking ahead, it seems unlikely that imperial and US gallons will be harmonized any time soon. The units are firmly entrenched in consumer usage and trade in the countries where they are standard. However, a few developments may continue to improve international standardization:
- Increasing use of metric units for trade and packaging reduces reliance on gallons.
- If the US ever converted to the metric system, US and imperial gallons would likely harmonize.
- Improved global communications makes consumers more aware of converting between gallon types when traveling.
Barring metrication in the US, the long history of the imperial and US gallons suggest they will remain distinct units for the foreseeable future. However, most industries and consumers are now well adapted to working with multiple gallon sizes thanks to the rise of global trade using liters.
Conclusion
While the UK and US gallon units share a common name, they are fundamentally different units of volume based on independent historical development of imperial and customary measurement systems. The imperial gallon continues to be 20% larger than the US gallon, which impacts any calculations involving liquid volumes between the UK, US and other countries using these units. However, the widespread adoption of metric units like liters internationally has made the differences easy to manage for most consumers and industries.
