In the UK, a gallon of petrol contains 4.54609 litres. This conversion rate has been standard since imperial gallons were replaced by US gallons in the UK in 1963. Let’s take a closer look at the history and details behind this measurement.
Quick Summary
Here are the key points about the litres in a UK gallon of petrol:
- 1 imperial gallon = 4.54609 litres
- This conversion rate has been set since 1963 in the UK
- Prior to 1963, there were different gallon measurements used – the imperial gallon was around 4.54609 litres, while the US gallon was 3.78541 litres
- The switch was made to match international standards, since the US gallon is the most common worldwide
- The imperial gallon is still used as a measure for draught beer and cider in the UK
The History Behind Gallons and Litres in the UK
First, it helps to understand the history behind the different gallon measurements in the UK and how we ended up at the current 4.54609 litre conversion rate.
Prior to 1963, the UK used the imperial gallon for liquid measurements, alongside other imperial units like pounds and feet. The imperial gallon dates back to the 1824 Weights and Measures Act and was equivalent to 4.54609 litres.
However, the imperial system was not universally used worldwide. The US had adopted a smaller gallon measurement known as the US gallon. This was equivalent to 3.78541 litres – around 20% smaller than the imperial gallon.
By the mid-20th century, the US gallon had become the international standard. Most Commonwealth nations like Canada, Australia and New Zealand had shifted to using the US gallon as well.
To align with international conventions, the UK made the switch from imperial gallons to US gallons in 1963. This changed the official conversion rate from around 4.54609 litres per gallon to 3.78541 litres per gallon overnight.
However, the switch only applied to sales of petrol and diesel fuel. Other liquid goods like draught beer and cider still use the imperial gallon to this day.
Pre-1963: The Imperial Gallon
- The imperial gallon was used for all liquid measurements
- 1 imperial gallon = 4.54609 litres exactly
- This gallon definition traces back to an 1824 UK weights and measures standard
- The imperial system was used throughout the British Empire at the time
Post-1963: The US Gallon
- The UK legally adopted the US gallon for petrol and diesel in 1963
- 1 US gallon = 3.78541 litres
- Matched the main international standard at the time
- The imperial gallon remained for draught beer and cider sales
Why the Change Occurred
Shifting gallon definitions allowed the UK to align with most other major economies at the time. By adopting the US gallon, the goal was to simplify international trade and cooperation.
Some of the specific reasons behind the switch include:
- Harmonisation – Matching the primary international standard made trade and commerce easier
- Simplicity – Reduced the burden of operating in multiple measurement systems
- Commercial benefits – Facilitated better deals and partnerships overseas
- Industry shift – Most UK vehicle companies already produced to US gallon standards
The imperial system still had merits, but economic factors made alignment with the US gallons a priority. The UK had also recently declined to adopt the metric system in 1965, showing a preference to maintain imperial measures.
How the Changeover Occurred
The process of switching from imperial gallons to US gallons began decades before 1963 but ramped up through the 1950s and early 1960s.
Early 20th Century
The US gallon gained traction internationally in the early 1900s. It became the dominant gallon measurement in North America after widespread adoption in Canada and Mexico.
Most oil companies in the UK switched their equipment and operations to be compatible with US gallons during this period.
World War II Era
Wartime cooperation with the US exposed more British industries to American engineering standards.
After WWII, NATO alliance building led to greater alignment of military equipment and fuel supplies based on US liquid gallons.
Post-War Shift
In the 1950s, the UK government conducted studies on fully converting to US gallons across sectors.
A voluntary switch was initiated in 1959 before compulsory legal metrology changes were implemented in 1963.
The 1973 entry into the EU common market reinforced the use of international standards like the US gallon in Britain.
Impact on Consumers
For most drivers, the switch from imperial gallons to US gallons at the pumps was largely invisible. Fuel continued to be sold by the gallon so there was no need to modify vehicles or equipment.
The only noticeable change was a reduction in fuel economy of around 20%. Where a car may have achieved 25 miles per imperial gallon before, it would now effectively achieve 21 miles per US gallon.
This fuel economy drop reflected the smaller volume of petrol being sold in each US gallon compared to the old imperial measurement.
Aside from this, most consumers could essentially continue on as normal despite the behind-the-scenes changes.
Ongoing Use of Imperial Gallons
Although petrol and diesel switched to US gallons in 1963, some liquid goods continued to be sold in imperial gallons in the UK:
- Draught beer and cider – Still sold in imperial pints and gallons to this day
- Milk – Gradually switched to litres and metric measurements during the 1970s and 1980s
- Spirits – Largely converted to metric, but some niche spirits still use imperial fluid ounces
Imperial gallons remain an official legal measurement in the UK, although nearly all goods now use metric units like litres and ml instead.
UK Gallons vs US Gallons
To summarize, there are two different gallon definitions relevant in the UK context:
Imperial Gallon
- Equivalent to 4.54609 litres
- Used in the UK prior to 1963
- Remains for some limited cases like draught beer and cider
US gallon
- Equivalent to 3.78541 litres
- Adopted as standard for petrol and diesel in 1963
- Matches most international applications
Choosing one universal gallon measurement simplified commerce and trade, even though it changed the conversion into litres overnight.
Litres in a Gallon of Petrol Today
In modern times, a gallon of petrol in the UK contains exactly 3.78541 litres. This US gallon definition has been firmly in place for 60 years.
While imperial gallons are still used for some niche purposes, any gallon of general motor fuel like petrol or diesel will be based on the US liquid gallon standard.
Converting Gallons to Litres
To switch between gallon and litre values, you simply need to use the 1 US gallon = 3.78541 litres conversion rate:
- 1 gallon = 3.78541 litres
- 2 gallons = 7.57082 litres
- 3 gallons = 11.35623 litres
This makes it easy to convert fuel consumption and efficiency readings between the two measurement systems.
Typical Fuel Tank Size
As examples, here are some typical car petrol tank sizes in both gallons and litres:
| Gallons | Litres |
| 10 gallons | 37.85 litres |
| 15 gallons | 56.78 litres |
| 20 gallons | 75.71 litres |
Regardless of the measurement system, understanding your car’s fuel tank capacity helps when refilling and budgeting for petrol costs.
Why the US Gallon is Used
There are a few key reasons why the US gallon has become nearly universal for modern commerce:
- Economies of scale from standardisation
- Simplicity compared to regional gallon variants
- Vast majority of countries have adopted it
- Aligned with dominant producer (the United States)
The US gallon is now used for petrol and diesel sales in the UK, Canada, Australia, New Zealand, South Africa, Taiwan, Hong Kong, Jordan and many more countries.
The European Union uses litres but still recognizes the US gallon for cross-border trade purposes.
Should the UK Go Back to Imperial Gallons?
There have been occasional discussions around restoring imperial gallons in the UK, but no concrete plans to make this change.
Switching back from US gallons would have limited advantages compared to the drawbacks:
Potential Benefits
- Alignment with older UK customs and heritage
- Improved fuel economy ratings by around 20%
- Nostalgia and familiarity among older consumers
Drawbacks
- Would compromise international trade and legal standards
- Confuse younger generations used to metric and US gallons
- Disruptive to revert fuel pumps, vehicle gauges, etc.
- Isolated the UK from most other markets
Based on these factors, the UK is highly likely to maintain the US gallon for petrol and diesel for the foreseeable future.
Should the UK Go Fully Metric?
Another alternative would be for the UK to complete the shift to metric units by switching petrol and diesel to litres instead of gallons.
This would align with the broader European market but still has some downsides:
Potential Benefits
- Harmonise fully with the European Union
- Part of a global trend towards metrication
- Easier conversion between units
- Consistent with UK currency changeover to decimal system
Drawbacks
- Loss of cultural identity linked to imperial system
- Disruption to switch fuel pumps and vehicle displays again
- Public confusion over change in gauges and efficiency
- Isolated from Commonwealth and American markets
Overall, the practical obstacles and backlash from consumers would likely prevent a full shift to litres for fuel in the UK.
Conclusion
In summary, a gallon of petrol in the UK contains 3.78541 litres – the same as a US gallon.
This conversion has been firmly in place since 1963 when Britain switched from imperial gallons in order to align with international standards.
The vast majority of markets now use US gallons for fuel measurements. As a result, reverting to imperial gallons or switching fully to litres seems unlikely at this point.
Understanding the litre-gallon relationship allows drivers to easily convert fuel volumes and mileage readings as needed.
While the UK may retain vestiges of imperial measurements for certain goods, petrol and diesel will continue to adhere to the convenient and widely adopted US gallon standard.
