What Is Imperial Gallon (imp gal)?
An imperial gallon is a unit for measuring liquid volume. It is bigger than the US gallon and is mainly linked with older British measurements and some uses today in a few countries.
Definition
The imperial gallon is a standard amount of space that a liquid can fill. Today it is defined using the metric system.
By the modern exact definition:
- 1 imperial gallon = 4.54609 litres
- 1 imperial gallon = 4.54609 cubic decimetres
This means if you pour 4.54609 litres of water into a container, you have filled exactly 1 imperial gallon.
History / Origin
The imperial gallon comes from the United Kingdom. In the early 1800s different gallons were used for beer, wine and other goods. This caused confusion in trade and taxes.
In 1824 the British government passed the Weights and Measures Act. This law created a single new gallon for the whole British Empire. The law said that 1 imperial gallon was the volume of 10 pounds of pure water at a set temperature and pressure. This link to water made it easier to copy the standard.
Later, when the metric system became common, the imperial gallon was redefined more neatly in terms of litres. It is now exactly equal to 4.54609 litres. This keeps the value stable and easy to compare with metric units.
Symbol & Abbreviation
There are a few ways to write imperial gallon in short form.
- gal is the basic symbol for gallon.
- imp gal is often used when we must be clear that it is an imperial gallon and not a US gallon.
In tables and technical writing, you may see:
- gal (Imp)
- Imperial gal
When there is any chance of confusion with US gallons, it is safest to write imp gal.
Current Use Around the World
The imperial gallon started in the United Kingdom and was spread through the British Empire. Today it is less common because many countries use the metric system, but it has not fully disappeared.
United Kingdom
- Fuel at petrol stations is sold in litres.
- Many people still talk about car fuel use as miles per gallon, and here gallon usually means imperial gallon.
- Old books, recipes and tools may still use imperial gallons.
Canada
- Fuel is sold in litres.
- Some drivers and car makers still show fuel economy as miles per imperial gallon.
Other countries
- Some Caribbean and former Commonwealth countries once used the imperial gallon more widely.
- Most have now moved to litres for trade, but older people and older documents may still use imperial gallons.
In science and international trade, imperial gallons are rarely used. Litres and cubic metres are preferred.
Example Conversions
Below are some simple conversion examples to help you understand the size of an imperial gallon.
From imperial gallons to litres
- 1 imp gal = 4.54609 L
- 1 imp gal ≈ 4.55 L (rounded)
- 2 imp gal ≈ 9.09 L
- 5 imp gal ≈ 22.73 L
- 10 imp gal ≈ 45.46 L
From litres to imperial gallons
Use this rule:
- 1 L ≈ 0.22 imp gal
Examples:
- 5 L ≈ 1.10 imp gal
- 10 L ≈ 2.20 imp gal
- 50 L ≈ 11.00 imp gal
Imperial gallon to US liquid gallon
- 1 imp gal ≈ 1.20095 US gal
- 5 imp gal ≈ 6.00 US gal
- 10 imp gal ≈ 12.01 US gal
US liquid gallon to imperial gallon
- 1 US gal ≈ 0.83267 imp gal
- 5 US gal ≈ 4.16 imp gal
To convert by hand:
- Multiply imperial gallons by 4.54609 to get litres.
- Divide litres by 4.54609 to get imperial gallons.
Related Units
The imperial gallon is part of a group of volume units. Here are some closely linked units and how they relate to 1 imperial gallon.
- Litre (L)
1 imp gal = 4.54609 L
1 L ≈ 0.21997 imp gal - Imperial quart (imp qt)
1 imp gal = 4 imp qt
1 imp qt = 0.25 imp gal - Imperial pint (imp pt)
1 imp gal = 8 imp pt
1 imp pt = 0.125 imp gal - US liquid gallon (US gal)
1 imp gal ≈ 1.20095 US gal
1 US gal ≈ 0.83267 imp gal - Cubic metre (m³)
1 m³ = 1000 L ≈ 219.97 imp gal
1 imp gal ≈ 0.004546 m³
FAQs
Q: Is an imperial gallon the same as a US gallon?
A: No. An imperial gallon is larger. 1 imperial gallon is about 4.55 litres, while 1 US gallon is about 3.79 litres. So 1 imperial gallon is about 1.2 US gallons.
Q: How many litres are in 1 imperial gallon?
A: Exactly 4.54609 litres. For quick mental maths, many people use 4.55 litres as a close estimate.
Q: Where is the imperial gallon still used?
A: It is mostly used in the United Kingdom and some Commonwealth countries, mainly when talking about car fuel use in miles per gallon. Day to day sales of fuel usually use litres.
Q: Is the imperial gallon a metric unit?
A: No. The imperial gallon is part of the old British imperial system. It is now defined using litres, but it is not itself a metric unit.
Q: Why was the imperial gallon created?
A: It was created in 1824 so that the whole British Empire would use one clear gallon instead of several different gallons for different products. This made trade and taxes simpler.
Q: How can I quickly convert imperial gallons to litres in my head?
A: A simple trick is to multiply by 4.5. For example, 2 imp gal × 4.5 ≈ 9 L. This is a bit low but close for quick estimates. For exact work use 4.54609.
Q: How much does 1 imperial gallon of water weigh?
A: At normal room conditions, 1 imperial gallon of water weighs a little more than 4.5 kilograms, or about 10 pounds. This is linked to its original definition.
Q: Are imperial gallons used in cooking recipes?
A: Some old British or Commonwealth cookbooks may use gallons, quarts and pints that are imperial. Modern recipes mostly use millilitres, litres, cups and grams instead.
