The existence of different gallon measurements stems from historical development of measurement systems. The US gallon is based on the wine gallon of 231 cubic inches established in England by Queen Anne in 1707. When the United Kingdom reformed its weights and measures system in 1824, they created the Imperial gallon, defined as the volume of 10 pounds of water at a specific temperature. The US retained the original definition while the UK and most Commonwealth countries adopted the new Imperial measure. This divergence in standards continues today, with the US using the smaller US gallon and the UK using the larger Imperial gallon.

For quick mental conversions, you can use these approximations:

• For US gallons to liters: Multiply by 4 (or more precisely, 3.8)
• For liters to US gallons: Divide by 4 (or more precisely, 3.8)
• For Imperial gallons to liters: Multiply by 4.5
• For liters to Imperial gallons: Divide by 4.5

A helpful shortcut: 1 US gallon is about 3.8 liters, while 1 Imperial gallon is about 4.5 liters. For rough estimates, you can remember that an Imperial gallon is about 20% larger than a US gallon. If precision isn’t critical, these mental approximations can save you time in everyday situations.

Fuel economy is measured differently around the world, requiring conversions to make meaningful comparisons. In the US, fuel economy is typically expressed as miles per gallon (MPG), while most other countries use liters per 100 kilometers (L/100km). These systems are not only in different units but also inversely related – higher numbers are better for MPG, while lower numbers are better for L/100km. To convert US MPG to L/100km, divide 235.21 by the MPG figure. For example, 25 MPG equals 235.21 ÷ 25 = 9.41 L/100km. The conversion is further complicated when UK measurements use Imperial gallons, resulting in Imperial MPG being about 20% higher than US MPG for the same efficiency. Understanding these differences is essential when comparing vehicle specifications or fuel costs across countries.

Yes, water volume changes with temperature due to thermal expansion and contraction. Water’s density varies with temperature, with maximum density occurring at approximately 4°C (39.2°F). As water warms above this temperature, it expands, meaning the same mass of water occupies a larger volume. Conversely, when water cools (above 4°C), it contracts. This property is why the original definitions of volume measurements like the liter and gallon specified particular temperatures. The Imperial gallon, for instance, was defined as the volume of 10 pounds of water at 62°F (16.7°C). While these temperature-dependent variations are typically small for everyday measurements (less than 0.5% between refrigerator and room temperature), they can be significant in scientific and industrial applications where precision is crucial. Modern definitions of these units are now fixed and independent of the properties of water.

The liter is the predominant measurement for volume in most countries worldwide, as part of the metric system. Countries using the US gallon include:

• United States
• Liberia
• Myanmar (though transitioning to metric)

Countries that historically used the Imperial gallon but have now officially switched to liters include:

• United Kingdom (though gallons are still sometimes used informally)
• Canada
• Australia
• New Zealand
• Many Caribbean nations
• Most former British colonies

Interestingly, some countries like the UK have officially adopted the metric system, but still permit certain imperial measurements in specific contexts. For example, while fuel is sold in liters in the UK, fuel economy is often still discussed in miles per gallon using the Imperial gallon. This global variation highlights the importance of understanding unit conversions in our interconnected world.