Looking to compare two vehicles or find your real-world MPG from fillup data? See also: Miles Per Gallon Calculator and Gas Cost Calculator.
Panel 1 - Fuel Efficiency Input and Trip Parameters

Enter Your Vehicle Fuel Economy

Panel 2 - Global Fuel Efficiency Conversion Matrix

Live Equivalent Readings Across All Four Formats

US Standard
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US MPG
UK / Imperial
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UK MPG
European
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L/100km
Asia / Pacific
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km/L
Panel 3 - Trip Budget Telemetry

Road Trip Fuel Cost Estimate

Total Fuel Required
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Enter distance to calculate
Estimated Trip Cost
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Enter distance and price to calculate
Enter trip distance above to see fuel volume. Add a fuel price to see total cost.
Key Terms Explained

Fuel Economy Glossary

Definitions for every metric and measurement used by this tool.

MPG (Miles Per Gallon)
How many miles a vehicle can travel on one gallon of fuel. Higher is more efficient. The US and UK each define a gallon differently, so "MPG" alone is ambiguous without specifying which standard.
L/100km (Litres per 100 km)
The volume of fuel (in litres) consumed to travel 100 kilometres. Lower is more efficient. Used by European and many Asian automakers as the official fuel economy standard.
km/L (Kilometres per Litre)
How many kilometres a vehicle travels on one litre of fuel. Higher is more efficient. Common in Japan, India, and parts of Southeast Asia. Analogous to MPG but using metric units.
Imperial Gallon (UK)
The British unit of volume equal to exactly 4.54609 litres (about 277.4 cubic inches). Used for UK MPG ratings. Approximately 20 percent larger than the US liquid gallon.
US Liquid Gallon
The American unit of volume equal to exactly 3.78541 litres (231 cubic inches). Used for US MPG ratings and most North American fuel pump pricing.
Fuel Economy
The ratio of distance a vehicle travels to the amount of fuel it consumes. Often measured under standardised test-cycle conditions by government agencies, meaning real-world figures may differ.
Inverse Metric
When two measurements express the same concept in reciprocal form. MPG and L/100km are inverse metrics: as one rises, the other falls, but not at a constant ratio due to the unit conversion constants involved.
Thermal Efficiency
The percentage of fuel energy that is actually converted to mechanical work. A typical petrol engine achieves 20 to 40 percent thermal efficiency; the rest is lost as heat. Higher thermal efficiency directly improves MPG and L/100km figures.

The Complete Guide to Global Fuel Efficiency Standards

If you have ever rented a car abroad, bought an imported vehicle, or compared specs between an American, British, or European model, you have run into the problem this tool solves: fuel economy figures labelled in one country's units are genuinely confusing when you are used to another. A "50 MPG" rating means something very different on a UK sticker than on a US window label, and neither maps cleanly to the L/100km figure on a German data sheet. This guide explains the standards, the math, and how to use this converter confidently.

How to Use This Tool

Type your vehicle's fuel efficiency number into Panel 1 and choose its native format from the dropdown. The conversion matrix in Panel 2 updates instantly with equivalent values in all four formats. For trip planning, enter your intended driving distance and your local fuel price. The budget readout in Panel 3 shows both the total volume of fuel required and your estimated cost. Hit "Copy Budget Summary" to paste the estimate into a group chat for split-cost trips.

Why There Are Four Different Standards

The four formats reflect three separate historical measurement traditions. The United States adopted its own gallon (3.785 L) when it standardised units independently from Britain in the early 1800s. The UK retained its larger Imperial gallon (4.546 L) until metrication began in the 1970s. Continental Europe adopted the metric system far earlier and never used gallons at all, leading to L/100km as the natural default. Japan and India, meanwhile, adopted the metric system but expressed efficiency as km/L (analogous to MPG but metric), which is still the standard in those markets today.

Conversion Formulas Reference

The table below shows the exact formulas used by this calculator. Note that conversions involving MPG use division rather than multiplication because MPG and L/100km are inversely proportional (one rises as the other falls).

From To Formula
US MPGL/100km235.215 / US MPG
UK MPGL/100km282.481 / UK MPG
L/100kmUS MPG235.215 / L100
L/100kmUK MPG282.481 / L100
km/LL/100km100 / km_L
US MPGkm/LUS MPG x 0.425144
UK MPGkm/LUK MPG x 0.354006
US MPGUK MPGUS MPG x 1.20095

Real-World vs. Official Ratings

Official fuel economy ratings (WLTP in Europe, EPA in the US, JC08/WLTC in Japan) are measured on a test bench under controlled conditions. Real-world efficiency typically runs 10 to 25 percent lower due to driving style, traffic, climate control use, tyre pressure, load, and road gradient. For accurate budget planning, use the consumption figure your vehicle's trip computer reports rather than the window sticker number.

Frequently Asked Questions

The UK uses the Imperial gallon (4.54609 litres), while the US uses the US liquid gallon (3.78541 litres). Because the Imperial gallon is about 20 percent larger, a vehicle rated at 40 UK MPG would only achieve roughly 33 US MPG on the same tank of fuel. Always confirm which gallon standard a fuel economy figure uses before making comparisons or planning a budget.
L/100km (litres per 100 kilometres) measures how much fuel a vehicle consumes over a fixed distance, making it easy to directly compare fuel costs. A lower L/100km number is better. km/L inverts this relationship and behaves more like MPG, where a higher number is better. European regulators, automakers, and fuel-economy standards adopted L/100km because it scales linearly with fuel cost: cutting L/100km from 10 to 5 saves exactly twice the fuel over any given trip.
Enter your vehicle's fuel efficiency and its unit (US MPG, UK MPG, L/100km, or km/L) in Panel 1 of this tool. Then enter your total trip distance and the local fuel price per litre or per gallon. The Trip Budget Telemetry panel instantly calculates the total volume of fuel required and your estimated trip cost in the same currency as your fuel price input.
MPG and L/100km measure fuel economy from opposite directions. MPG asks how far you can go on one unit of fuel (higher is better). L/100km asks how much fuel you need to cover a fixed distance (lower is better). Because they are reciprocals of each other, doubling your MPG does not halve your L/100km by exactly half unless you also account for the unit conversion constants. The exact formulas are: L/100km = 235.215 / US MPG and L/100km = 282.481 / UK MPG.
Yes, both have a measurable impact. Running air conditioning typically reduces fuel efficiency by 5 to 25 percent depending on outside temperature, humidity, and driving conditions. Highway speed has a larger effect: aerodynamic drag increases with the square of velocity, so driving at 120 km/h (75 mph) can consume 20 to 40 percent more fuel than driving at 90 km/h (56 mph). For accurate trip budgeting, use real-world efficiency figures from your trip computer rather than the manufacturer's official test-cycle rating.