MPG & Fuel Economy Calculator

Convert between UK MPG, US MPG and L/100 km, or work out your real-world fuel economy from the distance you covered and the fuel you used. UK and US gallons are clearly distinguished.

Economy value

Results

UK MPG

40mpg

US MPG

33.31mpg

Litres per 100 km

7.06L/100km

How this calculator works

Fuel economy is expressed two ways: MPG (miles per gallon — higher is better) and L/100 km (litres per 100 km — lower is better). The UK and US gallons are different sizes, so the same fuel use gives different MPG figures depending on which gallon is used.

1 mile = 1.609344 km
1 UK gallon = 4.54609 litres
1 US gallon = 3.785411784 litres
L/100 km (from UK MPG) = 282.481 ÷ UK MPG
L/100 km (from US MPG) = 235.215 ÷ US MPG
US MPG = UK MPG × (UK gallon ÷ US gallon)

Worked example: 40 UK MPG = 282.481 ÷ 40 = 7.06 L/100 km, and 40 ÷ 1.20095 = 33.31 US MPG.

Measuring your real-world economy

The most reliable way to measure actual fuel economy is the brim-to-brim method: fill the tank until the pump clicks, reset your trip computer, drive normally, then refill to the same click. The litres put in on the second fill are the fuel used over the distance shown on the trip. Enter those two figures above to get your real-world MPG and L/100 km.

Remember that smaller L/100 km figures indicate lower fuel consumption, while higher MPG figures indicate better economy — the two scales run in opposite directions.

Frequently asked questions

Why are UK and US MPG different?

Because a UK gallon (4.54609 litres) is larger than a US gallon (3.78541 litres). The same fuel use therefore covers more miles per UK gallon than per US gallon, so a UK MPG figure is always higher than the equivalent US MPG figure.

Which should I use — MPG or L/100 km?

Whichever matches your fuel pump and dashboard. UK pumps sell fuel by the litre, so L/100 km is the natural unit; MPG remains common in conversation and older cars. This tool shows all three together so you can compare.

Is a higher L/100 km figure better?

No — it is the opposite of MPG. L/100 km is consumption, so a smaller number means less fuel used and better economy. A car doing 5 L/100 km uses less fuel than one doing 8 L/100 km.

Are the results rounded?

Yes. Calculations use full precision internally and only the displayed values are rounded to two decimal places.

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