Fuel Economy Calculator (km/L, L/100 km, MPG)

Understanding the MPG calculator — concept and principle

This calculator works out your car's real fuel economy from the distance driven and the fuel used, and shows it in every common unit: km per litre, litres per 100 km, and miles per US and UK gallon. Use it after filling up to track economy, check a new car or compare with official figures.

One trip, four units: with the default inputs the result is 32.19 US mpg (38.66 UK mpg).

How it is calculated

Distance is converted to km (miles × 1.609344); fuel to litres (US gallon × 3.785411784, UK gallon × 4.54609).
km/L = km ÷ litres.
L/100 km = 100 × litres ÷ km.
US mpg = miles ÷ US gallons; UK mpg = miles ÷ UK gallons.

The principle

Fuel economy is distance per amount of fuel, or its reciprocal, fuel per distance. Different countries use different forms: km/L in Korea, Japan and India; L/100 km in Europe and Australia; mpg in the US and UK. Because the UK gallon is about 20% larger than the US gallon, the same car shows a higher mpg in UK units. The most accurate way to measure is "tank to tank": fill up, reset the trip meter, drive, fill up again, and use the fuel added the second time.

Worked example

Default inputs: 520 km on 38 litres.

km/L = 520 ÷ 38 = 13.68.
L/100 km = 100 × 38 ÷ 520 = 7.31.
Miles = 520 ÷ 1.609344 = 323.1; US gallons = 38 ÷ 3.785 = 10.04; US mpg = 32.19.
UK gallons = 38 ÷ 4.546 = 8.36; UK mpg = 38.66.

Each point is the calculator re-run with only “Fuel used” changed; the gold point is the default example. Change the inputs above and this chart follows your numbers.

Fuel used	Fuel economy (km/L)
9.5	54.74
19	27.37
28.5	18.25
38	13.68
47.5	10.95
57	9.12
76	6.84

How to read the result

Compare the result with your own earlier fill-ups rather than only with the official figure, which comes from a standard test cycle. Short trips, cold weather, motorway speeds, air conditioning and heavy loads all raise consumption. A sudden drop in economy can be a sign of under-inflated tyres or a fault worth checking.

Common mistakes

Measuring over a partial tank without filling to the same level.
Mixing US and UK gallons.
Comparing L/100 km (lower is better) with km/L (higher is better).
Judging economy from a single short trip.

Keep in mind: Fuel economy varies with driving conditions. The result reflects the trip you measured, not a guaranteed figure.

Frequently asked questions

How do I calculate my car's fuel economy?

Divide the distance by the fuel used, measuring from one full tank to the next.

How do I convert km/L to mpg?

Multiply km/L by about 2.352 for US mpg or 2.825 for UK mpg.

Why is UK mpg higher than US mpg?

The UK gallon is about 20% larger, so more miles fit in one gallon.

Sources: NIST Special Publication 811 (2008), Appendix B (1 mile = 1.609344 km; 1 US gallon = 3.785411784 L; 1 UK gallon = 4.54609 L)

Checked 9 Nov 2026. Exact unit definitions from NIST SP 811 (2008), Appendix B: 1 mile = 1.609344 km, 1 US gallon = 3.785411784 L, 1 UK gallon = 4.54609 L.

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