Fuel Economy Calculator
Find fuel economy from the distance driven and the fuel used. Or find the fuel needed and the fuel cost from the economy, the distance and the fuel price per liter or per gallon.
How the results are calculated
- Fuel economy (km/L) = distance (km) ÷ fuel (L)
- L/100 km = 100 ÷ fuel economy (km/L). The fuel used to drive 100 km. A smaller number means better economy.
- Fuel used (L) = distance (km) ÷ fuel economy (km/L)
- Fuel cost = fuel used (L) × price per liter
The calculation uses exact fractions, and the fuel cost is rounded once, to 2 decimal places, at the very end (to the nearest value by default; rounding down and up are also available). If you enter a price per gallon, it is first converted to a price per liter by dividing by the number of liters in the gallon you chose, and then used in the same calculation.
km/L, L/100 km and mpg
Fuel economy is written differently in different places. km/L is common in Japan, L/100 km in much of Europe, and mpg (miles per gallon, the miles you can drive on one gallon) in the United States and the United Kingdom. A larger number is better for km/L and mpg, and a smaller number is better for L/100 km.
There are two kinds of mpg, US and UK, because the gallon is a different size in each:
| Unit | Size |
|---|---|
| 1 mile | 1.609344 km (the international mile; exactly 1,609.344 m) |
| 1 US gallon (US gal) | 3.785411784 L (231 cubic inches) |
| 1 imperial gallon (UK gal) | 4.54609 L (about 1.2 times the US gallon; exactly 4.54609 L) |
For the same car, the UK mpg is the larger number (15 km/L is about 35.28 mpg in US terms and about 42.37 mpg in UK terms). When you compare figures from different countries, check whether they are US or UK mpg.
Examples
- Find fuel economy: 300 km driven on 20 L gives 15 km/L (6.67 L/100 km, 35.28 mpg US, 42.37 mpg UK).
- Find fuel cost: driving 300 km at 15 km/L with a fuel price of 1.70 per liter uses 20 L, and costs 34.00 (0.1133 per km).
- Using miles and gallons: driving 100 miles on 5 US gallons is 20 mpg (US), or about 8.5 km/L.
- With a price per gallon: driving 300 miles at 30 mpg (US) with a price of 3.50 per US gallon uses 10 US gallons and costs 35.00.
- From L/100 km: 6.67 L/100 km is about 15 km/L (100 ÷ 6.67).
Notes
- Real fuel economy changes with driving style, road conditions, temperature, load and the use of air conditioning. A rated economy, measured under set test conditions, is not necessarily what you will get in practice.
- To find fuel economy from fuel added, fill the tank completely, and next time fill it completely again. Use the amount added the second time and the distance driven in between. This reduces errors, although the amount that fits in a "full" tank varies with how the pump stops.
- Distance, fuel and economy must be greater than 0. The fuel price must be 0 or more. All can have up to 9 digits before the decimal point and 6 decimal places. Use a period as the decimal separator. Commas are accepted only as thousands separators.
- The price is used exactly as you enter it. Add taxes, fees and discounts yourself. The tool does not use a currency symbol.
- If you only need to convert lengths or volumes, use the Length Converter or the Volume Converter. To convert speeds, use the Speed Converter.
Sources for the values used
The size of the mile and the gallons are definitions from the sources below (checked on November 1, 2026).
- 1 mile = 1,609.344 m and 1 imperial gallon = 4.54609 L: NIST SP 811, Appendix B.8. The rows for the mile ("mile (mi)", 1.609 344 E+03 m) and for the imperial gallon ("gallon [Canadian and U.K. (Imperial)]", 4.546 09 E+00 L) are printed in boldface, which the appendix uses for exact factors.
- 1 US gallon = 231 cubic inches, which is 3.785411784 L (with 1 inch = 25.4 mm): NIST Handbook 44, Appendix C (2026 edition). The 231 cubic inches and the 3.785 411 784 L figure were read in the document file on November 1, 2026. NIST SP 811 lists 3.785 412 L for the US gallon, which is this value rounded to seven digits.
Frequently asked questions
What is the full-tank method?
You fill the tank completely, and next time you fill it completely again. You divide the amount you added the second time by the distance driven in between to get the fuel economy. Write down the distance each time you fill up.
Can the fuel economy be 0?
No. At 0, the car would not travel and the fuel used could not be found, so the tool shows an error. A distance of 0 also cannot give a fuel economy (it would be 0 km/L and L/100 km could not be found).
How different are US mpg and UK mpg?
The imperial gallon (4.54609 L) is about 1.2 times the US gallon (3.785411784 L). For the same economy, the UK mpg is therefore about 1.2 times the US mpg.
Does the type of fuel matter for the cost?
The tool calculates with the price you enter, and it does not distinguish fuel types. Enter the price for the fuel you use.