Fuel Consumption Calculator
Result
Liters per 100 kilometers
- Miles per US gallon
- 30.00 mpg
- Miles per imperial gallon
- 36.03 mpg (UK)
- Kilometers per liter
- 12.75 km/L
Fuel consumption is a single number with four common spellings, and a fuel consumption calculator rewrites whichever one you have into the others: mpg in US or imperial gallons, kilometers per liter, and liters per 100 km. The last two look like they should agree and they are exact opposites — a car doing 20 km/L is doing 5 liters per 100 km, while a thirsty truck at 15 liters per 100 km works out at 6.7 km/L. That inversion is the reason a conversion is worth doing rather than guessing: the two scales run in opposite directions, so an improvement is a bigger number on one and a smaller number on the other. The table below covers the range cars and light trucks actually fall in, and the calculator handles anything outside it, including the imperial gallon, which is 20% larger than the US one and makes every UK fuel economy figure look better than the same car's figure in America.
Fuel economy conversion table
| L/100 km | MPG (US) | MPG (UK) | km/L |
|---|---|---|---|
| 4 | 58.8 | 70.62 | 25 |
| 5 | 47.04 | 56.5 | 20 |
| 6 | 39.2 | 47.08 | 16.67 |
| 7 | 33.6 | 40.35 | 14.29 |
| 8 | 29.4 | 35.31 | 12.5 |
| 9 | 26.13 | 31.39 | 11.11 |
| 10 | 23.52 | 28.25 | 10 |
| 12 | 19.6 | 23.54 | 8.33 |
| 15 | 15.68 | 18.83 | 6.67 |
| 20 | 11.76 | 14.12 | 5 |
Rounded to two decimals. Read across a row to see the same fuel economy in all four units — the two metric columns are exact reciprocals, so a row where they match (10) is the point where the scales cross.
Formula
L/100 km = 100 × 3.785411784 ÷ 1.609344 ÷ mpg
- mpg
- Fuel economy in miles per US gallon
- 3.785411784
- Liters in a US gallon (231 cubic inches)
- 1.609344
- Kilometers in a mile
Use it when a figure arrives in a unit you do not think in — a review quoting mpg, a rental agreement quoting liters per 100 km, a dashboard set to the wrong country. It converts fuel economy as a rate; it does not tell you what a trip will use, which needs the distance as well.
Worked examples
A US figure of 30 mpg
- The figure you have: 30 mpg
- Liters per 100 km: 235.2146 ÷ 30 = 7.84
- Imperial mpg: 30 × 1.20095 = 36.03
- Kilometers per liter: 100 ÷ 7.84 = 12.75
The US figure is echoed back unchanged rather than recomputed, so the row you typed in never drifts to 29.9999 by a rounding step.
A European figure of 7.5 liters per 100 km
- The figure you have: 7.5 liters per 100 km
- US mpg: 235.2146 ÷ 7.5 = 31.36
- Imperial mpg: 282.4809 ÷ 7.5 = 37.66
- Kilometers per liter: 100 ÷ 7.5 = 13.33
Going the other way the constant changes: 282.4809 is the imperial version of 235.2146, because it is built on the 4.54609 liter imperial gallon instead of the 3.785411784 liter US one.
Limitations
This page converts a rate and nothing else. It will not tell you whether a figure is honest — a manufacturer's number comes from a test cycle, and a dashboard's number comes from the car's own fuel metering, and neither is the same measurement as dividing gallons by miles at the pump. The table is rounded to two decimals, so reading a value off it gives you a figure that agrees with the calculator to that precision rather than to every digit. Imperial gallons are included because the UK still quotes fuel economy that way in conversation, but the UK sells fuel in liters, so a fill-up there will not match the imperial figure directly.
Frequently asked questions
- How do I convert mpg to l/100 km?
- Divide 235.2146 by the mpg figure: 30 mpg is 7.84 liters per 100 km. For imperial mpg divide 282.4809 instead. Both constants come from dividing the liters in the relevant gallon by the kilometers in a mile, then multiplying by 100.
- How do I go from km/l to mpg?
- Multiply by 2.35215 for US gallons, or by 2.82481 for imperial ones. Going the other way, 30 mpg is 30 × 0.425144 = 12.75 km/L. The two scales both reward efficiency, so the conversion is a plain multiplication rather than an inversion.
- Is a higher number always better?
- Only on the miles per gallon and kilometers per liter scales — there, bigger is better. Liters per 100 km is the same quantity turned upside down, so there a smaller number is the better car. A page that quotes a fuel economy figure without saying which way it runs has told you nothing.
- How many liters is an imperial gallon?
- 4.54609 liters, against 3.785411784 for a US gallon — about 20% more. That single difference is why the same car is quoted at 30 mpg in the United States and 36 mpg in Britain, and it is the most common source of confusion in any fuel economy comparison across the Atlantic.
- Why does the table stop at 20 liters per 100 km?
- Because that is already a heavy truck or a car towing hard, and the rows either side of it are more useful than more rows further out. The calculator itself takes any figure from 0.1 to 1000 in any of the four units, so nothing in the table is a limit of the tool.
References
- The International System of Units (SI Brochure) — Bureau International des Poids et Mesures
- Fuel Economy — U.S. Department of Energy and Environmental Protection Agency