Energy and vehicle · Fuel
Convert a fuel-economy figure
The four usual notations for one number. The US gallon and the imperial gallon give two different mpg figures for the same car.
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How the calculator works
Everything routes through litres per 100 kilometres. Converting to kilometres per litre is a reciprocal: 5 L/100 km is 20 km/L, while 10 L/100 km is only 10. Doubling consumption does not halve distance in a linear way. For mpg, the gallon (3.785411784 L in the US, 4.54609 L in the UK) combines with the mile (1.609344 km), producing the familiar identity 235.215 ÷ mpg US = L/100 km.
Formula and method
km/L = 100 ÷ (L/100 km)
A reciprocal relation, not a proportional one.
L/100 km = 235.214583 ÷ mpg US
Combines the US gallon and the international mile.
L/100 km = 282.481 ÷ mpg UK
The imperial gallon is larger, so UK mpg reads higher.
The factors are exact: the international mile is 1.609344 km, the US gallon 3.785411784 L and the imperial gallon 4.54609 L. The same car shows roughly 20% more mpg in a British brochure than in an American one, without driving any differently. The tool does not turn economy into money: that is the fuel cost calculator's job.
Worked example
6.5 L/100 km in mpg
value = 6.5, unit L/100 km.
- km/L: 100 ÷ 6.5 ≈ 15.4.
- mpg US: 235.215 ÷ 6.5 ≈ 36.2.
- mpg UK: 282.481 ÷ 6.5 ≈ 43.5.
About 36 mpg US, but 43 mpg UK.
30 mpg US in litres per 100 km
value = 30, unit mpg US.
- 235.215 ÷ 30 ≈ 7.84 L/100 km.
About 7.8 L/100 km.
Input notes
- Value
- The number alone. Its meaning depends on the unit chosen just below.
- Unit entered
- The unit of the value entered. The other three are computed.
Assumptions and limits
Assumptions
- International mile: exactly 1.609344 km.
- US gallon: 3.785411784 L; imperial gallon: 4.54609 L.
- Economy is an average, not an instantaneous reading.
Limits
- No split between urban, highway and combined cycles.
- No conversion into cost: use the fuel cost calculator.
- An electric car is measured in kWh per 100 km, not in litres.
How to read the result
A low figure is good in L/100 km, but the opposite holds in km/L and mpg, where higher is better. That inversion explains a common comparison error: going from 10 to 8 L/100 km saves far more fuel than going from 6 to 5, even though the gap on screen looks smaller.
Common mistakes
Comparing a US mpg with a UK mpg.
The imperial gallon is 20% larger: the two numbers are not comparable.
Assuming the relation is linear.
L/100 km and km/L are reciprocals. Always compare within one unit.
Using this page for an electric car.
An electric vehicle is measured in kWh per 100 km.
Comparison
| L/100 km | km/L | mpg US | mpg UK |
|---|---|---|---|
| 4 | 25.0 | 58.8 | 70.6 |
| 6.5 | 15.4 | 36.2 | 43.5 |
| 8 | 12.5 | 29.4 | 35.3 |
| 12 | 8.3 | 19.6 | 23.5 |
Related calculations
- Fuel costThree numbers are enough: distance, economy per 100 km and price per litre. The tool returns litres, total cost and cost per kilometre.
- EV charging costFrom 20 to 80% on a 60 kWh battery, the meter records more than the battery receives. The tool bills the energy actually drawn from the grid.
- Volume converterThe liter is 1 dm³, or 0.001 m³ exactly. The US gallon and the UK imperial gallon are not the same capacity: the tool separates them, with no ambiguous “gallon” option.
- Length converterEvery value goes through the meter, then to the target unit. The inch is 25.4 mm exactly since the 1959 agreement, as in the NIST SI conversion factors.
Frequently asked questions
Why does 235 appear in the mpg conversion?
It is the product of the factors.
235.214583 comes from 100 × 3.785411784 ÷ 1.609344, the US gallon in litres divided by the mile in kilometres. Dividing that number by an mpg figure gives litres per 100 km directly.
Is UK mpg better than US mpg?
No, the unit is simply larger.
The imperial gallon holds about 20% more than the US gallon. The same car therefore shows a higher UK mpg without using less fuel. Always check which gallon a source means.
What is 7 L/100 km in mpg?
About 33.6 mpg US.
235.215 ÷ 7 ≈ 33.6 mpg US, or about 40.4 mpg UK. In kilometres per litre that is a little over 14.3.
Author and update
Written by Rédaction HexaCalc (editorial team). Content last updated: August 25, 2026. No third-party medical or financial review is claimed.
- NIST Guide to the SI, Appendix B: Conversion Factors — National Institute of Standards and Technology
- The International System of Units (SI) — Bureau international des poids et mesures (BIPM)
What will the trip cost?
The fuel cost calculator turns this economy figure into money.
Fuel costCategory: Energy and vehicle