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To convert US MPG to liters per 100 kilometers, divide 235.215 by MPG. For UK imperial MPG, divide 282.481 by MPG instead. The gallon systems are different, so you must identify which one the original figure uses.
A car rated at 30 US MPG is approximately 7.84 L/100km. A figure of 30 imperial MPG is approximately 9.42 L/100km. Entering the wrong gallon system can therefore make a fuel budget noticeably inaccurate.
The two conversion formulas
| Original measurement | Convert to L/100km |
|---|---|
| US miles per gallon | 235.215 ÷ US MPG |
| UK imperial miles per gallon | 282.481 ÷ imperial MPG |
The reverse operation uses the same numerator: US MPG = 235.215 ÷ L/100km. Thus 8 L/100km is approximately 29.40 US MPG. For imperial MPG, 282.481 ÷ 8 gives approximately 35.31.
These constants come from unit conversion, not a prediction of vehicle performance. A mile is 1.609344 kilometers; a US liquid gallon is 3.785411784 liters, while an imperial gallon is 4.54609 liters. NIST’s conversion references document the underlying length and volume units.
Why MPG gets bigger while L/100km gets smaller
MPG measures the distance a vehicle covers using one gallon. More distance per gallon indicates lower fuel use for the same journey. L/100km measures how much fuel is needed to cover a fixed distance. A smaller number therefore indicates lower fuel use.
They are reciprocal descriptions. Doubling US MPG from 20 to 40 halves consumption from about 11.76 to 5.88 L/100km. Adding ten MPG does not always save the same quantity of fuel: the starting efficiency matters.
A conversion table for US MPG
| US MPG | Approximate L/100km |
|---|---|
| 20 | 11.76 |
| 25 | 9.41 |
| 30 | 7.84 |
| 35 | 6.72 |
| 40 | 5.88 |
| 50 | 4.70 |
Values are rounded to two decimal places. Do not use this table for imperial MPG without changing the conversion factor.
Turn the result into an example trip cost
Suppose a trip is 600 kilometers, the vehicle averages 30 US MPG and fuel costs an illustrative 1.60 currency units per liter. Using the less-rounded conversion:
- Consumption: 235.214583 ÷ 30 ≈ 7.84049 L/100km.
- Fuel needed: 600 ÷ 100 × 7.84049 ≈ 47.04 liters.
- Fuel cost: 47.0429 × 1.60 ≈ 75.27 currency units.
To check another journey, use the Fuel Cost Calculator with a matching distance, efficiency and fuel-price unit. The price above is only a calculation example, not a current pump-price claim.
Which MPG figure should you convert?
A city, highway or combined rating describes a different test mix. Match the figure to the question and avoid comparing one vehicle’s highway figure with another’s city figure. The EPA’s gasoline label guide explains the separate rating fields and consumption information on the U.S. label.
For a personal trip estimate, your own recent measured fuel use under similar conditions can be a useful input. The conversion remains exact in principle, but the input consumption and expected trip conditions introduce uncertainty.
Common entry mistakes
Do not treat km/L as L/100km: convert by dividing 100 by km/L. Do not enter an electric vehicle’s MPGe as though it were gasoline MPG for a pump-fuel purchase. And do not enter a per-gallon price beside a per-liter fuel quantity.
Keep the original unit label with the number, convert once, and round the final budget. The Fuel Cost Calculator can then turn the consistent inputs into a practical estimate.