How it’s calculated
Miles per gallon and km per liter measure distance per fuel (higher is better). Liters per 100 km measures fuel per distance (lower is better), so it is the reciprocal, not a multiple. The constants come from the exact definitions of the mile (1.609344 km), US gallon (3.785411784 L) and imperial gallon (4.54609 L).
Example: 30 mpg (US) → 235.215 ÷ 30 = 7.84 L/100 km = 12.75 km/L = 36.03 mpg (UK).
Frequently asked questions
How do I convert mpg to L/100 km?
Divide 235.215 by US mpg (or 282.481 by UK mpg). For example, 25 mpg (US) = 235.215 ÷ 25 ≈ 9.41 L/100 km.
Why is UK mpg higher than US mpg for the same car?
An imperial gallon (4.546 L) is about 20% larger than a US gallon (3.785 L), so a car goes farther per UK gallon. 30 US mpg ≈ 36 UK mpg.
Is a lower L/100 km better?
Yes. L/100 km is fuel used per distance, so lower means more efficient. With mpg and km/L, higher is better.
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Sources
- NIST Handbook 44, Appendix C: General Tables of Units of Measurement — U.S. National Institute of Standards and Technology
- NIST Guide to the SI, Appendix B: Conversion Factors (SP 811) — U.S. National Institute of Standards and Technology
- FuelEconomy.gov: The official U.S. government source for fuel economy information — U.S. Department of Energy and U.S. EPA
Formulas are taken from the free public references above. Results are provided “as is” for informational and educational purposes only. See our disclaimer.
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