MPG vs. L/100km: Convert and Compare Fuel Consumption

Choose the original quantity and gallon convention before comparing fuel figures. MPG and km/L express distance per fuel; L/100km expresses fuel per distance. A larger MPG or km/L value covers more distance per fuel amount, while a smaller L/100km value uses less fuel per distance. The fuel calculator offers both MPG conventions.

Derive the reciprocal formula

NIST’s named factors, NIST Handbook 44 Appendix C and the reviewed unit definitions distinguish a US liquid gallon of 3.785411784 L, an Imperial gallon of 4.54609 L and an international mile of 1.609344 km.

US L/100km = (100 × 3.785411784 / 1.609344) / US MPG
Imperial L/100km = (100 × 4.54609 / 1.609344) / Imperial MPG
L/100km = 100 / (km/L)

The US numerator is approximately 235.2145833333; the Imperial numerator is approximately 282.4809363318. The shorter displayed constant 235.215 is rounded. Calculations use the underlying definitions, not that shorter value. Reverse a reciprocal conversion by dividing the same numerator by the L/100km value.

Work an example

For a hypothetical 30 US MPG value, the full derivation gives approximately 7.840486111 L/100km. The default calculator shows 7.8405, using four decimal places. At 40 US MPG the corresponding value is approximately 5.880364583 L/100km.

Changing the gallon convention re-expresses the same consumption: 30 US MPG is approximately 36.02849777 Imperial MPG. A changed number after conversion does not indicate changed performance.

Compare equal distances

For hypothetical constant consumption, liters used = L/100km × distance in kilometers / 100.

Over 1000 km, improving from 15 to 20 US MPG reduces calculated fuel from approximately 156.8097 L to 117.6073 L, saving 39.2024 L. Improving from 35 to 40 US MPG reduces it from approximately 67.2042 L to 58.8036 L, saving 8.4005 L. Each change is 5 MPG, but the fuel savings differ because the relationship is reciprocal.

These are arithmetic examples at stated consumption. They are not claims about advertised vehicle models or predictions for a particular trip. EPA/DOE label guidance explains why a consumption metric helps comparisons over fixed distances.

Reference values

MPG (US liquid gallons)	L/100km, at most ten significant digits
15	15.68097222
20	11.76072917
30	7.840486111
35	6.720416667
40	5.880364583
50	4.704291667

Each row follows the definition-derived arithmetic used by the calculator; displayed repeating results are rounded. Extra digits do not establish the original measurement’s accuracy.

Preserve the rating context

Unit conversion removes a difference in units only. Keep the original rating source, test cycle and conditions when comparing values from different specifications. Converting an EPA MPG rating into L/100km does not reproduce another jurisdiction’s testing or sticker value.

For electric energy accounting, read MPGe explained. For gallon definitions, use volume units; for the calculation and rounding policy, see methodology.

Reviewed October 6, 2026.