Fuel economy converter (mpg to L/100 km)

Convert mpg to L/100 km, L/100 km to mpg and km/L for US and UK gallons, using L/100 km = 235.215 ÷ US mpg or 282.481 ÷ UK mpg.

Updated 27 September 2026Checked against 4 worked examples

Converted value

L/100 km

Shown to 12 significant figures, half-up

Conversion: 30 mpg (US) ≈ 7.840486 L/100 km
Exactness: Exact definitions, repeating decimal

30 mpg (US) is about 7.840486 L/100 km. Because one unit counts fuel per distance and the other distance per fuel, a bigger number in one means a smaller number in the other.

Reciprocal relationship: better economy means fewer litres

30 mpg (US) in every fuel economy unit (4 rows)

Unit	Value	Definition
kilometre per litre (km/L)	12.75431	Base unit
litre per 100 km (L/100 km)	7.840486	km/L = 100 ÷ (L/100 km), exact
mile per US gallon (mpg (US))	30	1 mpg (US) = 1.609344/3.785411784 km/L, exact
mile per imperial gallon (mpg (UK))	36.0285	1 mpg (UK) = 1.609344/4.54609 km/L, exact

How it's calculated S

Convert mile per US gallon to kilometre per litre
$30\ \text{mpg (US)} \times \tfrac{1.609344}{3.785411784}\ \tfrac{\text{km/L}}{\text{mpg (US)}} = 12.7543112229\ \text{km/L}$
1 mpg (US) = 1.609344/3.785411784 km/L, exact
Convert kilometre per litre to litre per 100 km
$\text{L/100 km} = \frac{100}{12.7543112229} = 7.84048611111\ \text{L/100 km}$
km/L = 100 ÷ (L/100 km), exact
Why the numbers move in opposite directions
$\text{L/100 km} = \frac{235.214583333}{\text{mpg (US)}}$
L/100 km measures fuel used per distance; mpg (US) measures distance per unit of fuel. They are reciprocals: their product is always 235.2146, so doubling one halves the other.
Both units are defined exactly, but the converted value is a repeating decimal; it is shown to 12 significant figures.

About the fuel economy converter

Fuel economy comes in two shapes. Miles per gallon and kilometres per litre measure distance per unit of fuel; litres per 100 km measures fuel per distance. The two are reciprocals, so L/100 km = 100 ÷ (km/L), and a US mpg figure converts as L/100 km = 235.215 ÷ mpg.

The default converts 30 US mpg to 7.84 L/100 km. Comparing cars across markets is the usual reason: the US quotes mpg with a 3.785 L gallon, the UK quotes mpg with a 4.546 L gallon while selling fuel by the litre, most of Europe uses L/100 km and India uses km/L.

Because the scales are reciprocal, equal steps in mpg are not equal fuel savings: going from 10 to 20 mpg saves 5 gallons per 100 miles, while going from 30 to 50 mpg saves 1.33. The chart plots that curve.

Worked examples

30 mpg (US) in L/100 km

Value: 30 mpg (US)
From: mile per US gallon (mpg (US))
To: litre per 100 km (L/100 km)

Converted value: 7.8404861111 L/100 km
Exactness: Exact definitions, repeating decimal

Checked against: 100 ÷ (30 × 1.609344 km / 3.785411784 L) = 112903/14400 (Python 3.8 fractions from the legal definitions)

10 L/100 km in mpg (US)

Value: 10 L/100 km
From: litre per 100 km (L/100 km)
To: mile per US gallon (mpg (US))

Converted value: 23.5214583333 mpg (US)

Checked against: (100 / 10) km/L ÷ (1.609344/3.785411784) = 112903/4800 (Python 3.8 fractions from the legal definitions)

40 mpg (UK) in mpg (US)

Value: 40 mpg (UK)
From: mile per imperial gallon (mpg (UK))
To: mile per US gallon (mpg (US))

Converted value: 33.3069673852 mpg (US)

Checked against: 40 × 3.785411784 / 4.54609 (Python 3.8 fractions from the legal definitions)

Crossover: 10 km/L equals 10 L/100 km

Value: 10 km/L
From: kilometre per litre (km/L)
To: litre per 100 km (L/100 km)

Converted value: 10 L/100 km
Exactness: Exact

Checked against: 100 / 10 = 10 — the one value where km/L and L/100 km coincide

Questions

How do I convert mpg to L/100 km?

Divide 235.215 by a US mpg figure, or 282.481 by a UK mpg figure. 30 US mpg is 7.84 L/100 km and 50 UK mpg is 5.65 L/100 km. The constants are 100 × 3.785411784 ÷ 1.609344 and 100 × 4.54609 ÷ 1.609344, built from the exact gallon and mile.

Why is UK mpg higher than US mpg for the same car?

Because the imperial gallon is 20% larger: 4.54609 L against 3.785411784 L. A car doing 40 mpg on UK figures does 33.3 mpg in US gallons, and 40 US mpg is 48.0 UK mpg. Both describe the same fuel use; 40 UK mpg is 7.06 L/100 km.

Why does going from 10 to 20 mpg save more fuel than 30 to 50 mpg?

Because fuel used is distance divided by mpg. Over 100 miles, 10 mpg burns 10 gallons and 20 mpg burns 5, a saving of 5; 30 mpg burns 3.33 and 50 mpg burns 2, a saving of 1.33. Larrick and Soll called this the “MPG illusion” (Science, 2008); L/100 km avoids it because it measures fuel directly.

How do I convert km/L to L/100 km?

Divide 100 by the km/L figure: 20 km/L is 5 L/100 km and 12.5 km/L is 8 L/100 km. At 10 km/L the two numbers are equal. To go from km/L to US mpg, multiply by 2.35215, so 15 km/L is 35.3 US mpg.

Is a lower L/100 km better?

Yes. L/100 km counts fuel used, so a smaller number means less fuel: 5 L/100 km is twice as economical as 10 L/100 km. With mpg and km/L a larger number is better. The product of the two stays fixed, 235.215 for L/100 km and US mpg, which is why one falls as the other rises.

How accurate is the fuel economy converter?

Accuracy depends on your inputs and the method's assumptions. Decimal arithmetic uses 50 significant digits, but estimates, numerical methods and source data can be less precise; the displayed rounding does not remove those limits. It is checked against 4 worked examples whose answers come from independent sources; for example, “30 mpg (US) in L/100 km” is checked against 100 ÷ (30 × 1.609344 km / 3.785411784 L) = 112903/14400 (Python 3.8 fractions from the legal definitions).

Where does the method come from?

NIST SP 811 (2008), Guide for the Use of the SI, Appendix B — conversion factors; NIST Handbook 44, Appendix C — General tables of units of measurement.

About this calculator

\begin{gathered} \text{L/100 km} = \dfrac{100}{\text{km/L}} \\ \text{km/L} = \text{mpg}_{\text{US}} \times \dfrac{1.609344}{3.785411784} = \text{mpg}_{\text{UK}} \times \dfrac{1.609344}{4.54609} \\ \text{L/100 km} \approx \dfrac{235.215}{\text{mpg}_{\text{US}}} \approx \dfrac{282.481}{\text{mpg}_{\text{UK}}} \end{gathered}

Sources

Checked against references

4 worked examples with independently sourced answers ship with this calculator. They run in the test suite; you can run them here too.