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.
By AdminDrafted with AI writing toolsUpdated Checked examples: 4
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.
| 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 |
1 mpg (US) = 1.609344/3.785411784 km/L, exact
km/L = 100 ÷ (L/100 km), exact
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.
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.
The shortened coefficients and displayed converted decimals below are rounded approximations; the complete gallon-to-mile expressions define the exact factors.
Divide 235.215 by miles per US gallon: 30 mpg = 235.215 ÷ 30 = 7.84 L/100 km. For UK (imperial) mpg, divide 282.481 instead. The scales are reciprocals, so a higher mpg always means a lower L/100 km, and no single multiplier converts between them.
The constants come from the exact mile, 1.609344 km, and the two gallons: the US gallon of 231 cubic inches, 3.785411784 L, and the imperial gallon of 4.54609 L (NIST SP 811, Appendix B.8).
So 100 × 3.785411784 ÷ 1.609344 = 235.2146 for US mpg, and 100 × 4.54609 ÷ 1.609344 = 282.4809 for UK mpg.
Use the same constants the other way round: US mpg = 235.215 ÷ (L/100 km), and UK mpg = 282.481 ÷ (L/100 km).
Kilometers per liter (km/L), used in India, measures distance per liter, like mpg. L/100 km = 100 ÷ (km/L), so 20 km/L is 5 L/100 km and 12.5 km/L is 8 L/100 km; at 10 the two numbers are equal.
To get US mpg, multiply km/L by 2.352146: 15 km/L is 35.28 US mpg.
An imperial gallon is about 20% larger than a US gallon, so the same car shows a higher mpg in the UK, which sells fuel by the liter but still quotes imperial mpg. Multiply UK mpg by 0.832674 for US mpg, or US mpg by 1.200950 for UK mpg.
So 40 UK mpg is 33.31 US mpg, and 40 US mpg is 48.04 UK mpg. Both describe the same fuel use.
Repeating decimals are rounded to six places.
| Value | From | To | Converted economy |
|---|---|---|---|
| 30 | mpg (US) | L/100 km | 7.840486 |
| 10 | L/100 km | mpg (US) | 23.521458 |
| 40 | mpg (UK) | mpg (US) | 33.306967 |
| 20 | km/L | L/100 km | 5 |
| 5 | L/100 km | km/L | 20 |
| 50 | mpg (UK) | L/100 km | 5.649619 |
Fuel used on a trip is distance ÷ mpg, so gains at the low end save the most. Over 100 miles, going from 10 to 20 mpg saves 5 gallons, while going from 30 to 50 mpg saves 1.33 gallons.
Larrick and Soll called this the "MPG illusion" (Science, 2008). L/100 km avoids it by showing the saving directly: 30 US mpg is 7.84 L/100 km and 50 US mpg is 4.70 L/100 km, a cut of 3.14 L per 100 km.
For US mpg in one direction, use mpg to L/100 km or L/100 km to mpg. The gas mileage calculator works out mpg from odometer readings and fuel added, and the trip fuel cost calculator prices a journey.
Checked against: 100 ÷ (30 × 1.609344 km / 3.785411784 L) = 112903/14400
Checked against: (100 / 10) km/L ÷ (1.609344/3.785411784) = 112903/4800
Checked against: 40 × 3.785411784 / 4.54609
Checked against: 100 / 10 = 10 — the one value where km/L and L/100 km coincide
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.
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.
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.
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.
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 at approximately 235.215 for L/100 km and US mpg, which is why one falls as the other rises.
This calculator includes 4 worked examples with independently sourced answers. They run in the test suite, and you can run them here too.