Fuel Economy Converter
This fuel economy converter turns any fuel consumption figure into another unit instantly, moving between L/100km (litres per 100 kilometres), km/L (kilometres per litre), and miles per gallon in both US and UK forms. Enter a value, choose the unit you are converting from and the one you want to convert to, and the result appears right away. It is handy when comparing cars sold in different markets, planning a road trip abroad, or making sense of a spec sheet that quotes fuel economy in an unfamiliar unit.
Result
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mpg (US) uses the US gallon (3.785 L); mpg (UK) uses the imperial gallon (4.546 L).
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How it works
The tricky part is that L/100km measures fuel used per distance, while km/L and mpg measure distance travelled per fuel. These are inverse relationships, so you cannot convert them with a single multiplication factor the way you convert metres to feet — a bigger L/100km number means worse economy, while a bigger mpg number means better economy. The converter handles this by using L/100km as a common base and dividing: mpg (US) = 235.215 ÷ (L/100km), mpg (UK) = 282.481 ÷ (L/100km), and km/L = 100 ÷ (L/100km). For example, 8 L/100km equals 235.215 ÷ 8 ≈ 29.4 mpg (US). Because the relationship is reciprocal, halving your L/100km roughly doubles your mpg.
Fuel economy unit equivalents
Because L/100km and km/L are inverses, and mpg depends on the size of the gallon, a reference table is handy. These are the values you see most often on petrol and diesel spec sheets. Notice that as L/100km falls, both km/L and mpg rise: they all point to a more efficient car. A European car rated at 5 L/100km goes just as far as one advertised as 47 mpg (US) or 56.5 mpg (UK).
| L/100km | km/L | mpg (US) | mpg (UK) |
|---|---|---|---|
| 4 | 25.0 | 58.8 | 70.6 |
| 5 | 20.0 | 47.0 | 56.5 |
| 6 | 16.7 | 39.2 | 47.1 |
| 7 | 14.3 | 33.6 | 40.4 |
| 8 | 12.5 | 29.4 | 35.3 |
| 10 | 10.0 | 23.5 | 28.2 |
| 12 | 8.3 | 19.6 | 23.5 |
Example: fuel cost of a trip
To estimate how much you will spend on fuel for a trip, first work out how many litres you need and then multiply by the price per litre. The formula is: litres = distance × consumption ÷ 100, then cost = litres × price. For example, a 600 km trip at 7 L/100km needs 600 × 7 ÷ 100 = 42 litres; if a litre costs 1.50, the trip costs 42 × 1.50 = 63. If you work in km/L instead, divide the distance by the economy: 600 ÷ 14.3 ≈ 42 litres, the same result.
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Preguntas frecuentes
- Why can't I convert L/100km to mpg with a simple factor?
- Because they are inverse measures. L/100km is fuel per distance, while mpg is distance per fuel. When one goes up the other goes down, so instead of multiplying by a constant you divide a constant by your value: mpg (US) = 235.215 ÷ (L/100km).
- What is the difference between mpg (US) and mpg (UK)?
- They use different gallons. The US gallon is 3.785 litres while the imperial (UK) gallon is 4.546 litres, about 20% larger. So the same car shows a higher mpg figure in UK units than in US units even though the actual consumption is identical.
- How do I convert 8 L/100km to mpg?
- Divide 235.215 by 8 for US gallons, giving about 29.4 mpg (US). For UK gallons, divide 282.481 by 8, giving about 35.3 mpg (UK).
- What counts as good fuel economy?
- Roughly speaking, under about 6 L/100km (over 39 mpg US) is efficient for a petrol car, while over 10 L/100km (under 24 mpg US) is thirsty. Hybrids and small diesels often beat 5 L/100km.
- How do L/100km and km/L relate?
- They are reciprocals scaled by 100: km/L = 100 ÷ (L/100km). So 5 L/100km equals 20 km/L, and 10 L/100km equals 10 km/L. A lower L/100km and a higher km/L both mean better economy.