ElpiAutoFuel Cost Calculator
Distance plus fuel burned → real consumption in every unit, fuel needed, and trip cost.
Trip fuel & cost
35.3 UK mpg · 12.5 km/L
Unit converter — mpg, L/100km, km/L
Already know a figure and just want it in another unit? Type it here.
Fuel economy
Fuel economy conversion chart
The L/100km figures drivers see day to day, in all four units.
| L/100km | US mpg | UK mpg | km/L |
|---|---|---|---|
| 4.0 | 58.8 | 70.6 | 25.0 |
| 5.0 | 47.0 | 56.5 | 20.0 |
| 6.0 | 39.2 | 47.1 | 16.7 |
| 7.0 | 33.6 | 40.4 | 14.3 |
| 8.0 | 29.4 | 35.3 | 12.5 |
| 9.0 | 26.1 | 31.4 | 11.1 |
| 10.0 | 23.5 | 28.2 | 10.0 |
| 11.0 | 21.4 | 25.7 | 9.1 |
| 12.0 | 19.6 | 23.5 | 8.3 |
| 14.0 | 16.8 | 20.2 | 7.1 |
| 16.0 | 14.7 | 17.7 | 6.3 |
| 20.0 | 11.8 | 14.1 | 5.0 |
The math
How it is worked out
consumption (L/100km) = litres used / km × 100fuel for trip = distance / economy · cost = fuel × price
US mpg and UK mpg differ only because the gallon does — a US gallon is 3.785 L, an imperial gallon 4.546 L. L/100km is fuel per distance, so lower is better; mpg and km/L are distance per fuel, so higher is better. Converting between the two families is a reciprocal, which is why the scale is not linear.
The fill-up method (measured litres ÷ distance) gives your real consumption; a manufacturer or trip-computer figure is usually 10–20% optimistic.
Common questions
How do I calculate my car's fuel consumption?
Fill the tank, reset the trip meter, drive normally, then fill up again and note the litres (or gallons) it took. Consumption = fuel added ÷ distance since the last fill. In L/100km that is litres ÷ km × 100; for mpg it is miles ÷ gallons. This tool does the arithmetic and shows the result in every unit at once.
How is trip fuel cost worked out?
Fuel used = distance ÷ economy (or the litres you actually burned), then cost = fuel used × price per litre or gallon. Turn on "round trip" to double the distance, and enter a passenger count to split the cost.
How do I convert mpg to L/100km?
Divide 235.2 by the US mpg figure: 30 US mpg = 235.2 ÷ 30 ≈ 7.8 L/100km. For UK (imperial) mpg, divide 282.5 instead. The relationship is a reciprocal, so it is not linear — doubling mpg halves L/100km.
Why are US mpg and UK mpg different?
The gallon is different. A US gallon is 3.785 litres; an imperial (UK) gallon is 4.546 litres — about 20% larger. The same car does roughly 20% more miles per UK gallon, so 30 US mpg is about 36 UK mpg.
What is km/L?
Kilometres per litre — used in Japan, Korea, India and much of South-East Asia. It is 100 ÷ (L/100km): 8 L/100km = 12.5 km/L. Like mpg, higher is better.
What is good fuel economy?
For a modern petrol car, roughly 6–8 L/100km (30–39 US mpg, 35–47 UK mpg) in mixed driving is typical. Small hybrids reach 4–5 L/100km; large SUVs and trucks sit at 12+ L/100km. Real-world figures usually run 10–20% worse than the official rating.
Methodology
Conversions use exact definitions: 1 US gallon = 3.785411784 L, 1 imperial gallon = 4.54609 L, 1 mile = 1.609344 km. This gives 235.2146 for US mpg ↔ L/100km and 282.4809 for UK mpg. Trip cost multiplies fuel used by the price you enter; round trip doubles the distance; the per-person figure divides the total by the passenger count. Figures are shown to a practical precision.
Reviewed 2026-09-11.