Fuel Cost Calculator

Fuel economy is expressed in two fundamentally different, and slightly confusing, ways depending on where you are in the world, and this calculator handles both. In the US and UK, fuel efficiency is typically expressed as miles per gallon (mpg) — a "bigger is better" figure, where a higher mpg means the vehicle goes further on the same amount of fuel. In most of the rest of the world, efficiency is expressed as liters per 100 kilometers (L/100km) — a "smaller is better" figure, where a lower number means less fuel is needed to cover the same distance. These two units aren't just different units of the same thing; they're inverses of each other, which is exactly why converting between them isn't a simple multiplication and why mixing them up produces wildly wrong trip cost estimates. For the imperial (mpg) calculation, the logic is: fuel used equals distance divided by mpg (if your car does 30 miles per gallon and you're driving 300 miles, you'll use 300 ÷ 30 = 10 gallons), and total cost is fuel used multiplied by the price per gallon. For the metric (L/100km) calculation, the logic flips slightly because the unit itself is already "per 100km": fuel used equals (distance ÷ 100) multiplied by the L/100km figure (a car rated at 7 L/100km driving 500km uses (500 ÷ 100) × 7 = 35 liters), and total cost is that fuel used multiplied by the price per liter. A worked example in each system illustrates the calculation and the scale of costs involved. Driving 400 miles in a car rated at 32 mpg, with fuel priced at $3.60/gallon: fuel used = 400 ÷ 32 = 12.5 gallons, and total cost = 12.5 × $3.60 = $45.00. Now the metric equivalent: driving roughly 644 km in a car rated at 7.3 L/100km (a similar real-world efficiency to 32 mpg), with fuel priced at £1.45/liter: fuel used = (644 ÷ 100) × 7.3 ≈ 47 liters, and total cost = 47 × £1.45 ≈ £68.15. The pound and dollar figures aren't directly comparable here since they're different currencies and different local fuel prices, but the underlying mechanics — economy figure times or divided by distance, then multiplied by local price — are the same shape of calculation in both unit systems. A common misconception is trying to convert mpg to L/100km (or vice versa) using a simple linear scaling factor, when the correct conversion is actually a reciprocal relationship: L/100km = 235.215 ÷ mpg (for US gallons), or equivalently mpg = 235.215 ÷ L/100km. This is why a car that improves from 20 mpg to 40 mpg doesn't save half its fuel per 100km in a linear sense that matches, say, going from 10 L/100km to 5 L/100km — the relationship bends, and improvements at the low-mpg (thirsty) end of the scale save far more absolute fuel than the same mpg improvement at the high-mpg (efficient) end. This calculator sidesteps the need to convert between the two systems at all — pick the unit toggle matching your vehicle's spec sheet and enter the figures directly in that system, without needing a separate conversion step.