What will this trip actually cost in fuel?

How do I convert between MPG, L/100km, and km/L?

They measure efficiency in opposite directions — MPG and km/L are "distance per unit of fuel" (higher is better), while L/100km is "fuel per unit of distance" (lower is better) — which is why you can't just multiply by a fixed conversion factor between L/100km and the other two. This calculator handles the math internally based on whichever unit you select, so you don't need to convert manually.

Does this account for highway vs. city driving efficiency?

No — enter whichever efficiency figure best represents your actual trip; many vehicles list separate city and highway MPG or L/100km ratings. For a mixed trip, a combined or average rating will be more accurate than either extreme.

Why would I use this instead of just checking my car's trip computer?

A trip computer tells you what a trip already cost, or estimates range remaining — this tool is for planning ahead, before you drive: comparing routes, budgeting a road trip, or estimating commute costs at different fuel prices without needing to actually drive the distance first.

Worked example: a 500km round trip (1,000km total) at 6.5 L/100km and $1.50/liter needs 65 liters of fuel, for a total cost of $97.50.