How this calculator works
Pick US units (miles, mpg, price per gallon) or metric units (kilometers, liters per 100 km, price per liter), enter the trip distance, your vehicle's fuel economy, and the fuel price, and the calculator returns the fuel needed and the total cost. A one-way/round-trip toggle doubles both figures for a there-and-back journey.
Fuel economy is expressed two different ways depending on the country: mpg is a rate (more is better), while L/100km is a consumption figure (less is better) — the two are reciprocals of each other after converting units, not simple opposites, which is why swapping between them takes a real conversion rather than just 1 divided by the number.
The formulas
US: fuel needed (gal) = distance (mi) ÷ fuel economy (mpg); cost = fuel needed × price per gallonMetric: fuel needed (L) = distance (km) ÷ 100 × fuel economy (L/100km); cost = fuel needed × price per literA round trip doubles the distance used in both formulas before computing fuel and cost. Converting between mpg and L/100km exactly requires the international gallon (3.785411784 L) and mile (1.609344 km) definitions, not a rough approximation.
Worked example: 300-mile road trip
- Vehicle rated at 30 mpg, fuel priced at $3.50 per gallon, one-way distance 300 miles.
- Fuel needed: 300 ÷ 30 = 10 gallons.
- Cost: 10 × $3.50 = $35.
- For a round trip, double both: 20 gallons and $70.
Frequently asked questions
Why do I get a slightly different number if I convert mpg to L/100km by hand?
The conversion is not a simple reciprocal — it also involves converting gallons to liters and miles to kilometers. The exact formula is L/100km = (100 × 3.785411784) ÷ (1.609344 × mpg); a rough shortcut like '235 divided by mpg' is accurate to about three significant figures (roughly 0.1% off), which is fine for trip planning but not exact.
Does this account for highway versus city driving?
No — enter whichever fuel economy figure best matches your trip. Official window-sticker ratings usually list city, highway and combined figures separately; use the highway number for a mostly-highway road trip and the combined number otherwise.
Why might my actual fuel cost differ from the estimate?
Real-world fuel economy varies with speed, terrain, cargo weight, tire pressure, and driving style, typically by ten to twenty percent from the rated figure. This calculator projects a cost from the numbers you enter; treat it as a planning estimate, not a guarantee.
How do I estimate the cost of a multi-stop trip?
Add up the distance of every leg into a single trip distance, since fuel used depends only on total distance and average economy, not on how many stops you make along the way.
What if fuel prices differ along the route?
Enter an average price per gallon or liter across the trip for a reasonable estimate. For a trip that crosses regions with very different prices, splitting the trip into segments and running each with its local price gives a more accurate total.