To estimate road-trip fuel, divide the distance by your vehicle’s fuel economy, then multiply the fuel amount by its price. For a 300-mile trip at 30 miles per US gallon, that is 10 gallons. At an illustrative $3.50 per gallon, the fuel cost is $35.
The calculation is simple. Getting a useful result means choosing the right distance, matching the units and using an efficiency figure that resembles your actual driving. Here is how to do it in miles or kilometers, with a range when conditions are uncertain.
What numbers do you need before calculating gas cost?
Start with the total driving distance, expected fuel economy and price per gallon or liter. Include the return journey if you are calculating a round trip. Add planned side trips rather than relying on the straight-line distance between two places.
Use the price for the fuel your vehicle requires, in the currency you intend to calculate. All prices below are made-up examples, not current pump quotes. If you will buy fuel at several prices, estimate each purchase or route section separately and add the costs.
The DOE and EPA trip calculator lets you enter a route and personalize vehicle MPG and fuel prices. A calculator still depends on those inputs. Check that it includes your stops and whether its distance is one-way or round-trip.
How do you calculate gallons and cost from MPG?
Gallons needed = miles ÷ MPG. Then fuel cost = gallons × price per gallon. The gallons must use the same definition as the MPG figure.
Suppose a weekend route totals 450 miles, your car usually averages 30 US MPG on comparable journeys, and you assume $3.60 per US gallon. The estimate is 450 ÷ 30 = 15 US gallons, followed by 15 × $3.60 = $54. That is the estimated value of fuel consumed over the whole route.
For comparison, these are illustrative fuel costs for 300 miles at $3.50 per US gallon. More miles per gallon means less fuel needed for the same distance.
| US MPG | US gallons | Fuel cost |
|---|---|---|
| 20 | 15 | $52.50 |
| 25 | 12 | $42.00 |
| 30 | 10 | $35.00 |
| 40 | 7.5 | $26.25 |
How does the calculation work with liters?
If your vehicle reports liters per 100 kilometers, calculate liters needed as distance in kilometers × L/100 km ÷ 100. A 600-kilometer trip at 7.5 L/100 km uses an estimated 45 liters. At an illustrative 1.70 currency units per liter, the total is 76.50 in that currency.
If your efficiency is kilometers per liter, divide kilometers by km/L. At 12 km/L, a 600-kilometer journey needs an estimated 50 liters. Multiplying by the same illustrative 1.70 per liter gives 85.00.
The direction matters: higher MPG or km/L indicates greater distance from a unit of fuel. Lower L/100 km indicates less fuel used over a fixed distance. Do not divide distance by an L/100 km figure as though it were MPG.
How do you convert MPG, km/L and L/100 km?
For US MPG, use these practical conversion formulas. The constants are rounded; keep extra digits during calculation and round your final estimate.
L/100 km ≈ 235.215 ÷ US MPG
US MPG ≈ 235.215 ÷ L/100 km
km/L ≈ US MPG × 0.425144
US MPG ≈ km/L ÷ 0.425144
km/L = 100 ÷ L/100 km
L/100 km = 100 ÷ km/L
For example, 30 US MPG is approximately 7.84 L/100 km or 12.75 km/L. These describe the same efficiency. The inverse formulas are useful when comparing a vehicle specification with the units shown on your dashboard.
A US gallon is exactly 3.785411784 liters; an imperial gallon is exactly 4.54609 liters. For imperial MPG, use approximately 282.481 ÷ MPG to get L/100 km, or 282.481 ÷ L/100 km to get imperial MPG. Thus 30 imperial MPG is about 9.42 L/100 km, not 7.84. Check which gallon a specification uses before comparing vehicles.
These factors follow NIST’s unit conversions. If your route and efficiency use different distance units, our miles-to-kilometers guide explains the distance conversion.
Which fuel-economy figure should you use?
Recent fuel use on similar journeys is often a useful starting point. To calculate measured MPG across comparable full-to-full refills, divide the miles traveled by the gallons used. In metric units, divide kilometers by liters for km/L, or multiply liters by 100 and divide by kilometers for L/100 km.
Across several records, divide the total distance by the total fuel rather than simply averaging individual MPG figures. Match the observation period: the fuel added at a refill replaces fuel consumed since the preceding comparable fill.
Published ratings help with comparison, but the DOE and EPA explain that actual mileage varies with driving conditions, vehicle condition and other factors. A recent urban average may not represent a highway trip, and a highway rating may be optimistic for stop-and-go travel.
How can you turn one estimate into a useful range?
Choose plausible efficiency and price assumptions based on your vehicle and route. For a 450-mile trip, suppose you test 27–33 US MPG and $3.40–$3.80 per US gallon. The lower-cost case is 450 ÷ 33 × $3.40 = $46.36. The higher-cost case is 450 ÷ 27 × $3.80 = $63.33.
That produces an illustrative range of roughly $46–$63. It is a scenario comparison, not a guaranteed minimum or maximum. Detours, traffic and different pump prices can move the actual result outside it. Update the assumptions when better information becomes available.
Does trip fuel cost include the first tank or other expenses?
Fuel consumed and cash paid during the trip can differ. Starting with fuel already in the tank does not make that fuel free, but you may not need to buy all the estimated gallons while traveling. Ending with more or less fuel also changes what your receipts total.
Keep tolls, parking, accommodation, food and rental charges separate. Add them only when building a broader trip total. For a final check, confirm the full route distance, gallon type, efficiency units and fuel price before multiplying. This estimate does not determine refueling stops or guarantee driving range.
For a paper record of distances and fill-ups, you can browse vehicle mileage log books on Amazon. A notebook or spreadsheet you already have works too.
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