How to Estimate the Fuel Cost of a Road Trip

The two-step formula

First find how much fuel the journey needs, then multiply by the price. The first step depends on how you state fuel economy. With miles per gallon (or km per liter), divide. With liters per 100 km (or gallons per 100 miles), multiply.

fuel needed = distance ÷ mpgOr: fuel needed = distance ÷ 100 × liters per 100 km.

trip cost = fuel needed × price per unit

Keep units consistent. Miles go with gallons, kilometers go with liters, and the price must be per gallon or per liter to match. Mixing a liter price with a gallon figure is the most common source of a wildly wrong answer.

Count the whole distance, not just the headline number. A route of 380 miles is 760 miles if you drive home the same way, and local driving at the destination, detours and the stretch from your door to the start of the route all add up. It is easy to forget the return leg and halve the budget by accident.

A worked example in miles and in kilometers

US units. A round trip of 380 miles each way is 760 miles. The car averages 28 mpg and fuel costs 3.45 per gallon. Fuel needed is 760 ÷ 28 = 27.14 gallons. The cost is 27.14 × 3.45 = 93.64, which works out to about 0.123 per mile (3.45 ÷ 28).

Metric units. A round trip of 1,220 km in a car using 7.2 L/100 km at 1.62 per liter needs 1,220 ÷ 100 × 7.2 = 87.84 liters, costing 87.84 × 1.62 = 142.30.

Fuel economy has a large effect on the total. For the same 760 miles at 3.45 per gallon:

Price matters too, but less dramatically: every 0.10 change in the per-gallon price moves the 27.14-gallon trip by about 2.71.

Why add a buffer

The economy figure you plug in is an average, and a trip rarely matches your usual driving. Fuel use usually rises with higher steady speeds, headwinds, hills, a heavy load, a roof box, air conditioning and cold weather. Stop-start traffic also tends to cost more than easy cruising. Official test ratings are produced under standard conditions, so your own measured figure, from a few full tanks, is a better input.

A simple habit is to add 10% to the fuel estimate. For the 760-mile example that is 27.14 × 1.10 = 29.86 gallons and a budget of 103.01 instead of 93.64. If you end up spending less, nothing is lost.

Fuel prices also differ along a route, and they can be higher at highway service areas. If you can, use an average of prices you expect to pay rather than a single pump price from home.

Splitting the cost and counting other expenses

A common approach is to divide fuel and tolls equally among everyone in the vehicle, driver included. With the buffered budget of 103.01 and four people, each pays 103.01 ÷ 4 = about 25.75. In the metric example, the same split of 142.30 gives 35.58 each.

Fuel is not the whole bill. Depending on the trip you may also face tolls, parking, ferry or vehicle fees, and accommodation. Wear on tires and servicing is a real cost of driving too, although it is harder to attribute to a single journey. Decide up front which of these you are sharing so nobody is surprised at the end.

Planning fuel stops

Range is tank size multiplied by economy. A 13-gallon tank at 28 mpg gives 13 × 28 = 364 miles. Most drivers prefer not to run close to empty, so if you plan to refuel when a quarter of the tank remains, your practical range is 0.75 × 364 = 273 miles between fill-ups.

Use the fuel calculator to turn a distance, an economy figure and a price into fuel needed and total cost, and the MPG calculator to measure your own economy from a fill-up so the estimate reflects your real vehicle.