Fuel Cost Calculation: How to Estimate the Cost of a Trip

Fuel cost for a trip is the most ordinary multiplication in personal finance: how far you drive, how much your car burns per mile, and what each gallon costs. Add a few stops with different prices and it's the same three numbers, summed up.

The basic fuel-cost formula

For a single trip, fuel cost is three numbers multiplied together:

cost = distance / efficiency × price_per_unit

Drive 300 miles at 30 mpg with gas at $3.50/gal: 300 / 30 = 10 gallons, and 10 × $3.50 = $35. That's it. The distance divided by your car's efficiency gives the fuel you'll burn; multiply by the price and you have the cost. Everything else is unit conversion.

Why efficiency is in the denominator

Notice that efficiency divides the distance, it doesn't multiply it. That's because "miles per gallon" is a rate of distance per unit of fuel — to get the fuel, you flip it. The mental model: a higher mpg number means less fuel for the same distance, so efficiency shrinks the result. A 30 mpg car burns half the fuel of a 15 mpg truck over the same 300 miles.

This trips people up because "miles per gallon" sounds like more is better, and it is — but only because it appears in the denominator. Doubling your mpg halves your fuel, which halves your cost. That's why a fuel-efficient car saves so much over a year of driving.

Unit conversions: mpg vs L/100km

The formula looks different in countries that use litres per 100 km, because there efficiency is fuel per distance, not distance per fuel. The math becomes:

fuel = distance / 100 × L_per_100km

Here efficiency multiplies, because the unit is already "fuel per distance." A car that uses 7 L/100km over 500 km burns 500 / 100 × 7 = 35 litres. The two systems describe the same physics from opposite ends; you just have to know which convention your efficiency number uses before you plug it in. The fixed conversions are: 1 mile = 1.609 km, 1 gallon = 3.785 litres.

Multi-stop trips: why each leg matters

A real road trip isn't one number. You drive 120 miles around town at 22 mpg on pricey city gas, then 400 highway miles at 32 mpg on cheaper rural gas, then 60 miles back at 28 mpg. Each leg has its own distance, efficiency, and price — so you compute the cost per leg and add them up. The total is the sum, and the per-leg breakdown shows you where the money actually went (usually the long highway stretch).

This also reveals the savings from planning fuel stops. If rural gas is $0.30/gal cheaper, filling up before you leave the cheap zone beats topping up at a highway services station at $1 more per gallon. Multi-stop modeling turns "I think gas is cheaper out there" into a number.

Cost per mile — the number that scales

Once you have the total cost and the total distance, dividing gives the cost per mile — the single number that lets you compare any trip, any car, any fuel price. A 30 mpg car at $3.50/gal costs about $0.117 per mile; a 15 mpg truck at the same price costs about $0.233 per mile. Multiply by your annual mileage and you see why fuel-efficient cars save thousands over a year. Cost per mile is also how employers reimburse travel and how you decide whether a trip is worth it at all.

What the formula leaves out

Fuel is the biggest variable cost of a trip but not the only one. Wear and depreciation (roughly $0.10–0.20 per mile for an average car), tolls, parking, and the value of your time all belong in a full cost-of-driving number. But fuel is the part you control in the moment — by choosing where to fill up, combining trips, and driving gently — so it's the right number to start with.

Try it yourself

The Fuel Cost with Stops Calculator totals fuel cost across a multi-leg trip where each leg can have its own distance, efficiency, and price, with miles/km and mpg/L-per-100km units. For a single straightforward trip, the Fuel Cost Calculator does the basic one-leg formula, and the Currency Converter handles the cross-border version where the gas price is in a different currency.