What a Trip Really Costs in Fuel

Fuel is the only trip expense you can price to the cent before you leave the driveway. Tolls, parking, and coffee are variable; the fuel bill is just distance, efficiency, and price — three numbers you already know, multiplied together.

Three inputs, one multiplication

The calculator runs a single chain: distance ÷ efficiency gives the fuel burned, and fuel × price gives the cost.

fuel = distance / mpg          (or: distance / 100 × L/100km)
cost = fuel × price per gallon (or litre)

Drive 400 miles at 28 mpg with gas at $3.45 a gallon: 400 / 28 = 14.29 gallons, and 14.29 × $3.45 = $49.29. That is the entire calculation. (The general anatomy of this formula — why efficiency sits in the denominator, and how mpg and L/100km describe the same physics from opposite ends — is covered in the fuel-cost deep dive; this article is about the tool and the trip math around it.)

The same trip, metric edition

In litres-per-100km countries the efficiency multiplies instead of divides. A 600 km trip in a car rated at 8.4 L/100km with fuel at $1.75 a litre burns 600 / 100 × 8.4 = 50.4 litres, costing 50.4 × $1.75 = $88.20. Same physics, different convention — the tool accepts either and does the arithmetic the right way for whichever you pick.

Mixed units are handled, not punished

Real inputs are messy: a Canadian driving a US-spec car has kilometres on the odometer, mpg on the window sticker, and gas priced per litre. The tool normalizes behind the scenes using fixed conversions — 1 mile = 1.609344 km and 1 gallon = 3.785411784 litres — so you can enter distance in km with an mpg efficiency, or miles with L/100km, and still get a correct cost. The result always reports both views: the 400-mile example returns 643.74 km, 14.29 gallons, and 54.08 litres alongside the $49.29, which matters when you cross a border mid-trip and the pump unit changes under you.

Stretch the number: round trips and passengers

The output is for one one-way leg, so two multipliers make it real. A round trip doubles it: the $49.29 example is $98.58 there and back. Splitting among passengers divides it: four people sharing that ride pay $12.32 apiece one-way. Annualize it and the number stops feeling small — a 400-mile round trip once a month is about $1,183 a year at these inputs, which is the figure that justifies asking whether a train, a carpool, or a closer destination changes the decision. The same annualized number is also the honest baseline for the "should I drive or fly?" comparison: once fuel for the drive is priced, an airfare or rail fare can be weighed against it directly instead of by vibes.

Which efficiency number should you enter?

The window sticker is a laboratory number; real driving usually runs 10-20% worse, especially in city traffic and winter. Using your actual observed consumption — from your car's trip computer or a hand calculation at the pump — errs on the expensive side, which is the safe direction for a budget. If the number is for a go/no-go trip decision rather than an expense report, optimism is the more expensive mistake.

Try it yourself

The Fuel Cost Calculator prices a single trip from distance, efficiency, and fuel price, in miles or kilometres, mpg or L/100km, gallons or litres — and shows the fuel used in both systems. For a multi-leg trip with different roads, efficiencies, or fuel prices per leg, the Fuel Cost with Stops Calculator totals them leg by leg, and the Fuel Economy Calculator turns your own fill-up records into the real efficiency number this tool deserves as input.