The trip cost formula, worked through
Pricing out a road trip before you leave comes down to one short chain: divide the distance by your fuel economy to get gallons needed, then multiply by the price per gallon.
Trip cost = (Distance ÷ MPG) × Price per gallon
Worked example: a 300-mile trip in a car that gets 30 MPG needs 300 ÷ 30 = 10 gallons. At $3.50 a gallon, that's 10 × $3.50 = $35 for the trip.
Every other mode on this calculator rearranges the same three numbers — distance, fuel economy, and price — to solve for whichever one you don't already have.
MPG and L/100km measure fuel economy in opposite directions
This is the single most common mix-up when comparing a US car's window sticker to a European or Canadian one. MPG (miles per gallon) is distance covered per unit of fuel — bigger is better. L/100km (litres per 100 kilometres) is fuel burned per unit of distance — smaller is better. They're measuring the same underlying efficiency, just pointed in opposite directions, so you can't convert between them with simple multiplication.
L/100km = 235.214 ÷ MPG MPG = 235.214 ÷ L/100km
A car rated at 30 MPG works out to 235.214 ÷ 30 ≈ 7.8 L/100km. A European car rated at 6 L/100km converts to 235.214 ÷ 6 ≈ 39.2 MPG.
The constant 235.214 falls out of the unit definitions themselves (litres per US gallon, kilometres per mile) — it isn't an approximation specific to any one car.
US gallons vs. imperial gallons
A US gallon (3.785 litres) and an imperial gallon used in the UK and historically in Canada (4.546 litres) differ by about 20% — enough that using the wrong one throws off both a fuel-cost estimate and an MPG figure by a similar margin. If a fuel economy number looks unusually good or bad compared to what you'd expect, checking which gallon it's quoted in is the first thing worth verifying.
Comparing routes, prices, and vehicles
Small differences compound over distance in ways that are easy to underestimate. A 10-cent-per-gallon difference between two gas stations sounds trivial, but over a 500-mile trip at 25 MPG (20 gallons), that's $2 — and over a year of regular commuting, the same gap can add up to real money.
The same logic applies to comparing vehicles before a purchase: a car rated at 25 MPG versus one at 35 MPG burns noticeably more fuel over the same distance, and running both through the cost-per-mile mode side by side turns that abstract MPG gap into an actual dollar figure per year of typical driving.
Related tools
This calculator assumes you already know (or want to estimate) your fuel economy. If you'd rather work it out from an actual fill-up — tracking real MPG over time rather than a manufacturer rating — the Gas Mileage Calculator is built for that. If what you actually need is a mileage reimbursement figure for work or taxes rather than a fuel cost, the Mileage Calculator handles rate-based reimbursement instead. And since underinflated tires are one of the quietest ways to lose fuel economy, it's worth checking the Tire Pressure Calculator too.