Fuel Cost Calculator
What a trip costs in fuel — one way, there and back, and split between everyone in the car. Enter the distance and the economy in whichever unit you think in, and the same trip is priced again in a second car so the difference between them is a number of dollars rather than a number of miles per gallon.
The trip and the car
How far it is one way. The round-trip figures are worked out for you below.
Set independently of the economy unit — miles on a km/L car is a perfectly normal combination.
This also sets what the fuel price is quoted per — a gallon or a litre.
What the car actually does, not the window sticker. If you have never measured it, the gas mileage calculator turns one fill-up into this number.
A second economy figure — the other car you are weighing up. Both trips are priced from the same distance and the same fuel price.
Whatever you actually pay. The currency symbol is cosmetic — the arithmetic works in any currency.
Everyone splitting the fuel, including the driver.
300 miles at 30 mpg (US) burns 10 US gal — $70.00 there and back, or $35.00 each once 2 of you split it.
- Fuel needed (one way)
- 10 US gal
- Cost per person (one way)
- $17.50
- Cost per mile
- $0.1167
- Round-trip fuel
- 20 US gal
- Round-trip cost
- $70.00
- Round trip per person
- $35.00
| Fuel economy | Fuel (one way) | One-way cost |
|---|---|---|
| This car — 30 mpg (US) | 10 US gal | $35.00 |
| Compared — 40 mpg (US) | 7.5 US gal | $26.25 |
- At 40 mpg (US) the same trip costs $8.75 less in fuel. Over a year of trips like this one, that is the figure to multiply — not the headline gap in the economy numbers.
- Fuel is one line of the cost of driving, and not the biggest one. The IRS standard mileage rate is set at roughly twice a typical fuel cost because depreciation, maintenance, tyres and insurance are in it too — so use this figure to split a tank, not to decide between driving and the train.
- Real economy falls away above about 55 mph, in cold weather, and with a roof box on. One steady number cannot see any of that.
How the fuel, the cost and the split are worked out
Three lines of arithmetic, in this order. Everything else on the page is one of these three run again with a different number in it.
fuel = distance ÷ economy • cost = fuel × price • each = cost ÷ people- distance
- One way, in miles or kilometres
- economy
- Distance per unit of fuel — or, for L/100 km, its reciprocal
- price
- Per US gallon, imperial gallon, or litre
Internally every input is converted to kilometres and litres using exact definitions — the international mile is 1609.344 metres, the US gallon 3.785411784 litres, the imperial gallon 4.54609 litres — and the answer is converted back to the unit the price was quoted in. That is why a distance in miles and an economy in km/L give a sensible answer instead of a silent factor-of-1.6 error.
L/100 km is the one unit that is not a simple scaling of the others. It runs the other way: a bigger number is a worse car. The calculator inverts it before anything else happens, which is also why a 2 L/100 km improvement means something very different at 5 than it does at 15.
300 miles, 30 mpg, $3.50 a gallon
The fuel is 300 ÷ 30 = 10 US gallons. At $3.50 that is $35.00 one way, $70.00 there and back. Two people in the car makes it $17.50 each one way and $35.00 each for the round trip, and the cost of a single mile is $35.00 ÷ 300 = 11.67 cents.
Now the comparison. The same 300 miles in a 40 mpg car needs 7.5 gallons and costs $26.25 — a saving of $8.75. A third less fuel sounds decisive and is worth less than a restaurant starter on one trip. Where it does matter is repetition: 12,000 miles a year at 30 mpg is 400 gallons and $1,400, against 300 gallons and $1,050 at 40 mpg, so the same gap is $350 a year. That is the number to weigh against a difference in purchase price, not the one from a single drive.
And the honest footnote to both figures: at 11.67 cents a mile, fuel is a small fraction of what those 300 miles actually cost you. The IRS set its 2025 business standard mileage rate at 70 cents a mile. That rate is not a fuel figure — it is fuel plus depreciation, maintenance, tyres, insurance and registration, which is what a mile really costs to produce.
Three fuel tools, and the one question each answers
- Gas mileage calculator — use it first if you do not know your economy. It measures what your car actually does from one fill-up: distance covered, fuel it took to fill back up, done. The figure it gives you is the figure to type into the economy box on this page.
- Fuel economy converter — use it when the only problem is units. It turns 30 US mpg into imperial mpg, km/L and L/100 km and nothing else. No distance, no price, no trip.
- This page — use it when you know the economy and want money: what a specific trip costs, what each passenger owes, and what the gap between two cars is worth over a distance you actually drive.
What this calculator assumes
- One steady economy figure for the whole trip. Real consumption rises with speed above about 55 mph, in cold weather, on hills, with a full car or a roof box, and in stop-start traffic. A motorway run and a city crawl at the same nominal mpg are not the same trip.
- One fuel price for the whole trip, which is rarely true over a long drive and across state or national borders.
- Fuel only. No tolls, parking, ferries, congestion charges, or any of the ownership costs that make a mile cost several times what the fuel does.
- The round-trip figure is simply double the one way. It does not assume a different route home, a lighter car, or an empty tank at the far end.
- The currency symbol is cosmetic. The arithmetic never touches it, so a price in euros per litre gives an answer in euros.
- An even split between everyone in the car, driver included. Nothing here apportions wear, depreciation or the driver’s time.
Fuel cost FAQ
How much does it cost to drive 100 miles?
Divide 100 by your miles per gallon, then multiply by the pump price. At 30 mpg and $3.50 a gallon that is 3.33 gallons and $11.67 — about 12 cents a mile. At 20 mpg it is 5 gallons and $17.50; at 45 mpg it is 2.22 gallons and $7.78. The spread between a thirsty car and an efficient one on a short hop is smaller than people expect, which is exactly why the comparison row on this page is worth reading over a real distance rather than a hundred miles.
Is the fuel cost the real cost of the trip?
No, and it is not close. Fuel is the part you pay for at the time, which is why it feels like the whole bill. Add maintenance, tyres and repairs — the costs that accrue with distance rather than with time — and the variable cost of a mile is roughly double the fuel alone. Add depreciation, insurance and registration and you get something like the IRS standard mileage rate, which for 2025 was 70 cents a mile against a fuel cost near 12. Use the number on this page to split a tank with passengers. Do not use it to decide whether driving beats the train.
Should I use the window sticker mpg or my own?
Your own, if you have measured it. EPA and WLTP figures come from a standardised test cycle, which is what makes them comparable between cars and what stops them matching your commute. Real economy falls off above about 55 mph, in cold weather, with a roof box, in stop-start traffic, and with underinflated tyres. Measure one tank properly and you will usually find you are a few miles per gallon below the sticker.
How do I split fuel fairly with passengers?
The cost-per-person figure here divides the whole tank by everyone in the car, driver included, which is the split most people mean by 'chipping in for petrol'. If you want to be stricter, the driver is already carrying the wear-and-tear share that fuel does not cover, so an even split is generous to nobody and awkward for no one. For a one-way ride where the driver was going anyway, the honest number is closer to the extra fuel your weight and the detour caused, which is small.
Why does the fuel price change unit when I change the economy unit?
Because people buy fuel in the unit they measure it in. Someone thinking in US mpg pays by the US gallon; someone thinking in L/100 km pays by the litre. Letting you pick the two independently would only create combinations that are wrong — a price per litre applied to a per-gallon economy figure is off by a factor of nearly four. The distance unit is genuinely independent, though, so miles with a km/L car works fine.
Is it cheaper to drive or to fly?
Fuel alone almost always makes driving look cheaper, and that is the trap. Compare the fare against the fuel cost plus the distance-related share of maintenance and depreciation — in practice, two to three times the fuel figure — plus parking, tolls and a day of your time each way. Driving usually still wins for two or more people over a few hundred miles. Alone, over a thousand, it rarely does.
Sources and review notes
- IRS standard mileage rates — the all-in per-mile figure, set at 70 cents for business use in 2025 and revised annually
- NIST Special Publication 811 — the exact definitions used here for the mile, the US gallon and the imperial gallon
- US DOE and EPA, fueleconomy.gov — why your mileage differs from the window sticker
- AAA, Your Driving Costs — the annual breakdown that separates fuel from maintenance, tyres, depreciation and insurance
The arithmetic here is a division and two multiplications, and the unit factors behind it are exact by definition rather than measured, so nothing on this page goes stale. The IRS rate quoted above does change every year; it is cited to show the scale of what a fuel-only figure leaves out, not as an input to the calculation.