How to use the fuel cost calculator
- Trip fuel cost: type the distance in miles or km, and tick Return trip if you are coming back the same way. Type your car's fuel economy and pick the unit: mpg with US gallons, mpg with UK gallons, or liters per 100 km. Then type the fuel price and say whether it is per US gallon, per UK gallon or per liter.
- Split the cost by changing Split between to the number of people sharing the ride.
- MPG from a fill-up: fill the tank, reset the trip meter, drive, then fill the tank again. Type the trip meter reading and the amount of fuel the pump put in the second time.
The result updates as you type and always shows your fuel economy in all three units, so you can compare a US figure with a UK or European one. Use Copy result to paste the cost into a message.
How the trip cost is worked out
Fuel cost is the fuel the trip needs times the price of that fuel:
- With mpg: fuel = distance in miles ÷ mpg, in gallons.
- With L/100 km: fuel = distance in km × L/100 km ÷ 100, in liters.
- Cost = fuel × price per gallon or liter.
When the units do not match, for example UK mpg with a price per liter, the calculator converts everything to kilometers and liters first, using the exact figures below, then converts back. The cost is rounded to the cent only at the end.
| Trip | Fuel needed | Cost |
|---|---|---|
| 300 miles at 25 mpg (US), $3.50 per US gallon | 300 ÷ 25 = 12 US gallons | $42.00 |
| 300 miles at 45 mpg (UK), £1.40 per liter | 300 ÷ 45 = 6.667 UK gallons = 30.31 liters | £42.43 |
| 500 km at 6.5 L/100 km, 1.80 per liter | 500 × 6.5 ÷ 100 = 32.5 liters | 58.50 |
| 120 miles each way (240 miles) at 30 mpg (US), $3.25 per US gallon | 240 ÷ 30 = 8 US gallons | $26.00 |
Split between 3 people, the last trip's $26.00 is $8.666... each, which cannot be paid in whole cents. The calculator shares it as $8.67, $8.67 and $8.66, so the three shares add up to exactly $26.00, and it tells you that two people pay 1 cent more.
US gallons and UK gallons are different sizes
A US gallon and a UK (imperial) gallon are not the same amount of fuel, which is why two mpg figures for the same car can disagree.
- US gallon: NIST, the US National Institute of Standards and Technology, defines it as "231 cubic inches (exactly)", which its tables give as 3.785 411 784 liters.
- UK gallon: UK law, the Weights and Measures Act 1985, defines the gallon as "4.54609 cubic decimetres". A cubic decimeter is one liter, so a UK gallon is 4.54609 liters.
- Mile: the same in both countries. NIST gives 1 mile as "1.609 344 kilometers (exactly)".
NIST's own tables note that the British gallon is "about 20 % ... larger than the United States gallon". Divide one by the other and the UK gallon holds 1.2009 times as much.
Why a UK mpg figure is higher than a US one
Miles per gallon counts how far the car goes on one gallon. A UK gallon holds about 20% more fuel, so the same car goes about 20% further on it, and its mpg figure is about 20% higher. The car is not more efficient; the gallon is bigger. A car rated at 40 mpg in a UK brochure would show about 33.3 mpg on a US scale.
| mpg (UK gallons) | mpg (US gallons) | L/100 km |
|---|---|---|
| 30 | 24.98 | 9.42 |
| 36.03 | 30 | 7.84 |
| 40 | 33.31 | 7.06 |
| 50 | 41.63 | 5.65 |
| 60 | 49.96 | 4.71 |
To convert by hand: US mpg = UK mpg × 0.8327, and UK mpg = US mpg × 1.2009. Liters per 100 km works the other way round, because a lower figure means less fuel: L/100 km = 235.215 ÷ US mpg, or 282.481 ÷ UK mpg. NIST's SP 811 guide gives the US version as "divide 235.215 by number of miles per gallon".
How to work out your real MPG
The fuel economy printed for a car comes from a standard test, and your own driving can give a different figure. Your real figure is easy to measure:
- Fill the tank until the pump clicks off, and reset the trip meter.
- Drive as normal, ideally for most of a tank.
- Fill up again until the pump clicks off. Note the miles on the trip meter and the gallons or liters the pump put in.
- Divide the miles by the gallons. That is your mpg.
For example, 350 miles on 11 US gallons is 31.82 mpg (US), or 7.39 L/100 km. In the UK, 400 miles on 55 liters is 12.10 UK gallons, which is 33.06 mpg (UK). The same drive measured in US gallons would be 27.53 mpg, which shows again how much the gallon size matters.
Things that change the answer
- Driving style and conditions. Speed, hills, traffic, a loaded car, a roof box and air conditioning all use more fuel. Treat the result as an estimate and round up for a budget.
- Prices along the way. The calculator uses one price for the whole trip. On a long drive, prices can differ from place to place, so use a typical price for the route.
- Splitting the cost. The split is the fuel only. Tolls and parking are not included; add them to the total yourself before sharing.
- A distance of 0 gives a cost of 0. A fuel economy of 0 is not accepted, because no car runs on nothing.
Questions and answers
How do I calculate fuel cost for a trip?
Divide the distance by your mpg to get the gallons needed, then multiply by the price per gallon. 300 miles at 25 mpg needs 12 gallons, and at $3.50 a gallon that is $42.00.
Why is UK mpg higher than US mpg?
A UK (imperial) gallon is 4.54609 liters and a US gallon is 3.785411784 liters, so the UK gallon is about 20% bigger. A car goes further on a bigger gallon, so its mpg figure is higher: 40 mpg (UK) is about 33.3 mpg (US).
How do I convert mpg to L/100 km?
Divide 235.215 by the US mpg, or 282.481 by the UK mpg. 30 mpg (US) is 7.84 L/100 km, and 40 mpg (UK) is 7.06 L/100 km.
Can I use a price per liter with an mpg figure?
Yes. Pick per liter as the price unit. The calculator converts the gallons to liters for you, which is how most UK drivers need it, since fuel is sold by the liter but cars are quoted in mpg.
How do I split gas money between passengers?
Set Split between to the number of people. The cost is shared in whole cents; if it does not divide evenly, one or two people pay 1 cent more and the calculator says so, so the shares add up exactly.
Sources
Checked on 7 October 2026. If a rule has changed, please tell us.