Fuel Cost and Road Trip Calculator
What a journey costs in fuel, split between everyone in the car.
A long-term average beats the official number.
Miles per US gallon
Doubles the distance.
Fuel cost
$34.00
- Each person
- $34.00
- Fuel used
- 10 gallons
- Rate
- $0.11 per mile
- Distance
- 300 miles
Everything here runs on your device. Nothing you enter is uploaded or stored.
Enter a distance, your car’s fuel economy and the pump price, and this works out what the journey costs in fuel. It handles US and UK gallons as the different units they are, converts between MPG and litres per 100km properly, and splits the total between however many people are in the car.
How to use it
- Enter the one-way distance and choose miles or kilometres.
- Pick the fuel economy scale you know: miles per US gallon, miles per UK gallon, or litres per 100 kilometres.
- Enter your car’s figure. A long-term average from the trip computer is more reliable than the official number.
- Enter the pump price and select whether it is quoted per litre or per gallon.
- Tick the return box for a round trip, and set the number of passengers to split the cost.
Why five more MPG is worth less than you think
Miles per gallon is an inverted measure, and that single fact makes it consistently misleading.
Fuel economy expressed as MPG tells you distance per unit of fuel. What you actually pay for is fuel per unit of distance — the other way round. Because the relationship is a reciprocal rather than a straight line, a fixed improvement in MPG saves wildly different amounts depending on where you started.
Take a 12,000-mile year at $3.40 per US gallon. A car doing 15 MPG burns 800 gallons, costing $2,720. Improve it to 20 MPG and it burns 600 gallons, costing $2,040 — a saving of $680. Now take a car already doing 40 MPG: it burns 300 gallons for $1,020. Improve that by the same five MPG to 45 and it burns 267 gallons, costing $907. The saving is $113.
The same five MPG improvement is worth six times more on the thirsty car. This is why replacing an old truck with a slightly less thirsty one saves more fuel than replacing an efficient hatchback with a hybrid, and it is completely invisible if you think of MPG as a linear score.
Litres per 100 kilometres does not have this problem, because it measures fuel per distance directly. A car at 10 L/100km uses exactly twice the fuel of one at 5 L/100km, and dropping any car by 1 L/100km over the same distance saves the same absolute number of litres. That is why most of the world, and every engineer, prefers it — and why this calculator shows you the real value of an MPG change against your own numbers rather than leaving you to guess.
What it does not do
It calculates fuel only. Tolls, parking, wear, tyres and depreciation are real costs of driving and none of them are here. It assumes a constant consumption rate, so it cannot model city stop-start against open motorway, and it does not know your car — the economy figure has to come from you. For electric vehicles the maths is different again, since you would need kWh per 100 miles and an electricity rate rather than a fuel price.
Nothing you enter is uploaded. The calculation runs entirely in your browser.
MPG against litres per 100km
| US MPG | UK MPG | L/100km |
|---|---|---|
| 15 | 18.0 | 15.7 |
| 20 | 24.0 | 11.8 |
| 25 | 30.0 | 9.4 |
| 30 | 36.0 | 7.8 |
| 35 | 42.0 | 6.7 |
| 40 | 48.0 | 5.9 |
| 50 | 60.0 | 4.7 |
| 60 | 72.0 | 3.9 |
Fuel volume units
| Unit | Litres |
|---|---|
| 1 litre | 1.000 |
| 1 US gallon | 3.785 |
| 1 UK gallon | 4.546 |
Questions
Is a US MPG the same as a UK MPG?
No, and the gap is large. A UK gallon is 4.546 litres against the US 3.785, so 40 UK MPG is only about 33 US MPG. Comparing a British car's economy figure with an American one directly overstates it by roughly 20%.
Why is litres per 100km better than MPG?
Because it is a direct scale. Fuel used rises in proportion to the number, so halving it halves your fuel bill. MPG is inverted, which makes equal-sized improvements worth very different amounts depending on where you start.
How accurate is the result?
The arithmetic is exact, but real consumption varies with speed, load, terrain, weather and tyre pressure. Motorway driving at 80mph can use 25% more fuel than at 60mph. Treat the figure as a good estimate rather than a promise.
Should I use my car's official economy figure?
Official figures are usually optimistic. If your car has a long-term average readout, use that. Otherwise a figure 10-15% below the official number is closer to real-world use for most cars.
Last updated