Three ways to measure fuel economy
Fuel economy is expressed differently around the world, and confusing the units leads to big mistakes:
- US miles per gallon (mpg) – miles driven on one US gallon (3.785 litres). Higher is better.
- UK miles per gallon (mpg) – miles on one imperial gallon (4.546 litres). Because the UK gallon is bigger, the same car shows about 20% more mpg in the UK than in the US. Fuel in the UK is priced per litre.
- Litres per 100 kilometres (L/100 km) – used in Europe, Australia and most other countries. Lower is better.
Select the system that matches your car’s figures and your fuel price; the calculator does the conversion.
The formulas
US: gallons = miles ÷ mpg; cost = gallons × price per gallon
UK: litres = miles ÷ mpg × 4.546; cost = litres × price per litre
Metric: litres = km × L/100 km ÷ 100; cost = litres × price per litre
US example: a 250-mile trip in a car that does 30 mpg uses 8.33 gallons. At $3.40 a gallon the fuel costs $28.33 one way, or $56.67 for the round trip. Split between two people: $28.33 each.
UK example: 180 miles in a car doing 45 mpg (UK) uses 180 ÷ 45 × 4.546 = 18.2 litres. At £1.45 a litre that is £26.37.
European example: 600 km at 6.5 L/100 km uses 39 litres; at €1.85 per litre the trip costs €72.15.
Converting between mpg and L/100 km
L/100 km = 235.21 ÷ US mpg
L/100 km = 282.48 ÷ UK mpg
| US mpg | UK mpg | L/100 km |
|---|---|---|
| 20 | 24.0 | 11.8 |
| 25 | 30.0 | 9.4 |
| 30 | 36.0 | 7.8 |
| 40 | 48.0 | 5.9 |
| 50 | 60.0 | 4.7 |
Getting a realistic estimate
- Use real-world economy. Official figures (EPA in the US, WLTP in Europe) are measured under test conditions. Real driving is often 10–20% thirstier, especially at motorway speeds above 70 mph (110 km/h), in winter or with a roof box. Measure your own figure at the pump with the fuel economy calculator.
- Price varies. Fuel prices differ between stations, regions and countries. For long trips, use an average or check prices along your route.
- Extra costs. Tolls, parking and congestion charges are not included.
- Driving for work? Fuel is only part of the cost of running a car. Employers usually reimburse a per-mile rate that also covers wear, insurance and depreciation; see the mileage reimbursement calculator.
Splitting fuel costs fairly
Splitting the fuel cost between the driver and passengers is common for carpools and road trips. Some groups also add a contribution towards wear and tear, often a few cents per mile, because the driver carries the depreciation. Agree on the method before you set off.
Read more about driving costs
Fuel is only part of what a car costs. If you live in the Netherlands, the Dutch road tax calculator shows the quarterly road tax per province, and the Dutch company car tax calculator works out the taxable benefit of a lease car. Planning a road trip abroad? Convert prices with the currency converter, because fuel prices per liter can differ by more than 30% between neighboring countries.
Frequently asked questions
How do I calculate the fuel cost of a trip?
Divide the distance by your car’s mpg to get the gallons needed, then multiply by the price per gallon. 300 miles at 25 mpg and $3.50 per gallon costs 12 × $3.50 = $42.
How much fuel will I use per 100 miles?
Divide 100 by your mpg. At 30 mpg you use 3.33 US gallons per 100 miles.
Why are UK mpg figures higher than US figures?
The imperial gallon used in the UK is 4.546 litres, while the US gallon is 3.785 litres. More fuel per gallon means more miles per gallon for the same car.
How do I convert mpg to L/100 km?
Divide 235.21 by the US mpg figure, or 282.48 by the UK mpg figure. 30 US mpg is 7.84 L/100 km.
Does driving faster use more fuel?
Yes. Air resistance rises sharply with speed. Most cars are most efficient between about 45 and 60 mph (70–100 km/h); driving at 80 mph instead of 65 mph can increase fuel use by 15–25%.
Last reviewed: 2026-10-06. Results are estimates for information only.