To find the fuel cost of a trip, divide the distance by your mpg to get gallons, multiply by 4.54609 to get litres, then multiply by the price per litre. A 250-mile trip at 45 mpg uses about 25.3 litres, which costs about £36.62 at an example price of £1.45 a litre.
UK gallons, US gallons and litres
Miles per gallon is only meaningful if you know which gallon is used. The UK (imperial) gallon is 4.54609 litres. The US gallon is 3.78541 litres, so a US gallon is smaller and US mpg figures look lower for the same car.
| Unit | Litres |
|---|---|
| 1 UK gallon | 4.54609 |
| 1 US gallon | 3.78541 |
British cars and the figures in UK adverts use UK mpg. If a figure came from an American source, convert it before comparing.
How to work out the cost of a trip
Use this method:
- Gallons used = miles ÷ mpg.
- Litres used = gallons × 4.54609.
- Cost = litres × price per litre.
Worked example with the Fuel cost and mpg tool: a 250-mile journey at 45 mpg uses 5.56 UK gallons, or 25.3 litres. At an example price of £1.45 per litre, that is £36.62, or about 15p a mile. The price is only an illustration, not a current price. Use the price from your own receipt or fuel app.
If you share the cost with passengers, divide the total by the number of people, as with a split bill.
How to work out your real mpg
The figure on your dashboard can be optimistic. A more reliable method is to fill the tank, note the mileage, drive as usual, then fill up again and record the litres added.
- Miles driven since the last fill-up ÷ gallons added = mpg.
- Gallons added = litres ÷ 4.54609.
For example, 250 miles on 38 litres works out at 29.9 mpg (UK), which is about 24.9 US mpg or 9.44 litres per 100 km.
Converting mpg, litres per 100 km and km per litre
Many countries quote consumption in litres per 100 km instead. The two are linked by a constant:
- Litres per 100 km = 282.481 ÷ mpg (UK).
- Kilometres per litre = 100 ÷ litres per 100 km.
So 45 mpg (UK) is about 6.28 litres per 100 km. Note that for litres per 100 km a lower number is better, while for mpg a higher number is better.
What changes your fuel use
Real fuel use varies with a number of things, and the manufacturer’s quoted figure is measured in a test:
- Speed: driving faster, particularly on motorways, normally uses more fuel.
- Load and roof boxes, which add weight and drag.
- Short journeys with a cold engine.
- Traffic, stop-start driving and harsh acceleration.
- Tyre pressure and the condition of the car.
If you are comparing the full cost of running a car, fuel is only one part. For finance costs, see Loans, APR and car finance: comparing the real cost. If you drive an electric car, the arithmetic is similar to the one in How much does an appliance cost to run?, using kilowatt-hours and your own tariff.
Note: This guide offers general information and estimates, not advice. Real fuel use and prices vary, so check your own receipts and vehicle handbook.
Comparing two cars or two routes
You can use the same arithmetic to compare options. A car doing 35 mpg uses more fuel over 250 miles than one doing 45 mpg: 7.14 gallons against 5.56. Multiply the difference by 4.54609 to get litres, then by your price per litre, and you can see what the extra thirst costs over a year of driving. It also helps when deciding whether a longer, faster route is cheaper than a shorter, slower one, since speed and stop-start traffic both change consumption.
Remember to include more than fuel when you compare cars: insurance, tax, servicing and depreciation can easily outweigh a few mpg. Think of the fuel figure as one input in a wider budget.
Try the calculator
The Fuel cost and mpg tool works out the cost of a trip from distance, efficiency and price per litre, or finds your mpg from a fill-up. It accepts miles or kilometres and mpg, US mpg, litres per 100 km or km per litre.
Open the free Fuel cost and mpg
Frequently asked questions
Is a UK gallon the same as a US gallon?
No. A UK gallon is 4.54609 litres and a US gallon is 3.78541 litres, so the same car has a higher mpg figure in UK gallons than in US gallons.
How do I convert mpg to litres per 100 km?
Divide 282.481 by the UK mpg figure. For example, 45 mpg is about 6.28 litres per 100 km.
Why is my real mpg lower than the official figure?
Official figures come from test conditions. Driving style, speed, load, weather and short trips all use more fuel in everyday use.
What price per litre should I use?
Use the price you actually paid at the pump. Prices change often, so we do not quote a current figure.