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
Worked example 2: what a 10 mpg gap costs over a year
Say you drive 12,000 miles a year and fuel costs an example £1.45 a litre. That price is only for illustration, so swap in your own. Here is the same year in two cars, worked with our calculator.
| Car A | Car B | |
|---|---|---|
| Fuel economy | 35 mpg (UK) | 45 mpg (UK) |
| Fuel used in 12,000 miles | 342.86 UK gallons (1,558.7 litres) | 266.67 UK gallons (1,212.3 litres) |
| Cost per mile | about 19p | about 15p |
| Fuel cost for the year | £2,260.06 | £1,757.82 |
The gap is £502.24 a year, or roughly £42 a month. Notice that the saving from going from 35 to 45 mpg is bigger than the saving from going from 45 to 55 mpg would be, because gallons used fall by less each time. Consumption is easier to judge in litres per 100 km, where equal steps mean equal savings.
Step-by-step with litres per 100 km
If your car shows consumption in litres per 100 km (common on dashboards and in European cars), you can skip the mpg step altogether.
- Convert the distance to kilometres if needed (1 mile is about 1.609 km).
- Litres used = distance in km ÷ 100 × litres per 100 km.
- Cost = litres used × price per litre.
Example: 400 km at 6 litres per 100 km uses 24 litres. At £1.45 a litre that is £34.80, which is about 14p a mile. To check the answer against mpg, divide 282.481 by 6 to get roughly 47 mpg (UK).
Converting from US mpg is a common trap. A US-market car quoted at 37.5 US mpg is about 45 UK mpg, because a US gallon is smaller. Over 250 miles it costs £36.59 at the same example price, almost identical to the 45 mpg result in the main example.
Common mistakes when working out fuel costs
- Mixing gallons. Using 3.785 litres for a UK mpg figure understates your fuel use by about 17%.
- Forgetting the return leg. A day trip of 120 miles each way is a 240-mile bill.
- Using the best-case mpg. Use your own fill-up figure, not the brochure number, for anything you want to budget.
- Treating the price as fixed. Pump prices differ between stations and change week to week. Run the sum with a slightly higher price to leave a margin.
- Ignoring other running costs. Fuel is only part of the picture. Servicing, tyres, insurance and depreciation add to the cost of each mile, and finance costs are covered in Loans, APR and car finance: comparing the real cost.
Sharing the cost of a journey fairly
For lifts and car shares, divide the total fuel cost by the number of people in the car, including the driver. The driver is not usually charged extra for owning the car, but passengers can reasonably split fuel and any tolls or parking. Agree the figure before you set off, and use cost per mile (fuel cost divided by miles) if the route is likely to change. For a longer trip, add the time dimension with How flight times are worked out if you are comparing driving to the airport against other ways of travelling.
Glossary
- mpg (UK): miles per UK (imperial) gallon of 4.54609 litres.
- mpg (US): miles per US gallon of 3.78541 litres.
- L/100 km: litres of fuel used per 100 kilometres; lower is better.
- km/l: kilometres per litre; higher is better.
- Cost per mile: total fuel cost divided by miles driven.
Ways to cut your fuel bill without changing car
Small habits add up over a year of driving, and you can measure the effect with your own fill-up figures rather than trusting a claim.
- Keep tyres at the pressure given in the handbook, and check them regularly when cold.
- Remove roof boxes and heavy items you do not need. Extra weight and drag both cost fuel.
- Anticipate traffic so you brake and accelerate less sharply.
- Combine short trips into one longer journey so the engine is warm for most of it.
- Plan the route before you leave to avoid avoidable queues and detours.
To see whether a change worked, record miles and litres for three or four fill-ups before and after, work out mpg each time using the method above, and compare the averages. A single tank can mislead because of weather, traffic and how full you filled it. Gaining 2 mpg is worth roughly £75 (from 45 mpg) to £120 (from 35 mpg) a year at 12,000 miles and the example price above, by hand calculation.
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.
How much more fuel does a faster speed use?
It varies by car, but air resistance rises quickly with speed, so most cars use noticeably more fuel at motorway speeds than at moderate speeds. Test it yourself by recording a few fill-ups.
How do I convert km per litre to mpg?
Divide 100 by the km per litre figure to get litres per 100 km, then divide 282.481 by that number to get UK mpg. For example, 10 km per litre is 10 litres per 100 km, or about 28 mpg (UK).
Should I include the cost of the empty return journey?
Yes, if you have to drive it. For a business trip or a hire car, the return leg is real fuel you pay for even with no passengers.
Does this work for diesel and petrol alike?
Yes. The arithmetic is identical; only the price per litre and your car’s consumption figure change.