Quick answer
To calculate fuel cost, multiply the fuel you expect to use by the price of that fuel. In the UK, the awkward part is that car efficiency is often shown in MPG, but fuel is bought in litres.
Journey miles ÷ MPG × 4.54609 × fuel price per litre = estimated journey cost.
For electric cars, the formula is different because you use miles per kWh and electricity price per kWh rather than litres and gallons.
Want the answer without doing the maths?
Use the calculator to estimate petrol, diesel or EV journey costs, including cost per mile and split costs.
What you need before you calculate it
You only need three figures for a petrol or diesel journey:
- Journey distance: the number of miles you expect to drive.
- Fuel economy: your car’s MPG. Use your real MPG if you know it, not just the manufacturer’s claim.
- Fuel price: the pump price in pence per litre.
For an electric car, replace MPG with miles per kWh and replace pump price with electricity price per kWh.
Petrol and diesel fuel cost formula
The key conversion is that one UK gallon equals 4.54609 litres. That is why the calculation includes a gallons-to-litres step.
gallons used = journey miles ÷ MPG
litres used = gallons used × 4.54609
journey cost = litres used × fuel price per litreFor example, if you drive 100 miles in a car that averages 40 MPG and fuel costs £1.55 per litre:
- 100 ÷ 40 = 2.5 UK gallons used
- 2.5 × 4.54609 = 11.37 litres used
- 11.37 × £1.55 = £17.62 estimated journey cost
This also gives you your pence per mile. In this example, £17.62 ÷ 100 miles = 17.62p per mile.
Worked examples
These examples are rounded. Use your own local price and real fuel economy for a better estimate.
| Journey | Vehicle assumption | Price assumption | Estimated cost |
|---|---|---|---|
| 50 miles | Petrol car at 40 MPG | 155p per litre | About £8.81 |
| 100 miles | Diesel car at 50 MPG | 177p per litre | About £16.09 |
| 120 miles | EV at 3.5 miles/kWh | 26.11p per kWh | About £8.95 |
The diesel example uses less fuel than the petrol example because the MPG is higher, but it still depends on the diesel price, driving conditions and the car itself.
How to calculate EV journey cost
Electric car journey cost is usually simpler because you do not need litres or gallons. You need electricity used and the price per kWh.
kWh used = journey miles ÷ miles per kWh
journey cost = kWh used × electricity price per kWh
cost per mile = journey cost ÷ journey milesSo if an EV does 3.5 miles per kWh and electricity costs 26.11p per kWh, a 120-mile journey uses about 34.29 kWh and costs around £8.95.
Public rapid charging can cost much more than home charging. For a more detailed charging calculation, use the EV charging cost calculator.
Common mistakes that make the estimate wrong
How to split fuel costs fairly
If several people are sharing a journey, calculate the full cost first, then divide it by the number of people contributing.
cost per person = total journey cost ÷ number of people splitting the costFor example, a £36 fuel cost split between three people is £12 each. That is simple, but it may not cover parking, tolls, wear and tear or the driver’s time.
If you are calculating regular commuting or long-term travel, use the commute cost calculator instead because it can show weekly, monthly and annual costs.
Fuel cost is not the same as business mileage
Actual fuel cost tells you what the journey costs you. Business mileage rules are different. If you use your own car for eligible business journeys, HMRC mileage rules may allow a fixed mileage amount rather than reimbursing only the pump cost.
Do not assume normal home-to-work commuting is claimable. Use the mileage reimbursement calculator or read what counts as business mileage before treating a trip as a claim.
FAQs
How do I calculate fuel cost per mile?
First calculate the full journey cost, then divide it by the number of miles driven. A £18 fuel cost over 100 miles is 18p per mile.
Why do UK fuel calculations use 4.54609?
UK MPG is based on the UK imperial gallon. One UK gallon is 4.54609 litres, so you need that conversion when fuel is priced in litres.
Should I use the dashboard MPG or manufacturer MPG?
Use your real dashboard or fill-up MPG if you have it. Manufacturer figures are useful for comparison, but real-world driving can be different.
How do I calculate fuel cost for a return journey?
Double the one-way distance first, then run the same fuel cost formula. A 60-mile one-way trip is 120 miles return.
Is an electric car always cheaper per mile?
Not always. Home charging is often cheaper than petrol or diesel per mile, but public rapid charging can narrow the gap or remove it depending on price and efficiency.
Figures and sources to check
Fuel and electricity prices change, so treat examples as estimates and update the inputs before making a decision.
- UK Fuel Price Index — weekly average unleaded and diesel prices.
- GOV.UK advisory fuel rates — company-car advisory fuel and electric rates.
- Ofgem price cap unit rates — average electricity unit-rate assumptions.