How to Calculate Fuel Cost for a Journey
Knowing the fuel cost of a journey before you set off makes budgeting easier, helps you compare routes, and gives you a fair basis for splitting costs with passengers. The calculation brings together three inputs — distance, fuel economy and fuel price — and the result is an estimate rather than a guaranteed figure, because real-world driving conditions vary.
The basic calculation for petrol and diesel vehicles
The standard UK approach uses miles per gallon (MPG) as the efficiency measure and fuel price in pence per litre. The steps are:
1. Calculate fuel needed (litres): Distance (miles) ÷ MPG × 4.54609
2. Calculate cost: Fuel (litres) × Price per litre
The 4.54609 figure converts imperial gallons to litres. UK MPG figures use imperial gallons, not US gallons, so this conversion is essential if you are working in litres.
Worked example — petrol car
A 200-mile motorway journey in a car averaging 45 MPG, with petrol at £1.45 per litre:
Fuel needed: 200 ÷ 45 × 4.54609 = 20.2 litres. Cost: 20.2 × £1.45 = £29.29.
Cost per mile: £29.29 ÷ 200 = approximately 14.6p per mile.
Calculating fuel cost for an electric vehicle
Electric vehicle efficiency is measured in miles per kilowatt-hour (miles/kWh) or kilowatt-hours per 100 miles (kWh/100mi). The cost calculation uses your electricity unit rate in pence per kWh.
Formula: Cost = (Distance ÷ Miles per kWh) × Unit rate (p/kWh) ÷ 100
Example: A 200-mile journey in an EV averaging 3.5 miles/kWh, charging at 28p/kWh: Energy needed = 200 ÷ 3.5 = 57.1 kWh. Cost = 57.1 × 28p = £16.00. Considerably cheaper than the petrol equivalent — but only if charging at home rates. Rapid motorway chargers can cost 70-80p/kWh or more, which changes the comparison significantly.
Understanding MPG figures
Manufacturer-quoted MPG figures are measured under standardised test conditions that rarely reflect real-world driving. Motorway driving at a steady speed typically produces better economy than urban stop-start conditions. Cold weather, roof boxes, full loads, air conditioning and aggressive acceleration all reduce economy.
The most accurate MPG figure to use is your own recent average, available from your car's trip computer or calculated manually from fuel receipts. If you only have the manufacturer's figure, applying a reduction of 15-20% gives a more realistic estimate for mixed driving.
Cost per mile and HMRC mileage rates
Cost per mile is useful for comparing vehicles, planning running costs, and calculating reimbursement. HMRC's approved mileage rates for business travel in a private car are currently 45p per mile for the first 10,000 miles and 25p per mile above that. These rates are set to cover fuel plus a contribution to vehicle wear — they are not a pure fuel cost calculation.
If you need to charge a passenger a fair share, the pure fuel cost per mile is more appropriate than the HMRC rate.
Splitting journey costs fairly
Dividing the total fuel cost equally between all occupants (including the driver) is the most common approach for shared journeys. Some groups prefer passengers to contribute only, leaving the driver to absorb the cost. The Mile Minder tool calculates cost per person for any number of passengers.
For regular car-sharing arrangements, tracking actual fuel spend over time rather than estimating each journey separately gives a more accurate basis for contributions.
Factors that affect real fuel consumption
- Speed: Fuel consumption rises sharply above 60-65 mph on most vehicles. Motorway driving at 70 mph uses noticeably more fuel than at 60 mph.
- Traffic: Stop-start conditions are significantly less efficient than steady-speed driving, because energy is wasted in braking and more is needed to accelerate repeatedly.
- Load: A fully loaded car uses more fuel than a near-empty one. Roof boxes and bike racks add aerodynamic drag even when empty.
- Temperature: Cold engines are less efficient. Short journeys in cold weather produce much worse economy than long journeys in warm weather.
- Tyre pressure: Under-inflated tyres increase rolling resistance and fuel consumption. Checking pressure before a long journey is a simple way to optimise economy.
Tolls, parking and other journey costs
Fuel is usually the largest variable cost of a journey but not the only one. Motorway tolls, bridge tolls, parking charges and congestion zone fees are separate costs that can be significant for some routes. For a complete journey budget, add these to the fuel estimate rather than treating fuel cost as the total.
Common questions about journey fuel costs
Should I include the return journey?
If you are budgeting for a round trip, yes — double the one-way distance before calculating, or run two separate calculations if the routes differ.
Why does my actual fuel spend differ from the estimate?
Real fuel economy varies with conditions. The estimate is a planning tool, not a guaranteed total. Tracking actual spend on a few journeys gives you a better personal MPG figure to use in future estimates.
Are UK and US MPG figures the same?
No. UK MPG uses the imperial gallon (4.546 litres). US MPG uses the US gallon (3.785 litres). The same MPG number represents different fuel efficiency in the two systems — UK MPG is about 20% higher for the same physical consumption.
How do I find my car's actual MPG?
Fill the tank completely, reset the trip meter, drive normally, then fill to the same level. Divide the miles driven by the gallons used. One UK gallon equals 4.546 litres, so divide litres by 4.546 to convert.
Does driving style really make a difference?
Yes, significantly. Smooth acceleration, earlier gear changes, anticipating traffic and maintaining a steady speed can improve economy by 10-20% compared with an aggressive driving style.
Related tools and guides
Use Mile Minder to calculate journey fuel cost instantly for petrol, diesel or electric vehicles. For your overall household budget including travel costs, try Budget Wise. For electricity running costs at home, see the electricity cost guide. Browse all LibraireStudios guides for more practical help.
LibraireStudios tools are designed for everyday planning and general information. Fuel prices change frequently — always use current pump prices for accurate results.