Everyday Life · Practical Life · Transport & Fuel
Fuel Cost Calculator
Calculates the total fuel cost for a journey based on distance, fuel efficiency, and price per litre or gallon.
Calculator
Formula
D = distance travelled (km or miles), E = fuel efficiency (km per litre, miles per gallon, or L per 100 km), P = price per unit of fuel (per litre or per gallon). When efficiency is expressed as L/100km, the formula becomes: Fuel Cost = (D / 100) × E × P.
Source: Standard automotive fuel economy calculation used by the UK Department for Transport and the US EPA fuel economy guidelines.
How it works
Fuel cost is determined by three factors: how far you travel, how efficiently your vehicle uses fuel, and how much that fuel costs. By combining these three values, you can calculate exactly how many litres (or gallons) your trip requires and multiply that by the current pump price to find the total spend. This is fundamental to vehicle running cost analysis and personal travel budgeting.
The core formula is Fuel Cost = (Distance ÷ Efficiency) × Price per Unit, though the exact form depends on which efficiency metric you use. For L/100km (common in Australia, Europe, and Canada), fuel used equals Distance (km) ÷ 100 × Efficiency value. For km/L, fuel used equals Distance ÷ Efficiency. For MPG (common in the UK and USA), fuel used in gallons equals Distance (miles) ÷ MPG, which is then converted to litres if needed. The calculator handles all unit conversions internally, including miles to kilometres (1 mile = 1.60934 km), US gallons to litres (1 US gal = 3.78541 L), and UK gallons to litres (1 UK gal = 4.54609 L).
Beyond the headline cost, the calculator also outputs cost per kilometre and cost per mile — figures that are particularly useful when comparing different vehicles or fuel types. Fleet operators use cost-per-km figures to assess total vehicle operating costs, while individual drivers use them to compare petrol, diesel, hybrid, and electric alternatives on a fair per-distance basis.
Worked example
Suppose you are driving from London to Edinburgh, a distance of approximately 530 km (330 miles). Your car's official fuel efficiency is 7.2 L/100km, and the current petrol price is £1.54 per litre.
Step 1 — Calculate fuel used:
Fuel used = (530 ÷ 100) × 7.2 = 5.3 × 7.2 = 38.16 litres
Step 2 — Calculate total fuel cost:
Total cost = 38.16 × £1.54 = £58.77
Step 3 — Calculate cost per km and cost per mile:
Cost per km = £58.77 ÷ 530 = £0.111 per km
Cost per mile = £58.77 ÷ 330 = £0.178 per mile
Now compare with a more fuel-efficient vehicle at 5.5 L/100km:
Fuel used = (530 ÷ 100) × 5.5 = 29.15 litres
Total cost = 29.15 × £1.54 = £44.89 — a saving of £13.88 on a single trip.
Limitations & notes
This calculator uses a fixed efficiency figure for the entire journey, which does not reflect real-world variation. Fuel consumption changes significantly with speed (motorway driving is typically more efficient than urban stop-start), air conditioning use, vehicle load, tyre pressure, gradient, and driving style. Studies by the AA and RAC suggest real-world consumption can be 10–25% higher than official manufacturer figures, particularly for older NEDC-tested vehicles. For a more realistic estimate, use your vehicle's on-board trip computer reading from a similar previous journey rather than the manufacturer's stated figure. This calculator also does not account for toll costs, parking fees, or vehicle wear — for full journey cost analysis, consider adding these separately. Electric vehicle running costs require a different calculation based on kWh per 100 km and electricity price per kWh rather than fuel volume.
Frequently asked questions
How do I convert MPG to L/100km for this calculator?
You can convert UK MPG to L/100km using the formula: L/100km = 282.48 ÷ MPG(UK). For example, 40 MPG (UK) equals 282.48 ÷ 40 = 7.06 L/100km. However, this calculator accepts MPG directly so no conversion is needed — simply select 'Miles per Gallon' as your efficiency type.
Why is there a difference between US MPG and UK MPG?
A US gallon is 3.785 litres while a UK (imperial) gallon is 4.546 litres — about 20% larger. This means a car rated at 40 MPG (UK) is equivalent to only about 33 MPG (US). Always check which gallon standard your vehicle's specification uses, as mixing them up can cause a significant error in your fuel cost estimate.
How accurate is the fuel cost estimate compared to what I actually spend?
The estimate is most accurate when you use a real-world efficiency figure from your vehicle's trip computer rather than the manufacturer's official figure. Official WLTP figures are generally within 5–10% of real-world use for modern vehicles, while older NEDC figures can underestimate consumption by up to 25%. Factors like heavy traffic, cold weather, motorway speeds above 70 mph, and a full vehicle load all increase fuel consumption beyond the stated figure.
Can I use this calculator to compare petrol vs diesel running costs?
Yes. Run the calculation twice — once with your petrol vehicle's efficiency and current petrol price, and once with a diesel vehicle's efficiency and diesel price. Diesel vehicles typically achieve 15–25% better fuel economy but diesel fuel is often priced 5–10p per litre higher in the UK. The cost-per-km output makes it straightforward to compare the two directly regardless of the unit differences.
How do I calculate fuel cost for a return trip?
Simply double the one-way distance before entering it into the calculator. For example, if your destination is 150 km away, enter 300 km as the distance to get the full round-trip fuel cost. You can also run the calculator twice if the return journey involves different road types or loading conditions that affect your vehicle's efficiency.
Last updated: 2025-01-15 · Formula verified against primary sources.