The basic formula for petrol cost
To find out how much petrol will cost you, divide the distance you plan to drive by your car's fuel consumption rate, then multiply by the current petrol price per litre. The formula is: (Distance ÷ Fuel consumption) × Price per litre = Total cost.
For example, if you drive 300 kilometres, your car uses 7 litres per 100 kilometres, and petrol costs £1.50 per litre, the calculation is: (300 ÷ 100) × 7 × £1.50 = £31.50. This gives you the petrol cost for that specific journey.
The accuracy of this calculation depends on knowing three things: your actual distance, your car's real fuel consumption, and the current petrol price. Each of these can vary, so understanding where to find reliable figures matters more than the maths itself.
Key Takeaways
- Divide your total distance by 100, multiply by your car's litres-per-100km figure, then multiply by the price per litre to find the cost of a single journey.
- Your car's fuel consumption varies by driving conditions, so check your manual for the official combined figure or track your own consumption over several fill-ups.
- Petrol prices change daily at different stations, so use a price-comparison site or check your local supermarket fuel prices to find the current rate in your area.
- For regular commuting, calculate your weekly or monthly cost by multiplying your daily distance by the number of days you drive, then explore the formula.
Finding your car's fuel consumption rate
Every car has a fuel consumption figure, but the one printed in the manual or on the manufacturer's website is a test result, not what you will see in real driving. The official combined figure assumes a mix of motorway and urban driving under controlled conditions. Your actual consumption depends on how you drive, traffic conditions, road type, and whether you carry extra weight.
The most reliable way to find your real consumption is to fill your tank completely, drive normally for at least 200 kilometres, then fill up again and note how many litres you added. Divide the distance by the litres to get your consumption per kilometre, then multiply by 100 to express it as litres per 100 kilometres. Repeat this over several weeks to account for seasonal variation and different driving patterns.
If you want a quicker estimate, use the manufacturer's combined figure but add 10 to 20 percent to account for real-world conditions. Many modern cars also display fuel consumption on the dashboard or through a connected app, though these figures can be optimistic by 5 to 15 percent.
Checking current petrol prices
Petrol prices vary between stations and change daily, sometimes several times a day. The price you saw yesterday is not the price you will pay today. Before calculating a journey cost, check what petrol actually costs near you right now.
Supermarket fuel is often cheaper than branded forecourts, sometimes by 10 to 20 pence per litre. Use a price-comparison site such as PetrolPrices.com or Confused.com to see prices at stations within a few miles of your location. These sites update frequently and let you filter by fuel type (unleaded, diesel, super unleaded) and station brand.
If you fill up regularly at the same station, you may have a loyalty card that gives a discount per litre. Factor this into your calculation if you use it consistently. For a one-off journey, check prices along your planned route as well as near home, because a cheaper station further along the motorway might save you money despite the extra distance.
Calculating cost for regular commuting
If you drive the same route most days, calculate your weekly or monthly petrol cost by working out your total distance first. Measure the distance from home to work and back, then multiply by the number of days you drive each week. For example, a 25-kilometre round trip driven five days a week is 125 kilometres per week.
explore the petrol cost formula to this weekly total: (125 ÷ 100) × 7 × £1.50 = £13.13 per week. Multiply by 52 to find your annual cost, or by 4.3 to estimate a monthly average. This gives you a realistic budget for fuel rather than guessing.
Commuting costs can change seasonally because fuel consumption rises in winter (cold engines run less efficiently) and petrol prices fluctuate. Recalculate every few months to stay accurate, or use a slightly higher estimate to account for price rises and seasonal variation.
Accounting for driving conditions that affect consumption
Your fuel consumption is not fixed. Motorway driving at a steady speed uses less petrol than stop-start urban driving, even over the same distance. Idling in traffic, frequent acceleration, and driving in cold weather all increase consumption. If your journey involves a mix of road types, you may need to calculate sections separately.
A 100-kilometre motorway journey at 70 mph might use 6 litres, but the same distance through city traffic could use 9 litres. If you drive 50 kilometres on the motorway and 50 kilometres in town, use different consumption figures for each section. Check your car's dashboard display or your own tracking data to see how consumption changes between motorway and urban driving.
Carrying extra weight, towing a trailer, or driving with roof racks also increases consumption. If you regularly carry passengers or luggage, factor in an extra 5 to 10 percent. For occasional trips with heavy loads, calculate those journeys separately from your normal commute.
Using online calculators and apps
Several free online tools let you enter your distance, fuel consumption, and petrol price, then calculate the cost automatically. Google Maps shows distance between two points; you can then plug that into a petrol cost calculator. Apps like Fuelly let you log each fill-up and track your consumption over time, which removes guesswork from the formula.
These tools are useful for checking your mental maths, but they are only as accurate as the figures you enter. If you use an outdated petrol price or an optimistic fuel consumption rate, the result will be misleading. Always verify the inputs before trusting the output.
Some car manufacturer apps (such as those linked to connected car services) show real-time fuel consumption and can estimate range on your current fuel level. These are helpful for planning long journeys, but remember they are estimates based on your recent driving pattern, not predictions of what will happen on a different route or in different conditions.
Planning for fuel costs on longer journeys
For a long trip, break the route into sections and calculate the cost for each. A 500-kilometre journey might include 200 kilometres of motorway, 200 kilometres of A-roads, and 100 kilometres of urban driving. Use your car's consumption figures for each road type, then add them together.
Check petrol prices along your planned route, not just at home. A cheaper station 100 kilometres into the journey might be worth planning a stop for, especially on a long trip. Some motorway services are significantly more expensive than nearby supermarket forecourts just off the motorway, so a small detour can save money.
Budget for a small margin above your calculated cost. Unexpected traffic, poor weather, or a less efficient route than planned can increase consumption. Adding 10 percent to your calculated cost gives you a realistic buffer without being wasteful.
Frequently Asked Questions
How do I know if my car's fuel consumption is accurate?
Track your own consumption over several fill-ups by dividing the distance driven by the litres added at each fill-up. If your average differs from the manufacturer's figure by more than 15 percent, your driving style or conditions are significantly different from the test conditions. Use your own tracked figure for calculations rather than the official one.
Should I use the combined, urban, or motorway fuel consumption figure?
Use the combined figure for mixed driving. If your journey is mostly motorway, use the motorway figure (usually lower). If it is mostly urban, use the urban figure (usually higher). For a mix, calculate each section separately and add them together.
Does petrol price include tax?
Yes, the price per litre shown at the pump includes fuel duty and VAT. You cannot separate these out for your calculation — use the displayed price as it is.
Why is my actual fuel cost higher than my calculation?
The most common reasons are that your real fuel consumption is worse than you estimated, petrol prices have risen since you calculated, or you are driving in conditions (heavy traffic, cold weather, extra weight) that increase consumption. Recalculate using your actual consumption figures and current prices.
Can I calculate petrol cost if I don't know my exact distance?
Use Google Maps or your sat-nav to measure the distance before you travel. If you are calculating for a regular commute, measure the route once and use that figure each time. For a rough estimate without exact distance, use your journey time and average speed to estimate distance, but this is less accurate than measuring.