1. Build the full distance
Count the outbound and return journey, local driving at the destination and likely detours. If a mapping app shows 430 km one way, the trip is already at least 860 km before local travel. Add a modest distance buffer rather than assuming every route decision will be perfect.
2. Use your real fuel consumption
A vehicle label or brochure provides a standard comparison figure, but your recent average is usually more useful for budgeting. Divide litres used by kilometres travelled and multiply by 100 to calculate L/100 km. Use several fill-ups if possible, because one tank can be distorted by traffic or measurement timing.
Speed, hills, tyre pressure, air conditioning, roof boxes and heavy loads can all affect consumption. If the trip conditions are harder than normal, use a slightly higher L/100 km figure.
3. Calculate litres before cost
For a 900 km trip at 7.5 L/100 km, expected fuel use is 900 × 7.5 ÷ 100, or 67.5 litres. Keeping this intermediate number visible makes the estimate easier to update when the price changes.
4. Use a realistic fuel price
Prices can differ by location and fuel grade. Use the grade your vehicle requires and a recent price from the route or destination. Multiplying 67.5 litres by 2.18 gives an estimated cost of 147.15 in the same currency as the entered price.
5. Add a travel buffer
A 10% planning buffer would bring 147.15 to about 161.87. Keep tolls, parking, maintenance and food separate so the fuel estimate remains easy to audit. After the trip, compare actual litres and distance with the plan; that record will improve the next estimate.