A fuel cost calculator can answer two different budget questions: what fuel will this trip consume, and how much extra cash would a fill before departure need? Enter distance, consumption, price, starting fuel, your chosen minimum ending stock and assumed usable capacity. These estimates cannot establish driving range or safety.
The distinction matters when fuel is already in the tank. Consumed fuel still has a value even when no purchase is needed. This calculator separates that value from additional pump cash, using a simple stock balance rather than treating every journey as a fresh tank purchase.
Trip fuel use vs refill cash
Budget scenario only: compare fuel used with extra pump cash for one fill BEFORE departure. Enter all six estimates; unknown is not zero.
R is your chosen minimum ending stock, not an exact target or a recommendation. Actual ending stock can exceed R. No range or safety assessment. Non-fuel costs and unknown price changes are excluded, not treated as zero.
This tool runs in your browser. Your entries are not sent or saved. All amounts are in Australian dollars (AUD).
Load example fills hypothetical figures. They are not current rates or recommendations.
Method and limits
Use = distance × L/100km ÷ 100. Buy = max(0, use + R − I). Headroom = C − I. If buy exceeds headroom, this one-fill BEFORE departure model does not apply. It does not model a single fill mid-route.
End = I + buy − use; stock change = end − I. Use value = use × price; pump cash = buy × price; stock value change = stock change × the SAME price. Use value = cash − stock value change. Do not add use value and cash.
Australian Government Green Vehicle Guide fuel calculator supports the base fuel-use/cost formula and units only. The inventory extension is ours, not government-endorsed. Input limits define this tool, not vehicle data. No live prices or historical purchase-cost account.
Loading the calculator.
What do I enter in the fuel cost calculator?
Use kilometres for trip distance, litres per hundred kilometres for assumed consumption, and AUD per litre for price. Include the distance you intend to budget for, rather than assuming a return journey has been included automatically.
- Distance, D: your trip estimate in km.
- Consumption, c: your assumed L/100km, not a vehicle-specific prediction supplied by this tool.
- Price, p: your AUD/L assumption, used both for the planned purchase and as a common comparison value for fuel stock.
- Initial stock, I: assumed usable litres held before the proposed fill.
- Minimum ending stock, R: the litres you choose to leave after the trip, not a recommended reserve.
- Usable capacity, C: your assumed usable tank capacity in litres.
A fuel gauge mark does not provide exact litres. Treat starting stock and usable capacity as assumptions, not precise measurements inferred from the display. Initial stock and minimum ending stock must each be non-negative and no greater than assumed usable capacity.
Check price units before calculating. A hypothetical price of 189.9 cents/L converts to AUD 1.899/L by dividing by 100. Entering the cents figure as dollars would distort the result. Every price in the worked examples below is fictional, not a national average or live quote. Your entered price remains an assumption for this calculation.
How is trip fuel use different from refill cash?
The base calculation is distance × consumption ÷ 100, giving litres consumed. Multiplying those litres by the AUD/L price gives consumed fuel value. This uses the distance, consumption and price relationship in the Australian Government’s Green Vehicle Guide calculator.
Our additional stock-and-cash model asks how much must be bought before departure to cover that use while retaining your chosen minimum ending stock. It subtracts fuel already held, then sets any negative purchase requirement to zero. Government sources do not endorse this extension.
Calculation details and stock reconciliation
Fuel use = D × c ÷ 100. Minimum purchase = max(0, use + R − I). Available headroom before departure = C − I. The model applies only when the minimum purchase does not exceed that headroom.
For an applicable scenario: consumed fuel value = use × p; additional pump cash = purchase × p; ending stock = I + purchase − use; stock-value change = (ending stock − I) × p.
The reconciliation is consumed fuel value = additional pump cash − stock-value change. Use unrounded values for that identity, then round money for display. Small displayed rounding differences are not extra costs.
This is one planned fill before departure, with no other fuel purchases during the modelled trip. It does not find a suitable fill somewhere along the route. If the required purchase exceeds available headroom, the scenario is outside this model.
Why can a trip consume AUD 80 but need only AUD 50?
Take a hypothetical 500 km trip at 8 L/100km and AUD 2/L. Assume initial stock of 25 L, a chosen minimum ending stock of 10 L and usable capacity of 50 L.
Fuel use is 500 × 8 ÷ 100 = 40 L, worth AUD 80. The minimum purchase is 40 + 10 − 25 = 25 L, costing AUD 50. Headroom is 50 − 25 = 25 L, so this example fits the before-departure fill model exactly.
Ending stock is 25 + 25 − 40 = 10 L. Stock falls by 15 L, valued at AUD 30 using the same assumed price. The reconciliation is AUD 80 = AUD 50 − (−AUD 30).
The missing cash has not become a discount. Part of the trip uses stock already held. Do not add AUD 80 and AUD 50 together: they describe consumption and purchasing, not separate expenses to combine.
Does no refill mean the journey is free?
Keep the hypothetical 500 km trip, 8 L/100km, AUD 2/L, 10 L minimum ending stock and 50 L capacity, but start with 50 L. Fuel use remains 40 L, worth AUD 80.
No purchase is needed: max(0, 40 + 10 − 50) = 0 L, so additional pump cash is AUD 0. Ending stock is 10 L and its value falls by AUD 80. No checkout payment does not mean a free journey.
A shorter hypothetical trip also shows why the ending-stock input is a minimum. At 100 km, 8 L/100km and AUD 2/L, with 50 L initially, a 10 L minimum and 50 L capacity, use is 8 L worth AUD 16.
No purchase is needed, and ending stock is 50 − 8 = 42 L, above the chosen minimum. The model does not remove surplus fuel or invent a refund to force ending stock down to 10 L.
Why might pump cash exceed the fuel consumed?
For another hypothetical 100 km trip at 8 L/100km and AUD 2/L, assume only 5 L initially, a 10 L minimum ending stock and 50 L capacity. The trip still consumes 8 L worth AUD 16.
The purchase becomes 8 + 10 − 5 = 13 L, costing AUD 26. It fits within 45 L of headroom. Ending stock is 5 + 13 − 8 = 10 L: a stock increase of 5 L, valued at AUD 10.
The reconciliation is AUD 16 = AUD 26 − AUD 10. The extra payment builds stock rather than increasing this trip’s consumption. That stock-value increase is a comparison entry, not profit, a refund or cash savings.
What if the required purchase will not fit?
Extend the first hypothetical trip to 800 km, keeping 8 L/100km, AUD 2/L, initial stock of 25 L, minimum ending stock of 10 L and capacity of 50 L.
Use becomes 64 L, worth AUD 128. The required purchase is 64 + 10 − 25 = 49 L, but before-departure headroom is only 25 L. This is outside the model; do not treat a hypothetical AUD 98 purchase as an executable fill.
That result does not say the trip is impossible. It says the requested stock balance cannot be achieved with this one-before-departure-fill assumption. The calculator does not plan intermediate stops, assess fuel availability or establish safe range.
Which assumptions should I check before budgeting?
The Green Vehicle Guide explains that NEDC and WLTP consumption figures come from laboratory testing. They support comparison; actual on-road consumption may vary. A label figure is not a promise that your trip will use exactly the calculated litres.
Keep the source of your consumption estimate visible when comparing scenarios. Changing that estimate changes both litres used and the required purchase. Changing starting stock can change checkout cash without changing the amount consumed by the trip.
The chosen price values old stock for comparison only. It does not reconstruct its historical purchase invoice or establish an actual refund, sale value or asset profit. Keep receipts separately if the question is what you actually paid.
For cash-flow planning, place the applicable additional pump cash alongside other spending in your Australian household budget review. Keep consumed fuel value separate so the same fuel is not counted twice.
Additional pump cash here means calculated litres purchased multiplied by the entered price. Card surcharges, transaction rounding, coupons and other adjustments are unverified and excluded, not set to zero. It is not a promised final checkout bill.
When does this fuel cost calculator not fit?
Do not use it as a fuel-stop, range or safety planner, or as advice on how much reserve to carry. Uncertain starting litres remain uncertain after calculation. Precise-looking outputs cannot validate uncertain inputs.
It also does not estimate total motoring or holiday costs. Tolls, parking, servicing, depreciation and other non-fuel expenses are excluded and unknown here, not assumed to be zero. Multiple purchase prices or intermediate fills need a different model.
Quick decision checklist
- Check distance, consumption and AUD/L units, and label each uncertain input as an assumption.
- Choose your own minimum ending stock without treating it as a reserve recommendation.
- Confirm the required purchase fits the available before-departure headroom.
- Budget additional pump cash separately from consumed fuel value, and account for excluded costs elsewhere.
Sources
- Green Vehicle Guide: Fuel cost and CO₂ calculator, for the base distance, consumption and price calculation and laboratory-use caution.
- Green Vehicle Guide: Fuel consumption label, for L/100km and NEDC/WLTP laboratory testing context.
This guide was prepared with AI assistance and source checking; it has not received professional financial or legal review.
