Fuel Cost Mistakes Australians Make and How to Avoid Them

Updated 13 September 2026 · Everyday

A Fuel Cost Calculator gives a fast answer, but the answer is only as good as what goes in. These are the mistakes we see most often with fuel cost calculations in Australia, and how to avoid them.

1. Forgetting public holidays

Day counts don’t know about state public holidays, so allow for them in deadlines.

2. Using the wrong units

Australia uses litres per 100 km for fuel and cents per kWh for electricity. Converting from other units introduces errors.

3. Stacking percentages

Percentage changes compound, so they can’t simply be added or subtracted.

4. Ignoring supply charges

Electricity bills include a daily supply charge on top of usage.

5. Overlooking the detail that matters most here

Fuel prices follow weekly cycles in most capital cities. Fill up at the bottom of the cycle to save.

How the trip fuel cost changes with fuel use

A common mistake is getting one input slightly wrong. Here is what happens to the trip fuel cost when “Fuel use (L/100km)” is off by up to 20% in either direction, with everything else held steady.

Fuel use (L/100km)	Trip fuel cost
6	$58.50
7	$68.25
8	$78.00
9	$87.75
10	$97.50

At 6 the result is $58.50; at 10 it is $97.50. If a small change in this input moves the answer a lot, it is worth double-checking that figure before you rely on the result.

A quick sense check

Fuel = distance × L/100km ÷ 100. Cost = fuel × price per litre. With typical inputs the calculator returns trip fuel cost of $78.00. If your own result looks wildly different, check that each figure is in the right unit and period (weekly, monthly or yearly) before drawing conclusions.