Common Percentage Mistakes (and a Better Way)

Updated 12 September 2026 · Everyday

A Percentage 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 percentage calculations in Australia, and how to avoid them.

1. Ignoring supply charges

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

2. Forgetting public holidays

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

3. Using the wrong units

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

4. Stacking percentages

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

5. Overlooking the detail that matters most here

A 10% rise followed by a 10% fall leaves you 1% behind where you started.

How the 25 as a % of 200 changes with total

A common mistake is getting one input slightly wrong. Here is what happens to the 25 as a % of 200 when “Total” is off by up to 20% in either direction, with everything else held steady.

Total	25 as a % of 200
160	15.63%
180	13.89%
200	12.5%
220	11.36%
240	10.42%

At 160 the result is 15.63%; at 240 it is 10.42%. 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

Percentage = value ÷ total × 100. Increase = total × (1 + p ÷ 100). Decrease = total × (1 − p ÷ 100). With typical inputs the calculator returns 25 as a % of 200 of 12.5%. 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.