Discount: What to Check Before You Trust the Number

Updated 26 August 2026 · Business and gst

A Discount 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 discount calculations in Australia, and how to avoid them.

1. Spending GST collected

GST belongs to the ATO. Keep it aside until your BAS is due.

2. Undercharging as a contractor

Your rate must cover super, leave, unbilled time and business costs, not just the salary you want.

3. Forgetting on-costs

An employee costs well above their salary once super, workers comp and payroll tax are included.

4. Mixing up margin and markup

A 50% markup is a 33% margin. Pricing with the wrong one can erase profit.

5. Overlooking the detail that matters most here

Two discounts don’t add up. 20% off then 10% off is 28% off, not 30%.

How the sale price changes with discount

A common mistake is getting one input slightly wrong. Here is what happens to the sale price when “Discount (%)” is off by up to 20% in either direction, with everything else held steady.

Discount (%)	Sale price
20	$160.00
22	$156.00
25	$150.00
28	$144.00
30	$140.00

At 20 the result is $160.00; at 30 it is $140.00. 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

Sale price = price × (1 − discount ÷ 100). With typical inputs the calculator returns sale price of $150.00. 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.