Getting the Most From a Profit Margin Calculator: Tips and Common Errors

Updated 21 August 2026 · Business and gst

A Profit Margin 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 profit margin calculations in Australia, and how to avoid them.

1. Mixing up margin and markup

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

2. Spending GST collected

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

3. Undercharging as a contractor

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

4. Forgetting on-costs

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

5. Overlooking the detail that matters most here

Work margins out on GST-exclusive prices. GST is collected for the ATO, not earned.

How the profit margin changes with selling price

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

Selling price ($)	Profit margin
80	25%
90	33.33%
100	40%
110	45.45%
120	50%

At 80 the result is 25%; at 120 it is 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

Margin = (price − cost) ÷ price. Markup = (price − cost) ÷ cost. With typical inputs the calculator returns profit margin of 40%. 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.