How to Work Out GST in Australia (And the Mistake Nearly Everyone Makes)
AustraliaAustralian GST is 10%. Adding it is easy — pulling it back out of a total is where people go wrong, and the error costs real money on every invoice.
Guides
The calculators give you the answer. These explain the maths behind it — including the mistakes that cost people money.
How Sales Tax, VAT and GST Maths Works
Adding tax is one multiplication. Pulling it back out of a total is where the errors live — and the method is the same whatever your rate happens to be.
30% Off Plus a Further 20% Off Is Not 50% Off
Stacked discounts multiply, they do not add. The combined saving is always smaller than the two numbers suggest — and retailers know exactly how good that looks.
Margin vs Markup: The Confusion That Quietly Kills Small Businesses
A 50% markup is a 33% margin. Businesses that treat them as the same number price their products too low and cannot work out why the money never arrives.
Percentage Increase vs Increase By a Percentage: Two Different Questions
These sound identical and are not. One finds the percentage, the other finds the new value — and mixing them up is the most common percentage error there is.
Percentage Points vs Percent: The Error You Read Every Day
A rate moving from 2% to 3% rose by one percentage point — or by 50%, depending on which you mean. Reporting picks whichever sounds better.
How to Work Out Percentages in Your Head
Four tricks that cover almost every percentage you will ever need to do mentally — including the reversal trick that makes hard ones trivial.
Reverse Percentages — Finding the Number Before the Change
You know the price after a discount and want the price before it. Subtracting the percentage back gives the wrong answer, and it is wrong in a predictable direction.
How to Split a Bill and Work Out a Tip
The maths takes five seconds. The awkward parts are uneven orders, tipping on tax, and rounding — which is where the table usually ends up short.
How to Work Out What Percent One Number Is of Another
Divide the part by the total and multiply by 100. The whole difficulty is knowing which number is the part — get that backwards and the answer is confidently wrong.
Why a 50% Rise Then a 50% Fall Does Not Get You Back
Percentages are not symmetrical. Up 50% then down 50% leaves you 25% below where you started — and the same asymmetry quietly distorts every set of figures you read.