How to Calculate Sales Tax on a Purchase
Sales tax is a percentage added to a price, and there are two calculations: adding it, and extracting it from a total that already includes it. The Sales Tax Calculator does both.
Adding tax
Multiply the price by one plus the rate. At 8 per cent, multiply by 1.08. A 50 item becomes 54.
Doing it as two steps - work out the tax, then add it - gives the same answer and takes longer. One multiplication is less error-prone.
Working backwards, where people go wrong
Given a total of 108 that includes 8 per cent tax, the pre-tax price is not 108 minus 8 per cent, which gives 99.36. It is 108 divided by 1.08, which is 100.
The reason is that the 8 per cent was calculated on 100, not on 108. Subtracting 8 per cent of the larger number removes too much. Divide to go backwards, multiply to go forwards, and that single rule prevents the commonest error in tax arithmetic.
The tax portion itself is then the total minus the pre-tax price - 8 in this case. If you need it in one step, multiply the total by the rate divided by one plus the rate.
This matters in practice when a receipt shows only a total, when you are reclaiming tax on expenses, or when a client pays a round figure and you need the net for your accounts.
Inclusive and exclusive pricing
Two conventions, and knowing which you are looking at avoids an unpleasant surprise.
Tax-exclusive pricing shows the price before tax and adds it at the till. This is normal in the United States and Canada, which is why the amount you pay is more than the shelf label.
Tax-inclusive pricing shows the final price. This is normal across Europe and in much of the world for consumer sales, and it is often a legal requirement for goods sold to consumers.
Business-to-business invoices are usually quoted tax-exclusive in both systems, with tax shown as a separate line - which is what the Invoice Generator produces.
Rates are not one number
In the United States, sales tax is set by the state and frequently added to by counties and cities, so a single transaction can carry several layered rates and the total varies between neighbouring towns. Some categories - groceries, medicine, children's clothing - are often exempt or reduced.
Elsewhere, VAT or GST is national with reduced rates for certain goods. In either system the rate depends on where the sale takes place and what is being sold, so a calculator needs the right rate as an input and cannot know it for you.
Common questions
Why does the total differ from my estimate?
Layered local rates, a category-specific rate, or rounding applied per line rather than on the total.
Is tax calculated before or after a discount?
After the discount, on the reduced price. The Discount Calculator covers the other side.
How do I find my local rate?
Your tax authority's own lookup. Rates change and third-party lists go stale.
Does shipping get taxed?
Depends on the jurisdiction. Some treat it as part of the sale, some do not.
Got a price to add tax to, or a total to unpick?