Sales tax, VAT and GST go by different names in different countries and are administered differently, but the arithmetic underneath is the same everywhere: a percentage added to a price. What varies is the rate, whether the price you are quoted already includes it, and who can reclaim it. Get those three things straight and the maths is a single multiplication.
Enter the rate that applies where the sale happens — this calculator does not assume a country, because rates range from zero to over 25% and there is no sensible default. If you are unsure of your rate, check the relevant tax authority rather than guessing; a wrong rate produces a confidently wrong number.
Adding tax to a price
Multiply by 1 plus the rate. At 10%, multiply by 1.1. At 20%, multiply by 1.2. At 8.25%, multiply by 1.0825.
$1,200 at 10%. 1,200 × 1.1 = $1,320, of which $120 is tax.
$49.99 at 20%. 49.99 × 1.2 = $59.99, of which $10 is tax.
Removing tax from a total
This is where most errors happen. If a total already includes tax, you divide by the factor — you do not subtract the rate.
$1,320 including 10% tax. Divide by 1.1: 1,320 ÷ 1.1 = $1,200 before tax, with $120 of tax. Taking 10% off $1,320 would give $1,188, understating the net figure by $12 and overstating the tax by the same amount.
$59.99 including 20% tax. Divide by 1.2: 59.99 ÷ 1.2 = $49.99 net, $10 tax.
The error grows with the rate. At 5% it is small enough to hide in rounding; at 20% it is over 3% of the total, which is more than enough to fail a reconciliation. Our reverse percentages guide explains why division is the correct operation here.
Inclusive versus exclusive pricing
Whether displayed prices include tax is a matter of local law and convention, and it is the single biggest source of confusion when buying across borders.
- In much of Europe, the UK, Australia and New Zealand, consumer prices are generally shown tax-inclusive — the shelf price is what you pay.
- In the United States and Canada, prices are usually shown tax-exclusive, with sales tax added at the register. The rate can vary by state, province, county and city, and can depend on what is being sold.
- Business-to-business quotes are frequently exclusive even in inclusive-pricing countries, so a quote that looks cheaper may simply be quoted differently.
If you are comparing two prices, confirm they are on the same basis before concluding anything.
Where people go wrong
Subtracting the rate to get the net figure. Covered above, and worth repeating because it is so common in hand-kept books.
Applying tax to a discounted price incorrectly. Usually the discount comes off first and tax is charged on the reduced price — so a discount reduces your tax too. Check the local rule if the amounts are material. The discount calculator handles the first half.
Assuming one rate covers everything. Many systems apply reduced or zero rates to food, books, children’s clothing, medicine or exports. A single site-wide rate is an assumption, not a fact.
Compounding tax on tax. Where two taxes apply, whether the second is charged on the first’s inclusive or exclusive amount is a jurisdictional rule. Do not assume they simply add.
Rounding at the wrong point. Rounding per line and rounding on the invoice total can give different results, and some tax authorities specify which is required. On a long invoice the difference is real money.
Region-specific guides
Because rates and rules are genuinely local, we keep them in separate guides rather than picking one country’s version as the default: Australian GST sets out the 10% case, and how sales tax maths works covers the general mechanics that apply anywhere.
Nothing here is tax advice. For filing, compliance or anything with a regulator attached, check with a qualified adviser in your jurisdiction — see our terms.