Working out how to add or remove GST/VAT is easier with GST/VAT Calculator, which answers as you type. Type Amount and Tax Rate (%) into GST/VAT; each keystroke re-runs the math. GST/VAT prints the result and its working. GST/VAT suits budgeting and planning checks. GST/VAT is privacy-first: Everyday Life inputs stay offline. GST/VAT is part of the Everyday Life collection on CalcProMaster, alongside free gst/vat calculator, free calculators and more. GST/VAT is designed for real people. Labels stay plain. No sign-up or email — GST/VAT answers as you type.
What the defaults give
$100 with 18% GST is $18.00 of tax and a $118.00 total.
The tax is charged on the pre-tax amount, which is why the total is exactly 118% of the price.
The formula
Tax = amount × rate ÷ 100
Total = amount + tax
Adding tax is straightforward. Removing it is the step people get wrong: dividing a tax-inclusive figure by 1.18 is correct, while subtracting 18% is not, because the 18% was charged on the smaller pre-tax base. On $118 the difference is 60 cents — small per bill, but it compounds across invoices and it is the usual reason a reconciliation does not balance.
Worked examples
$100 at 18% → tax $18.00, total $118.00
$50 at 18% → $9.00, $59.00
$200 at 18% → $36.00, $236.00
$100 at 9% → $9.00, $109.00
$100 at 36% → $36.00, $136.00
Doubling the rate doubles the tax; halving it halves the tax. The relationship is linear, so two items at 12% cost the same tax as one item at twice the price.
Rates worth knowing
GST is a single national rate in some countries and a slab structure in others. India runs 5%, 12%, 18% and 28% across categories, with 18% the standard rate on most goods and services; Australia and New Zealand use 10% and 15%; Singapore 9%; Canada 5% federal plus provincial tax; the UK uses 20% VAT with reduced rates of 5% and 0%. Always use the rate for the specific supply, not the headline rate.
For a business, GST is usually neutral: tax collected on sales is offset against tax paid on purchases, and only the difference is remitted. That is why an invoice needs the tax shown separately rather than buried in a total.
Rounding and thresholds
Tax authorities differ on rounding: some require per-line rounding to the nearest cent, others allow rounding on the invoice total. On a long invoice the two approaches can differ by more than one cent, which is enough to fail a matching check.
Registration thresholds also change what you should be charging at all. Below the threshold a supplier may not be registered and cannot charge GST, so a quote that includes it is wrong regardless of the arithmetic.
Frequently Asked Questions
How much GST is added to $100 at 18%?
$18.00, making the total $118.00. The tax is 18% of the pre-tax amount.
How do I remove GST from a total?
Divide by 1 plus the rate. $118 ÷ 1.18 is $100. Subtracting 18% from $118 gives $96.76, which is wrong.
Why is subtracting the rate incorrect?
The rate was charged on the smaller pre-tax base, so it is a smaller share of the total. Dividing by 1.18 reverses the multiplication exactly.
Does GST apply to services as well as goods?
Generally yes in countries with a GST or VAT, though some services are exempt or zero-rated. The rate depends on the specific supply.
Is GST a cost for a business?
Usually not. Tax collected is offset against tax paid on inputs, so only the net difference is remitted. The tax is a cost to the final consumer.
Days Between Dates and other Everyday tools
Days Between Dates · Poker Flush Odds Calculator · Gift Wrap Calculator · Age Calculator · Concrete Calculator · Sleep Cycle Calculator · Everyday Guides