Calculators
VAT and GST Calculator
Add tax to a price, or work out how much tax is already inside one. The second sum is the one people get wrong, and it is the one that matters at return time. Enter the amount and the rate below, then read on for why the two directions are not symmetrical.
By Arshad Hossain · Published
The short version
- Adding tax and removing it are different sums. Removing is gross × rate ÷ (100 + rate).
- A 20% rate means the tax is one sixth of the gross figure, not one fifth.
- Invoice both figures separately: the net, the tax, then the total.
- Show each rate separately when one document mixes rates.
VAT / GST calculator
Adding tax to a net amount
This is the straightforward direction. Multiply the net figure by the rate and add it on. At 20%, a net 1,000 carries 200 of tax and the customer pays 1,200. At 5%, the same net figure carries 50 and the customer pays 1,050.
On an invoice, show all three numbers on separate lines: the net subtotal, the tax as its own line with the rate stated, and the gross total. A single combined figure is not enough for a registered buyer to reclaim the tax, and in most jurisdictions it is not a compliant invoice.
Removing tax from a gross amount
This is where the mistake happens. To find the tax inside a gross figure you cannot simply take the percentage of it. The rate was applied to the net amount, not to the total, so the tax is a smaller share of the gross than the rate suggests.
The formula is tax = gross × rate ÷ (100 + rate). At 20%, that works out as gross ÷ 6. A gross 1,200 contains 200 of tax and 1,000 of net. Taking 20% of 1,200 gives 240, which is wrong by a fifth and would leave the return short.
At 5% the divisor is 21, at 12% it is 9.33, at 18% it is 6.56. The calculator handles all of them, but the shape of the sum is worth knowing: the higher the rate, the bigger the gap between the two methods.
| Rate | Tax inside a gross 1,200 | Wrong answer (rate × gross) |
|---|---|---|
| 5% | 57.14 | 60.00 |
| 12% | 128.57 | 144.00 |
| 18% | 183.05 | 216.00 |
| 20% | 200.00 | 240.00 |
When one invoice carries more than one rate
Mixed rates on a single document are normal rather than exceptional. Zero-rated delivery alongside standard-rated goods, or an Indian invoice carrying 5% on one line and 18% on another, both need each rate shown separately with the amount it was charged on.
A single blended figure hides the information the buyer needs to reclaim correctly and the information a tax authority expects to see. Calculate each rate against its own base, then show the bases and the tax amounts as separate lines in the totals block.
Rounding
Round the tax figure to two decimal places once, at the point you calculate it, rather than rounding the inputs first. Rounding a rate or a base before multiplying introduces an error that compounds across a long invoice.
Where a document mixes rates, round each rate group separately and then add the results, rather than rounding a single combined total. That way each line of the breakdown adds up on its own terms, which is what a reviewer will check.
VAT and GST calculations: common questions
How do I remove VAT from a total?
Divide the gross amount by 1 plus the rate as a decimal to get the net, then subtract that from the gross to get the tax. At 20%, divide by 1.2. A shortcut for 20% is that the tax is exactly one sixth of the gross figure.
How do I calculate 20% VAT?
To add it, multiply the net by 0.2 and add the result. To find it inside a gross figure, divide the gross by 6. Those give different answers from the same starting number, so be clear which figure you are holding before you start.
Is GST calculated the same way as VAT?
The arithmetic is identical. The differences are in the rates, what each rate applies to, and the invoice fields each regime requires. India uses 5, 12, 18 and 28 percent bands and splits the tax into CGST and SGST for supplies within one state.
What is the reverse charge and does it change the sum?
Under a reverse charge the buyer accounts for the tax instead of the seller. You invoice the net amount, charge no tax, and state that the reverse charge applies. Your calculation stops at the net figure and the buyer does the rest.
Should I round tax up or down?
Round to two decimal places using normal rounding rather than always rounding up, and round once at the end of the calculation. Consistency matters more than direction; what causes problems is rounding at several stages so the lines stop reconciling.
Frequently Asked Questions
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Can I use it for sales tax in the US?
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