Reverse GST calculator
Pull the tax back out of a GST-inclusive price — with the division most tools get wrong.
The mistake this page exists to fix
You have ₹1,180 including 18% GST and you need the amount before tax. Almost everyone's first instinct is to take 18% off ₹1,180, which gives ₹967.60. The correct answer is ₹1,000, and it is not close — the error is ₹32.40 on a single line item.
The reason is straightforward once you see it. The 18% was charged on ₹1,000, not on ₹1,180. Going forward you multiply the smaller number by 1.18. Going backwards you divide the larger number by 1.18. Subtracting the percentage from the total takes 18% of the wrong number, and it always undershoots.
The error grows with the rate
At 5% the naive method is out by about a quarter of a per cent. At 18% it is out by nearly three per cent. At 28% it is out by more than six per cent of the base amount. On a small receipt nobody notices; across a quarter of invoices it becomes a reconciliation problem that somebody has to find.
Because it is such a common error, this page deliberately shows what the wrong method would have produced, labelled as such, next to the right answer. If a figure you have been given matches that line, you now know exactly how it was arrived at.
When you actually need this
More often than you would think. Retail prices are quoted inclusive, so working out the taxable value from a shelf price needs the reverse calculation. So does reconciling a card statement against a GST return, reclaiming input tax from a receipt that only shows a total, and working backwards from a price a customer has agreed to in order to see what you actually keep.
The split still applies
Once the tax is extracted, it splits the same way it would have going forward: half CGST and half SGST within a state, or the whole amount as IGST between states. That is what has to appear on the invoice, and the totals are identical either way.
Any rate, any country
The arithmetic is not specific to India. The same division extracts VAT from a UK gross price, EU VAT from any member state's rate, or Australian GST from an inclusive figure. Set the split to "do not split" and the page works as a general tax-extraction calculator.
Rates change — type yours in
The rate is a field rather than a fixed table. Indian GST slabs were restructured in 2025, rates depend on HSN classification, and any calculator with a hard-coded slab list eventually gives a confident wrong answer. Confirm the rate that applies to your goods and enter it.
Common questions
How do I remove GST from an amount?
Divide by (1 + rate ÷ 100). For 18%, divide by 1.18. Do not subtract 18% of the inclusive amount — that undershoots, and by more the higher the rate.
What is the difference between CGST, SGST and IGST?
They are the same total tax, split differently. Within one state it is halved into CGST and SGST; between states it is a single IGST at the full rate. The customer pays the same either way.
Which GST rate applies to my product?
That depends on its HSN classification, and it changes. This calculator takes whatever rate you give it rather than guessing, because a hard-coded slab table goes stale and a wrong rate on an invoice is a real problem.
Can I use this for VAT?
Yes. Set the split to "do not split" and it works for VAT, sales tax or any consumption tax. The inclusive-to-exclusive arithmetic is identical everywhere.
Why does the page show a "common wrong answer"?
Because that figure circulates. If a number you were handed matches it, somebody subtracted the percentage from the inclusive total instead of dividing, and now you can point at where it came from.
Is anything I type stored?
No. It runs entirely in your browser and answers as you type.