Adding GST and removing it are not the same sum
Adding GST is straightforward: take the price, work out the percentage, add it on. Removing GST from a price that already includes it is where nearly every mistake happens, because you cannot simply subtract the same percentage. The GST was calculated on the lower figure, so taking 18% off the higher one removes too much.
The correct move is to divide rather than subtract. A price that includes 18% GST is 118% of the pre-tax price, so dividing by 1.18 recovers the original. On ₹1,180 that gives ₹1,000 and ₹180 of GST. Subtracting 18% instead would have given ₹967.60 — wrong by over ₹32, and wrong in a direction that understates your tax liability.
Using this GST calculator
Enter the amount and the rate. The Calculation setting in the customize panel decides which direction you are working in — adding GST to a price that excludes it, or removing GST from a price that already includes it. The results show the GST amount, the price before GST and the price after, so you can see the whole picture regardless of which end you started from.
The rate field defaults to 18%, the most common Indian slab, but takes any rate. The four standard slabs are 5%, 12%, 18% and 28%, and which applies depends on the goods or services — this calculator will not guess it for you, because getting the classification wrong is a compliance problem no calculator can solve.
CGST, SGST and the split
On an intra-state supply, GST is levied half by the centre and half by the state: 18% GST is 9% CGST plus 9% SGST. That is why the results show both, each at half the total — they are not additional taxes on top, they are the same tax divided between two authorities, and an invoice is expected to show them separately.
For an inter-state supply the split does not apply. A single IGST is charged instead, at the full rate, and collected by the centre before being apportioned. The total is identical either way, so the GST amount on this page is correct for both cases — only the presentation on the invoice changes.
Rounding, and a worked check
A useful habit: add GST and then remove it, and see whether you land back where you started. On ₹1,000 at 18%, adding gives ₹1,180; removing 18% from ₹1,180 gives ₹1,000 exactly. If a supplier's invoice does not reverse cleanly, the rate on it is probably not the rate you assumed.
Round figures are the exception rather than the rule, though. Dividing by 1.18 rarely produces a tidy number, so a GST-inclusive price of ₹500 breaks down to ₹423.73 plus ₹76.27 — correct, and not round. Invoices are generally rounded to the nearest rupee at the total, which is why a line-by-line recalculation can differ from an invoice by a rupee or two without either being wrong.
For a straightforward percentage of a number, the percentage calculator is quicker, and the discount calculator handles the related question of tax applied after a markdown.
FAQ
Is GST calculated before or after a discount?
After. GST applies to the transaction value — what the buyer actually pays — so a discount shown on the invoice reduces the taxable amount, and GST is charged on the reduced figure.
So a ₹1,000 item with a 10% trade discount is taxed on ₹900, giving ₹162 of GST at 18% and a total of ₹1,062. Charging GST on the full ₹1,000 first and discounting afterwards would overstate the tax. The one caveat is that the discount has to be recorded on the invoice itself to reduce the taxable value.
How do you add GST to a price?
Multiply by the rate and add it on.
Total = Price + GST
How do you remove GST from an inclusive price?
You divide — you do not subtract the percentage. This is the mistake almost everyone makes.
Take 1,180 at 18%. Subtracting 18% gives 967.60, which is wrong. Dividing by 1.18 gives 1,000, which is right.
| Method | Result | |
|---|---|---|
| 1,180 − 18% | 967.60 | Wrong |
| 1,180 ÷ 1.18 | 1,000.00 | Correct |
The reason: the 18% was charged on the smaller number. Taking 18% off the bigger number removes too much.
What are CGST, SGST and IGST?
They're the same tax, split differently depending on where the sale happens.
| Sale | Split |
|---|---|
| Within one state | Half CGST, half SGST |
| Between two states | All of it as IGST |
An 18% intra-state sale is 9% CGST and 9% SGST. The same sale across a state line is 18% IGST. The total you pay is identical either way, which is why this calculator shows one GST figure and halves it for you.
Which GST rate should I use?
The one that applies to what you're selling. The common slabs are 5%, 12%, 18% and 28%, with 18% the most widely applied, but rates are set per category and they change.
This calculator deliberately does not guess a rate for you from a product name. Look up the rate for your goods or services, type it in, and the arithmetic will be right.
Why doesn't adding then removing GST give a round number?
It does, exactly, because the two operations are inverses: multiply by 1.18, divide by 1.18, and you're back where you started.
What you may be seeing is rounding on an invoice. Tax is normally rounded to the nearest paisa or cent per line item, so a long invoice can drift by a rupee or two against a single calculation of the total. That's the rounding, not the formula.