What this calculates
Enter a price and a GST rate, and this shows the tax and the invoice total, updating as you type. It works in both directions: add GST to a pre-tax amount, or pull the GST back out of a price that already contains it.
Because it covers sales within a state, the tax is also shown as its two halves — CGST to the centre and SGST to the state — which is how it appears on an intra-state invoice. The rates offered are the slabs in force today, so a rate that has been withdrawn is not on the list.
The formula
Adding GST to a pre-tax amount is a percentage of that amount, added on top.
The tax, added on
GST = base × (rate ÷ 100)The invoice total
Total = base + GSTTaking GST back out of a tax-inclusive price is not the same operation reversed. The tax is a percentage of the base, not of the total, so you divide rather than subtract.
The pre-tax amount
Base = amount ÷ (1 + rate ÷ 100)The tax inside it
GST = amount − baseOn a sale within one state, the tax is halved between the two authorities.
The two halves
CGST = SGST = GST ÷ 2Worked example
Now run it backwards. Given only the ₹11,800 total and the 18% rate, dividing by 1.18 returns the base of ₹10,000 and leaves ₹1,800 of tax — the same two figures. The two modes are exact inverses, which is the quickest way to check an invoice you have been handed rather than one you are writing.
The wrong move is to take 18% off the ₹11,800 instead. That gives ₹9,676, understating the base by ₹324, because 18% of the total is a larger amount than 18% of the base.
When to use it, and the mistakes to avoid
Use it when raising an invoice, when checking one you have received, when a supplier quotes a price without saying whether GST is included, and when you need the CGST and SGST lines separately for your books.
The mistakes that cost the most:
- Subtracting the rate to get the base. Taking 18% off a GST-inclusive price does not undo adding 18% to the base. Divide by 1.18 instead.
- Not asking whether a quote includes GST. "₹10,000" and "₹10,000 plus GST" differ by ₹1,800 at 18%. Settle it before the work starts, not at the invoice.
- Using a slab that no longer exists. 12% and 28% were withdrawn on 22 September 2025. A template or spreadsheet still carrying them will quietly produce the wrong tax.
- Treating CGST and SGST as extra tax. They are two halves of one charge, not two charges. An 18% sale is 9% plus 9%, not 36%.
- Applying the split to an inter-state sale. Across state lines the same total is charged as a single IGST line. The tax is the same; the split is not.
- Rounding each half up. Two halves rounded independently can add to a paisa more than the tax itself. The halves should always add back to the GST figure.
FAQ
How do I calculate GST on ₹10,000?
Multiply by the rate and divide by 100. At 18%, GST on ₹10,000 is ₹1,800, so the invoice total is ₹11,800. On an intra-state sale that ₹1,800 is collected as ₹900 CGST plus ₹900 SGST — two lines on the invoice, one amount out of your pocket.
How do I remove GST from a price that already includes it?
Divide, do not subtract. Switch the calculator to 'this amount already includes GST' and it divides by 1 plus the rate: ₹11,800 at 18% comes back to ₹10,000, leaving ₹1,800 of tax. Taking 18% off ₹11,800 instead gives ₹9,676, which is wrong by ₹324 — the percentage is being taken from the wrong base.
What are the current GST slabs in India?
0%, 5%, 18% and 40%. The 12% and 28% slabs were removed by the 56th GST Council with effect from 22 September 2025, and 40% applies to a short demerit list rather than being a general slab. A few niche rates sit outside this structure, including 3% on precious metals and 0.25% on rough diamonds.
What are CGST and SGST, and why is the tax split in two?
On a sale within one state the tax is shared between the centre and the state, so a single 18% charge appears as 9% CGST and 9% SGST. It is one tax at one rate, shown as two lines. You pay the same either way — the split decides who receives it, not how much you hand over.
Does this calculator handle IGST on inter-state sales?
Not yet. It works out the tax at the rate you choose and splits it as CGST and SGST, which is the intra-state case. An inter-state sale is charged as IGST at the same full rate — so the GST and total figures here still hold, but the two halves do not apply and the invoice would carry a single IGST line instead.
Why is one half a paisa larger than the other?
Because an odd number of paise cannot be halved exactly. Extracting 18% from ₹999 gives ₹152.39 of tax, and the only way to split that into two amounts that still add to ₹152.39 is ₹76.20 and ₹76.19. The calculator puts the extra paisa on one side rather than rounding both halves up and inventing a paisa that nobody paid.