Two formulas, and the one people get wrong
Adding GST is easy. Multiply the price by the rate, then add it on. A ₹25,000 service at 18% carries ₹4,500 of GST, so the invoice total is ₹29,500, with ₹2,250 each of CGST and SGST.
Removing GST from a price that already includes it is where mistakes happen. The formula is base = inclusive price ÷ (1 + rate/100), and then GST = inclusive price − base. Take a bill of ₹1,180 at 18%. Divide by 1.18 and you get a base of ₹1,000, so the GST is ₹180. The shortcut many people use, 18% of 1,180, gives ₹212.40 and a base of ₹967.60. That is ₹32.40 off, because the 18% applies to the base, not to the total.
How the slabs work now
Since the 56th GST Council meeting on 3 September 2025, and effective 22 September 2025, the main slabs are 0%, 5%, 18% and 40%. Most items that sat at 12% moved to 5%, and most of those at 28% moved to 18%. The 40% rate covers sin and luxury goods such as tobacco, aerated drinks, large cars and motorcycles above 350cc. Some special rates remain, including 3% on gold and 0.25% on rough diamonds. Individual life and health insurance premiums were exempted. That is why this page has a Custom option: pick it for 3% and type the rate.
The calculator does not know which rate your product falls under. That depends on its HSN code (for goods) or SAC code (for services). If you issue invoices, look up the code on the GST portal or ask your accountant, and do not copy the rate from a competitor's bill.
Worked example: a ₹5,250 bill at 5%
A small business buys goods with a printed total of ₹5,250 at 5% GST. To find what it can claim as input tax credit, remove the GST. 5,250 ÷ 1.05 = ₹5,000. GST is ₹250, which splits into ₹125 CGST and ₹125 SGST when both buyer and seller are in the same state. Had the seller been in another state, the same ₹250 would appear as IGST on the invoice, a single line. The total payable is the same either way. What changes is which government receives which part, and how credit flows afterwards.
The same ₹1,000 base at the 40% slab gives GST of ₹400 and a total of ₹1,400. Here the higher the rate, the bigger the gap between the shortcut and the right answer: 40% of ₹1,400 would be ₹560 instead of ₹400.
CGST, SGST and IGST in plain terms
- Within one state (seller and place of supply in the same state): the rate is split equally. At 18%, that is 9% CGST to the Centre and 9% SGST to the state. In union territories without a legislature the state part is UTGST.
- Between states: IGST at the full rate, collected by the Centre and shared with the destination state.
- Place of supply matters, not the buyer's address on the bill. For services especially, the rules can be different from goods. If unsure, ask a tax professional.
Common mistakes
- Subtracting the rate from the inclusive price, as in the ₹967.60 example above.
- Calculating GST after a discount in the wrong order. If the discount is shown on the invoice, GST is generally charged on the reduced value. Check how your invoice treats it.
- Applying a flat 18% to everything because "most things are 18%". Food, books, medicines, hotel rooms and transport all have their own treatment.
- Rounding each line and then the total, which can leave a one-rupee difference from what billing software prints. This calculator rounds the GST to paise and splits it so that CGST + SGST always equals the GST exactly.
What it does not do
It handles one rate on one amount. It does not apply the reverse charge mechanism, cess, TCS, composition scheme rules, or different rates on a mixed supply. It is a check for individual bills, not a replacement for GST filing software. If you also run loan or investment numbers for the same business, the EMI calculator and the percentage calculator cover those side questions.
Questions people ask
How do I calculate GST from a MRP or inclusive price?
Divide by 1 plus the rate as a decimal. For 18%, divide by 1.18. For 5%, divide by 1.05. The difference between the MRP and the result is the GST contained in it.
Is the GST amount the same for CGST plus SGST and for IGST?
Yes. At 18% the total tax is 18% in both cases. Only the labels change: 9% plus 9% inside a state, 18% IGST between states.
Which GST rate should I choose?
The one that applies to your specific item or service, found by its HSN or SAC code. The default of 18% is only a starting point. If you are not sure, check the official GST rate finder or ask your CA.
Can I use this for a GST-registered invoice?
Use it to check the numbers, but the legal invoice must come from your billing or accounting system with the GSTIN, HSN or SAC and place of supply details.