Your bill says ₹1,180, “inclusive of all taxes”. How much of that is GST, and how much should the price fall when a rate is cut? Two short formulas answer both questions, and you can check any receipt with them in under a minute.
What GST actually is
The Goods and Services Tax came into force in India on 1 July 2017 and replaced a long list of older levies, including VAT, service tax, central excise duty, entertainment tax and octroi. It is charged at every stage of production, but businesses get it refunded along the way. The only person who pays it for good is the final consumer, which is why it matters on your bill.
Since 22 September 2025, the structure is much simpler than before. There are now four standard rates: 0% and 5% for essential goods and services, 18% as the standard rate, and 40% for luxury and sin goods. The old 12% and 28% slabs were removed. Some items still sit outside GST entirely, such as petrol, diesel, alcoholic drinks and electricity, and basics like fresh fruit and vegetables or dairy products are exempt.
The two formulas you need
- Adding GST to a pre-tax price: GST = price × rate ÷ 100. The final price is the pre-tax price plus that amount.
- Finding GST inside a final price: GST = final price × rate ÷ (100 + rate). The pre-tax price is final price × 100 ÷ (100 + rate).
In plain words: when the tax is already included, you cannot just take 18% of the total, because the total already contains the tax. You divide by 118 instead of 100, which “unpacks” the price into its two parts.
A real example with an 18% item
Say a product costs ₹1,000 before tax and falls in the 18% slab.
- GST: ₹1,000 × 18 ÷ 100 = ₹180
- Final price: ₹1,000 + ₹180 = ₹1,180
Now work backwards from the ₹1,180 on the bill: ₹1,180 × 18 ÷ 118 = ₹180 of GST, and ₹1,180 × 100 ÷ 118 = ₹1,000 before tax.
The common shortcut, 18% of ₹1,180, gives ₹212.40. That overstates the tax by ₹32.40 on a single item, an error that grows quickly on a large purchase or a business expense claim.
Why your invoice shows CGST and SGST
India uses a dual GST model. When the seller and buyer are in the same state, the tax is split into Central GST (CGST) and State GST (SGST), shared 50:50 between the central and state governments. On our ₹1,000 item, the invoice would show ₹90 CGST plus ₹90 SGST. That is not a double tax: it is the same ₹180, split in two halves. When the seller is in another state, or for imports, you see a single Integrated GST (IGST) line of ₹180 instead.
How much should a price fall after a GST cut?
This is where most people overestimate. A cut from 12% to 5% does not make the price 7% cheaper, because the drop is measured on the old final price, which already includes the old tax. The formula is:
New price = old final price × (100 + new rate) ÷ (100 + old rate)
- Item worth ₹2,000 before tax, moving from 12% to 5%: ₹2,240 becomes ₹2,240 × 105 ÷ 112 = ₹2,100. You save ₹140, which is 6.25% of what you used to pay, not 7%.
- Item worth ₹10,000 before tax, moving from 28% to 18%: ₹12,800 becomes ₹12,800 × 118 ÷ 128 = ₹11,800. That is a drop of about 7.8%, not 10%.
Knowing the exact figure is useful, because nobody automatically checks that a cut reaches you. After the 2025 reform, the Ministry of Finance said it would not revive the National Anti-Profiteering Authority and would rely on market competition to pass lower rates on to retail prices. If a shop still charges ₹2,240 for that item, the formula tells you exactly what the price should be.
Mistakes to avoid
- Applying the rate to a tax-inclusive price. Divide by (100 + rate), never by 100, when the tax is already inside the price.
- Subtracting rate points directly. A 10-point cut in the rate never means a 10% cheaper item.
- Assuming everything is taxed. Fuel, alcohol and electricity are outside GST, and many fresh basics are exempt, so a 0% line on a grocery bill is normal.
- Using pre-2025 rates. Any calculation that still uses 12% or 28% is based on slabs that no longer exist.
Once you have the two formulas, any bill becomes easy to read: divide by 118, 105 or 140 depending on the slab, and you know exactly how much of your money went to tax.
