The formula
When a price already includes GST:
Price before GST = inclusive price × 100 ÷ (100 + GST rate)
GST inside = inclusive price − price before GST
For ₹11,800 including 18% GST: ₹11,800 × 100 ÷ 118 = ₹10,000, and the GST inside is ₹1,800.
Why subtracting 18% gives the wrong answer
18% was added on top of the original price, not taken from the final price. If you do ₹11,800 − 18% you get ₹9,676 — that is ₹324 too low, because 18% of the bigger number is bigger. Dividing by 1.18 undoes the addition exactly.
Quick divisors to remember
| GST rate | Divide the inclusive price by | Share of the bill that is GST |
|---|---|---|
| 5% | 1.05 | 4.76% |
| 18% | 1.18 | 15.25% |
| 40% | 1.40 | 28.57% |
| 3% (gold) | 1.03 | 2.91% |
Examples
| Inclusive price | Rate | Before GST | GST inside |
|---|---|---|---|
| ₹11,800 | 18% | ₹10,000.00 | ₹1,800.00 |
| ₹1,050 | 5% | ₹1,000.00 | ₹50.00 |
| ₹999 | 18% | ₹846.61 | ₹152.39 |
| ₹1,40,000 | 40% | ₹1,00,000.00 | ₹40,000.00 |
| ₹52,000 | 3% | ₹50,485.44 | ₹1,514.56 |
Where this is useful
Shop prices and MRPs in India include GST, so this is how you find the tax inside a phone, a restaurant bill or a car quotation. Freelancers and small businesses use it when a client agrees an “all-inclusive” fee: the invoice must still show the taxable value and the GST separately. And if you claim input tax credit, the GST inside your purchase bills is the amount you can set off.
One last tip: round only at the end. GST is calculated on each line item and rounded to the nearest paisa, so working with the unrounded figure avoids small mismatches with the invoice.
Reverse GST for each slab, in words
At 5%, one twenty-first of a GST-inclusive price is tax (5 ÷ 105). At 18%, it is 18 parts out of 118 — a little over 15%. At 40%, it is 40 out of 140, or two-sevenths. Thinking of it this way helps when you only have a mobile calculator: multiply the inclusive price by the rate, then divide by (100 + rate).
For businesses, the taxable value you get from a reverse calculation is what goes in the “taxable value” column of your GSTR-1, and the GST goes in the CGST/SGST or IGST columns. Small differences of a paisa or two between your calculation and the invoice are normal; GST law lets suppliers round each line item. When the difference is bigger, it usually means part of the bill is exempt or taxed at another rate — a restaurant bill with packaged water, for example.
Key GST figures (checked 5 November 2026)
| Item | Figure |
|---|---|
| Main slabs | 5% (merit) and 18% (standard) |
| Special rate | 40% (luxury and sin goods) |
| Gold, silver, jewellery | 3% |
| Rough diamonds | 0.25% |
| In force since | 22 September 2025 (56th GST Council, 3 Sep 2025) |
| Latest review | 57th GST Council, 8 October 2026 — no rate changes |
FAQ
How do I remove 18% GST from ₹11,800?
Divide by 1.18: ₹11,800 ÷ 1.18 = ₹10,000 before GST, so the GST inside is ₹1,800.
How much GST is in a ₹999 item at 18%?
About ₹152.39. The price before GST is ₹846.61 (₹999 ÷ 1.18).
What share of an inclusive price is GST at 5%?
About 4.76% (5 ÷ 105). On ₹1,050 that is ₹50.
Sources (checked 5 November 2026): CBIC notification No. 9/2025-Central Tax (Rate) and related rate notifications, effective 22 September 2025; PIB release on the 56th GST Council meeting (3 September 2025); press briefing after the 57th GST Council meeting, New Delhi, 8 October 2026 (no rate changes, reported by Business Today and Upstox); CGST Act 2017 s.9 and IGST Act 2017 s.5.