Scope and assumptions
Uses the post-22-Sep-2025 GST slabs (5%, 18%, 40%, 0% exempt). GST is calculated on the taxable value; the combined rate is what the customer pays (CGST+SGST or IGST behind it).
Quick navigation
- Slabs overview → gst-india-percentage
- What falls in each slab → gst-slabs-india
- What GST is → what-is-gst-india
- The calculator → Percentage Calculator
Why GST maths is one formula with four rates
India's GST looks complicated because it has four consumer-facing rates (0%, 5%, 18%, 40%), but the arithmetic behind every one is identical: GST = price × rate. The rate is just a decimal swap — 0.18 for 18%, 0.40 for 40%. Since the 22 Sep 2025 reform removed the old 12% and 28% slabs, the only rates you meet are these four. This page is the calculation reference.
1. The formula
Once you know the slab rate r for the item:
GST = Price × r Total = Price × (1 + r)
r is the decimal rate (18% → 0.18). For intra-state sales the 18% is really 9% CGST + 9% SGST, but the math on the customer's bill is the same combined number.
2. Worked examples (post-2025 slabs)
| Price | Slab | GST | Total |
|---|---|---|---|
| ₹2,000 | 18% | ₹360 | ₹2,360 |
| ₹800 | 5% | ₹40 | ₹840 |
| ₹100 | 0% | ₹0 | ₹100 |
| ₹10,00,000 | 40% | ₹4,00,000 | ₹14,00,000 |
| ₹50,000 | 18% | ₹9,000 | ₹59,000 |
Check: ₹2,000 × 0.18 = ₹360; × 1.18 = ₹2,360. ✓ ₹10,00,000 × 0.40 = ₹4,00,000. ✓
3. Reverse: price from a GST-inclusive total
If a price is shown inclusive of GST and you need the pre-tax value:
Price = Total ÷ (1 + r)
Example: a ₹1,180 total at 18% → ₹1,180 ÷ 1.18 = ₹1,000. The GST portion = ₹1,180 − ₹1,000 = ₹180.
4. Common mistakes
- Using the old 12%/28% rates — gone since 22 Sep 2025; use 5/18/40.
- Forgetting the 1 + in the total — ₹2,000 × 0.18 = ₹360 tax, but total is ₹2,360.
- Quoting only CGST or SGST — the customer pays the combined slab.
- Applying 18% to a 40% demerit item — tobacco/luxury cars are 40%.
- Taxing exempt (0%) goods — milk, eggs, fresh produce are 0%.
5. Where this is used
Every GST invoice in India uses this: a shopkeeper adding 18% to a sale, a freelancer billing 5% on services, a car dealer charging 40% on a luxury car. The formula is the same; only the slab changes.
6. Mental check
GST = price × rate, total = price × (1 + rate). For ₹2,000 at 18%: 2,000 × 0.18 = ₹360, total ₹2,360. If you get ₹36,000 you forgot to use the decimal 0.18. Reverse: ₹1,180 ÷ 1.18 = ₹1,000 net. Always confirm the current slab on CBIC / GST Council before filing.
Reverse and re-check at a glance
| You have | Formula | Result |
|---|---|---|
| Net ₹2,000 @ 18% | × 1.18 | ₹2,360 total |
| ₹1,180 incl @ 18% | ÷ 1.18 | ₹1,000 net, ₹180 GST |
| ₹800 @ 5% | × 1.05 | ₹840 total |
| ₹10,00,000 @ 40% | × 1.40 | ₹14,00,000 total |
CGST / SGST split (intra-state)
| Slab | CGST | SGST |
|---|---|---|
| 18% | 9% | 9% |
| 5% | 2.5% | 2.5% |
| 40% | 20% | 20% |
Quick reference
| You want to… | Do this |
|---|---|
| Add GST | Price × (1 + rate) |
| Remove GST | Total ÷ (1 + rate) |
| Use the right rate | 0 / 5 / 18 / 40% (post-2025) |
| Avoid error | Don't use old 12% / 28% slabs |
FAQ
Q: What is the formula for GST India? GST = price × rate; total = price × (1 + rate), using the slab rate as a decimal.
Q: What are the current GST rates? 5%, 18%, and 40% (demerit/luxury), plus 0% exempt, after the 2025 reform.
Q: How much GST on ₹1,000 at 18%? ₹180, for a ₹1,180 total.
Q: How do I reverse a GST-inclusive price? Divide the total by (1 + rate); the difference is the GST.
Q: Why do I see CGST and SGST separately? Intra-state sales split the slab into central and state portions; the combined rate is unchanged.
Q: What is taxed at 40%? Tobacco, cigarettes, aerated drinks, and large/luxury cars.
Q: Is there still a 28% GST? No — the 28% slab was removed on 22 Sep 2025; most such goods moved to 18%.
Q: Where do I confirm the rate? CBIC (https://cbic-gst.gov.in/) and the GST Council (https://gstcouncil.gov.in/).
Sources: CBIC / GST India (https://cbic-gst.gov.in/); GST Council (https://gstcouncil.gov.in/); 2025 slab simplification effective 22 Sep 2025.
Disclaimer: GST rates and the 2025 reform are set by the GST Council and can change. Informational only, not tax advice. Confirm the current rate on the official CBIC / GST Council sites or with a tax professional.
Sources and references
- CBIC / GST Indiahttps://cbic-gst.gov.in/
- GST Councilhttps://gstcouncil.gov.in/