Complete guide
Reviewed July 2026GST (Goods and Services Tax) is India's unified indirect tax, charged as a percentage of a product or service's value. Whether you're a business issuing invoices, a shopper checking a bill, or a freelancer quoting a client, two questions come up constantly: how much GST to add to a price, and how much GST is already baked into a total.
This calculator does both. In exclusive mode it adds GST to a base amount (the invoicing case); in inclusive mode it extracts the GST already contained in a final price (the reverse-GST case). It works with every standard slab — 3%, 5%, 12%, 18% and 28% — and shows the base, the tax, and the total.
Below: the forward and reverse formulas, worked examples, the CGST/SGST/IGST split, the slab structure, and the mistakes that cause invoice errors.
GST formulas: adding and removing
Add GST (exclusive): GST = Base × rate/100 Total = Base + GST Remove GST (inclusive): Base = Total × 100/(100 + rate) GST = Total − Base
The forward direction is simple multiplication. The reverse — extracting GST from a tax-inclusive price — trips people up: you cannot just take 18% of the total, because the total already includes that tax. You must divide by (100 + rate), not multiply.
Worked examples
- Add GST: a service priced ₹1,000 at 18% → GST = 1000 × 0.18 = ₹180; total invoice = ₹1,180.
- Remove GST: a bill shows ₹1,180 inclusive of 18% → base = 1180 × 100/118 = ₹1,000; GST = ₹180.
- Common error: taking 18% of ₹1,180 = ₹212.40 is WRONG — that over-states the tax because the total already includes it.
- Different slab: ₹2,500 product at 12% → GST ₹300, total ₹2,800. Reverse: ₹2,800 ÷ 1.12 = ₹2,500 base.
GST slabs and the CGST/SGST split
18% is the workhorse slab covering most services and many goods. Rates and category placements are revised periodically by the GST Council, so verify the current rate for your specific HSN/SAC code before invoicing.
| Rate | Typical goods & services |
|---|---|
| 0% | Fresh produce, milk, unpacked grains, most books |
| 3% | Gold, silver and jewellery |
| 5% | Household essentials, economy transport, small restaurants |
| 12% | Processed foods, business-class air, some electronics |
| 18% | Most services, electronics, industrial goods (the common slab) |
| 28% | Luxury and 'sin' goods: cars, tobacco, aerated drinks |
How the tax splits: CGST, SGST and IGST
For a sale within the same state (intra-state), the GST is split equally between the Central government (CGST) and the State government (SGST) — so 18% becomes 9% CGST + 9% SGST. For a sale across state lines (inter-state), the whole amount is charged as a single Integrated GST (IGST). The total the customer pays is identical either way; only the government split differs.
Using this calculator
- Enter the amount — the base price if you're adding GST, or the final price if you're removing it.
- Pick the GST rate for your product/service category.
- Choose the mode: 'Add tax (exclusive)' to build an invoice total, or 'Remove tax (inclusive)' to extract GST from a displayed price.
- Read the base, tax and total. For intra-state sales, halve the tax figure to get CGST and SGST separately.
Common mistakes
- Extracting GST by taking the rate of the inclusive total — always divide by (100 + rate) instead.
- Using the wrong slab; check the current rate for your exact HSN/SAC code, as they change.
- Forgetting to split intra-state GST into CGST + SGST on the invoice.
- Charging IGST on an intra-state sale (or vice versa) — determined by the place of supply, not where you're based.
- Rounding each line differently; round the final tax and total consistently to avoid reconciliation mismatches.
Frequently asked questions
Glossary
- GST
- Goods and Services Tax — India's unified indirect tax on goods and services.
- GST-exclusive
- A price before tax; GST is added on top to reach the total.
- GST-inclusive
- A price that already contains the tax; the label is what you pay.
- CGST / SGST
- Central and State GST — the two equal halves of intra-state GST.
- IGST
- Integrated GST — the single tax on inter-state supplies.
- Input tax credit
- Offsetting GST paid on purchases against GST collected on sales.
- HSN / SAC
- Classification codes for goods (HSN) and services (SAC) that set the applicable rate.
- MRP
- Maximum Retail Price — a GST-inclusive ceiling price for consumer goods.
Key takeaways
Adding GST is multiplication (base × rate); removing it is division by (100 + rate) — never take the rate of a tax-inclusive total. Know your slab (18% is most common), split intra-state tax into equal CGST + SGST and inter-state into IGST, and always itemise tax on invoices to preserve input-tax credit. The master factor (1 + rate/100) handles both directions.
Enter your amount and rate above, pick add or remove — and next time a bill shows an inclusive total, you'll extract the exact GST in seconds.