GST Calculator: How It Works
GST arithmetic is only two operations — adding tax to a base price, and stripping tax out of a price that already includes it — but the second one is where nearly every manual error happens. This page covers both directions, the slab structure, and the CGST/SGST/IGST split that has to appear correctly on an invoice.
Adding GST versus removing it
To add GST to a price that excludes it:
GST = Base × Rate ÷ 100 · Total = Base + GST
To remove GST from a price that already includes it:
Base = Total × 100 ÷ (100 + Rate) · GST = Total − Base
The common mistake is taking 18% off an inclusive price. On an inclusive total of 1,180 at 18%, subtracting 18% gives 967.60 — wrong. The correct base is 1,180 × 100 ÷ 118 = 1,000, and the GST is 180. The error grows with the rate: at 28% it is over 6%.
The slabs
| Rate | Typically applies to |
|---|---|
| 0% | Fresh produce, unbranded staples, most books, certain healthcare and education services |
| 5% | Packaged essentials, economy transport, small restaurants, life-saving drugs |
| 12% | Processed foods, business-class air travel, some apparel and footwear tiers |
| 18% | The default for most goods and the great majority of services — IT, telecom, financial and professional services |
| 28% | Luxury and demerit goods; often with an additional cess |
Slab assignment is set by law and revised periodically, so verify the current rate for your specific HSN or SAC code before invoicing. This calculator applies whichever rate you enter.
CGST, SGST and IGST
The rate you charge is the same either way; only the split changes.
| Supply | Components on an 18% invoice |
|---|---|
| Within one state (intra-state) | CGST 9% + SGST 9% |
| Between states (inter-state) | IGST 18% |
| Union territory | CGST 9% + UTGST 9% |
Which applies is determined by the place of supply, not by where your office is. Getting this wrong is one of the most common invoice corrections, because the tax paid under the wrong head cannot simply be reclassified — it typically has to be refunded and re-paid.
A worked invoice
Consulting services, 50,000 before tax, supplied within the same state:
| Taxable value | 50,000.00 |
|---|---|
| CGST @ 9% | 4,500.00 |
| SGST @ 9% | 4,500.00 |
| Invoice total | 59,000.00 |
Supplied to another state, the same invoice shows IGST 18% = 9,000.00 and the same 59,000.00 total.
Input tax credit, in one paragraph
A registered business offsets the GST it paid on purchases against the GST it collected on sales, and pays the difference. Collect 9,000 and pay 5,400 on inputs, and 3,600 goes to the government. The credit is only available if your supplier actually filed and paid — which is why reconciling purchase records against the auto-populated return matters, and why an invoice from an unregistered supplier carries no credit.
Reverse charge
On certain supplies the buyer pays the tax directly instead of the seller collecting it. Common cases include specified services from unregistered suppliers, goods transport agency services, and legal services from an advocate. The invoice must state that tax is payable under reverse charge, and the buyer records it as both a liability and, where eligible, a credit.
Rounding and presentation
Tax amounts are normally rounded to two decimals per line and the invoice total to the nearest whole unit. Show the taxable value, each tax head with its rate, and the total separately — a single combined 'GST' line on an intra-state invoice is not compliant, because the CGST and SGST components must be visible.