How GST is added and removed
GST (goods and services tax) is charged as a percentage of the pre-tax price. This calculator has two modes, switched with the Add GST and Remove GST buttons, and a rate dropdown with the current Indian rates (the 5%, 18% and 40% slabs plus the special 3% and 0.25% rates) and a Custom rate option for anything else. The three results are the base amount, the GST amount and the total.
- Add GST (you enter the price excluding GST):
GST = base × rate / 100andtotal = base + GST. - Remove GST (you enter the price including GST):
base = total ÷ (1 + rate / 100)andGST = total − base.
Worked examples
Adding 18% to 1,000 gives GST of 180 and a total of 1,180. Going the other way, if a bill says 1,180 including 18% GST, the base is 1,180 ÷ 1.18 = 1,000 and the GST is 180.
The classic error is taking 18% off the inclusive total: 18% of 1,180 is 212.40, which would give a wrong base of 967.60. GST was charged on the base, not on the total, so you must divide by 1.18 instead. Likewise, if 1,000 is the tax-inclusive price at 18%, the base is 1,000 ÷ 1.18 = 847.46 and the GST is 152.54.
Side-by-side at each rate
| Rate | Add to base of 1,000 | Remove from total of 1,000 |
|---|---|---|
| 0.25% | GST 2.50, total 1,002.50 | Base 997.51, GST 2.49 |
| 3% | GST 30.00, total 1,030.00 | Base 970.87, GST 29.13 |
| 5% | GST 50.00, total 1,050.00 | Base 952.38, GST 47.62 |
| 18% | GST 180.00, total 1,180.00 | Base 847.46, GST 152.54 |
| 40% | GST 400.00, total 1,400.00 | Base 714.29, GST 285.71 |
Things to check before you rely on the number
- Which rate applies. The rate depends on the item or service classification (HSN or SAC code), and the GST Council revises slabs from time to time. The restructure that took effect on 22 September 2025 removed the old 12% and 28% slabs, moving most items to 5% or 18% and luxury and sin goods to a new 40% rate. Individual items are still reclassified from time to time, so confirm the current rate for your goods on the official GST portal. The dropdown is a calculator, not a rate lookup. For an older invoice issued at 12% or 28%, or for GST in another country, choose Custom rate and type the percentage.
- CGST, SGST and IGST. For a sale within one state, the total GST is split equally between central and state GST, so 18% appears on an invoice as 9% CGST plus 9% SGST. For a sale across states it is charged as a single IGST at the full rate. The total is the same either way.
- Rounding. This tool displays up to two decimal places. Invoices usually round each line or the invoice total as the law and your accounting software specify, so a tiny difference of a paisa is normal.
- Input credit. Registered businesses can often offset GST paid on purchases against GST collected on sales, so the GST on a bill is not necessarily a cost to them. This calculator only computes the tax on one amount.
For invoice layout and line-by-line tax see the guide on calculating GST on an invoice, and what GST is for background. To work out a rate yourself, use the percentage calculator.