GST is easy to calculate once you know two formulas: one to add GST to a price, and one to take it out of a price that already includes it. Most mistakes come from using the wrong one, usually by subtracting a percentage from an inclusive price. This guide shows both, with the CGST, SGST and IGST split.
Add GST to a price
GST amount = price × rate ÷ 100 Total price = price + GST amount
Example: a service costs ₹5,000 and GST is 18%. GST = 5,000 × 18 ÷ 100 = ₹900. The customer pays ₹5,900.
Remove GST from an inclusive price
Price before GST = inclusive price ÷ (1 + rate ÷ 100) GST amount = inclusive price − price before GST
Example: an MRP of ₹1,180 includes 18% GST. Price before GST = 1,180 ÷ 1.18 = ₹1,000, so the GST is ₹180.
CGST, SGST and IGST
- Same state (supplier and place of supply): charge CGST and SGST, each half of the rate. At 18%, that is 9% + 9%.
- Different states, imports and exports: charge IGST at the full rate, 18% in this example.
On ₹1,000 at 18%: CGST ₹90 + SGST ₹90 = ₹180 within a state, or IGST ₹180 between states. The total tax is the same; only the head it is reported under differs.
Current GST rates
Since 22 September 2025 (often called GST 2.0), most goods and services are taxed at 5% or 18%, with a 40% rate for luxury and sin goods. Special rates of 0.25% (rough diamonds) and 3% (gold and silver) remain, and many essentials are exempt. The older 12% and 28% slabs still appear on invoices raised before the change. Always confirm the rate for your product or service using its HSN or SAC code.
Rate quick table
| Price before GST | 5% GST | 18% GST |
|---|---|---|
| ₹1,000 | ₹50 → ₹1,050 | ₹180 → ₹1,180 |
| ₹10,000 | ₹500 → ₹10,500 | ₹1,800 → ₹11,800 |
| ₹50,000 | ₹2,500 → ₹52,500 | ₹9,000 → ₹59,000 |
Putting GST on a bill
A tax invoice must show the taxable value, the rate and the CGST, SGST or IGST amounts separately. The GST Invoice Generator does the split for you from the place of supply, and the GST invoice format guide lists every required field.
Frequently asked questions
How do I calculate 18% GST quickly in my head?
Take 10% (move the decimal), add most of another 10%, and trim: for ₹2,500, 10% is 250, 8% is 200, so GST is ₹450. For inclusive prices, divide by 1.18 or multiply by about 0.847.
Is GST charged on the discounted price?
Generally GST applies to the transaction value after a discount that is shown on the invoice at the time of supply. Check the rules for post-sale discounts with your accountant.