How to Calculate GST: Inclusive vs Exclusive Prices, CGST, SGST and IGST

Whether you run a shop, freelance, or just want to check a bill, you will regularly need to do two GST calculations: add GST to a base price, or work out how much GST is hidden inside a final price. Both take one line of maths.

Adding GST to a price (exclusive)

Example: a service costs ₹1,000 before tax at 18% GST. GST = ₹1,000 × 18 ÷ 100 = ₹180. The customer pays ₹1,180.

Removing GST from a price (inclusive)

This is where people make mistakes. You cannot take 18% of ₹1,180 — that gives ₹212.40, which is wrong. Use this instead:

Example: ₹1,180 × 18 ÷ 118 = ₹180. The base price is ₹1,180 − ₹180 = ₹1,000.

CGST, SGST and IGST

• Sale within the same state (intra-state): the GST is split equally into CGST (central) and SGST (state). At 18%, that is 9% + 9%.
• Sale to another state (inter-state): the whole amount is charged as IGST. At 18%, that is 18% IGST.
• Union territories without a legislature use UTGST in place of SGST.

The total tax is the same either way; the split only decides which government receives it and which input credit the buyer can claim.

Current GST rates

GST rates were rationalised from 22 September 2025. Most goods and services now fall under 5% or 18%, with a 40% rate for a small list of luxury and "sin" goods, and special rates for items such as gold. Many essentials are exempt. Rates are linked to the HSN code (goods) or SAC code (services), so always confirm the current rate for your specific item on the GST portal or with your accountant.

Common invoicing mistakes

1. Quoting a price without saying whether GST is included. Write "₹1,000 + GST" or "₹1,180 incl. GST".
2. Charging CGST + SGST on an inter-state sale (or IGST on an intra-state sale). The place of supply decides this.
3. Rounding each line differently from the invoice total. Round once, at the invoice level.
4. Missing the buyer's GSTIN on B2B invoices, which blocks their input tax credit.