Add GST to a price or take it out of a GST-inclusive price, and see the CGST + SGST split (sale within your state) or IGST (sale to another state). The Excel formulas for the same calculation are under the result.
How the GST calculation works
Adding GST: GST = price × rate. A ₹10,000 item at 18% has ₹1,800 GST, so the invoice total is ₹11,800.
Removing GST: a GST-inclusive price is not reduced by the rate directly. Divide by (1 + rate): ₹11,800 ÷ 1.18 = ₹10,000 taxable value, so the GST inside is ₹1,800. Taking 18% off ₹11,800 (₹2,124) is a common mistake.
CGST + SGST or IGST?
When the seller and the place of supply are in the same state, the GST is split equally into CGST (central) and SGST (state), so 18% becomes 9% + 9%. For a sale to another state, the whole rate is charged as IGST.
Do it in Excel
See GST inclusive and exclusive calculation in Excel and the GST sales register lesson with a practice file.