Adding GST
To add GST to a price before tax, multiply the price by the rate, divide by 100 and add the result: ₹1,000 at 18% carries ₹180 of GST, so it costs ₹1,180.
GST = price × rate ÷ 100
Taking GST out
When a price already includes GST, the price before tax is the price × 100 ÷ (100 + rate). For ₹1,180 including 18% GST that is ₹1,000, and the GST is the other ₹180.
Price before GST = price with GST × 100 ÷ (100 + rate)
CGST, SGST and IGST
On a sale within a state, GST is split equally between the centre (CGST) and the state (SGST): 18% is 9% and 9%. On a sale from one state to another, all of it is charged as IGST. Since the changes of September 2025, most goods and services are taxed at 5% or 18%, with 40% for a short list of luxury and sin goods; check the rate for your item, as some have rates of their own.