Goods and Services Tax (GST) is added to most things sold in India. Shop bills and invoices show prices in two ways, before tax (exclusive) or with tax already included (inclusive), and it is easy to mix them up.
Adding GST to a price
GST amount = price × rate ÷ 100, and the final price is the original price plus that amount.
Example: an item costs ₹1,000 before tax at an 18% rate. GST is ₹180, so the customer pays ₹1,180.
Finding GST inside an inclusive price
When the price already includes tax, don't just take 18% of it, that overstates the tax. Instead:
Price before tax = inclusive price × 100 ÷ (100 + rate)
Example: ₹1,180 including 18% GST gives 1,180 × 100 ÷ 118 = ₹1,000 before tax, so the GST is ₹180.
CGST, SGST and IGST
- For a sale within the same state, the tax is split equally into CGST (central) and SGST (state), 18% becomes 9% + 9%.
- For a sale from one state to another, the whole amount is charged as IGST.
Which rate applies?
GST rates depend on the product or service and are revised by the GST Council from time to time. Always check the current rate for your item on the official GST portal or with your accountant before preparing an invoice.
The GST Calculator does both directions for you, and the Invoice Generator builds a GST invoice with the CGST/SGST split.