How it works
To add GST, multiply the price by the rate and add the result. To remove GST from a price that already includes it, divide by (100 + rate) and multiply by 100 to get back to the price before tax.
For a sale within one state, the GST is split equally between CGST (central) and SGST (state or union territory). For a sale between states, the whole amount is charged as IGST.
The formula
Price before GST → GST = price × rate ÷ 100, total = price + GST.
Price with GST → price before GST = total × 100 ÷ (100 + rate), GST = total − price before GST.
A worked example
A price of ₹1,180 that includes 18% GST: the price before GST is 1,180 × 100 ÷ 118 = ₹1,000, and the GST is ₹180. Within a state that is ₹90 CGST and ₹90 SGST.
Frequently asked questions
Which GST rate applies to my product or service?
It depends on the item’s classification (HSN code for goods, SAC for services). Most goods and services moved to 5% or 18% from 22 September 2025, with 40% for a small set of items, and a few special rates such as 3% for gold and 0.25% for some stones still apply. Rates change, so confirm the rate for your item on the GST portal or with a tax professional.
What is the difference between CGST, SGST and IGST?
CGST goes to the central government and SGST to the state when a sale is within one state. IGST is charged on sales between states and on imports, and is shared between the centre and the destination state.
Does this make a valid tax invoice?
No. It only does the arithmetic. A tax invoice needs the details the law requires, such as GSTIN, invoice number and HSN or SAC codes.
How is rounding handled?
The figures are shown to the paisa. Invoices often round each line or the total to the nearest rupee, so a printed invoice can differ by a few paise.