How to calculate GST: adding it, removing it and avoiding the common mistake
Add GST to a price, take it out of a GST-inclusive amount, and split it into CGST and SGST. Clear examples and the one mistake almost everyone makes.
Whether you run a shop, prepare invoices or just want to know what an item “really” costs, you will often need to add GST to a price or take it back out. The arithmetic is simple, but one very common mistake produces wrong numbers on invoices every day. This guide shows both calculations with examples.
Rates change. GST rates are set by notification and have been revised several times, and different goods and services attract different rates. The examples use 18% only because it makes the arithmetic easy to follow. Always confirm the rate for your item in the official rate schedule or with your accountant.
Adding GST to a price
When the price is before tax, multiply by the rate and add the result:
GST = Price × Rate ÷ 100 Total = Price + GST
Example: a product priced ₹1,000 with 18% GST. GST = 1,000 × 18 ÷ 100 = ₹180. The total is ₹1,180. A shortcut: multiply by 1.18 to get the total directly.
Removing GST from a GST-inclusive price
When the price you have already includes GST, you divide, you do not subtract:
Price before GST = Total ÷ (1 + Rate ÷ 100)
Example: a bill total of ₹1,180 that includes 18% GST. Price before GST = 1,180 ÷ 1.18 = ₹1,000, so the GST in the bill is ₹180.
The common mistake
Many people try to remove GST by taking 18% off the total: 1,180 × 18% = ₹212.40, then 1,180 − 212.40 = ₹967.60. That is wrong. The 18% was calculated on ₹1,000, not on ₹1,180, so you have removed too much. The correct pre-tax price is ₹1,000.00, and the GST is ₹180. On a large invoice, this mistake adds up to real money, and it can lead to a mismatch with the GST return.
| Method | Result for ₹1,180 (18% GST included) |
|---|---|
| Correct: divide by 1.18 | ₹1,000.00 before GST, ₹180.00 GST |
| Wrong: subtract 18% of the total | ₹967.60 (off by ₹32.40) |
CGST, SGST and IGST
GST on a sale is split depending on where the buyer and seller are:
- Within one state: the tax is split equally between the Central GST (CGST) and the State GST (SGST). 18% GST becomes 9% CGST plus 9% SGST. On a ₹1,000 sale that is ₹90 plus ₹90.
- Between two states: the whole amount is charged as Integrated GST (IGST). 18% IGST on ₹1,000 is ₹180.
The total tax is the same either way. The split matters for how the invoice is written and where the tax goes.
Use the calculator
The GST calculator does both directions. Choose “Add GST” or “Remove GST”, enter the amount and pick a rate from the buttons or type your own, including rates like 0.25% or 3%. It shows the pre-tax price, the GST and the total, with the CGST and SGST split alongside IGST.
Related calculations
- Discounts and GST together. GST is normally charged on the price after the discount. Apply the discount first, then add the tax. The percentage calculator handles the discount step.
- Pricing for profit. If you set your selling price from cost, work out margin before tax. The margin and markup calculator shows both measures.
- Rounding. Round only the final amounts on an invoice, not intermediate steps, to avoid pennies of drift across many lines.
This article is general information about the arithmetic of GST, not tax advice. For compliance questions, such as which rate applies or how to file returns, speak to a chartered accountant or the official GST portal.