By the Tooliday Team. Last updated 14 November 2026.
Whether you run a small business, freelance, or just want to check a bill, you will often need to add GST to a price or find out how much tax is hidden inside a total. The maths is simple, but one common shortcut gives the wrong answer. This guide explains both directions with examples. You can check any figure with our GST Calculator.
What GST is, in short
GST (Goods and Services Tax) is a tax on the supply of goods and services in India. The rate depends on what is being sold. For most goods and services, the main rates in use are now 5% and 18%, with a higher 40% rate for certain luxury and demerit goods, after the rate changes of September 2025. Some items are exempt or taxed at special rates, and rates can change, so always confirm the rate for your item on the official GST portal or with a tax professional.
Adding GST to a price
GST amount = price × rate ÷ 100
Total = price + GST amount
Example: a service costs ₹1,000 before tax and the rate is 18%. GST is 1,000 × 18 ÷ 100 = ₹180, so the total is ₹1,180. A faster way is to multiply by 1.18.
Another example: goods priced at ₹2,100 with 5% GST. GST is ₹105, so the total is ₹2,205.
Removing GST from a total
This is the direction where mistakes happen. Many prices you see already include GST, and you may need the base price for your accounts.
Price before GST = total × 100 ÷ (100 + rate)
GST amount = total − price before GST
Example: a bill is ₹1,180 including 18% GST. Price before GST = 1,180 × 100 ÷ 118 = ₹1,000, and the GST inside it is ₹180.
The common mistake
It is tempting to take 18% of ₹1,180, which gives ₹212.40, and subtract it to get ₹967.60. That is wrong. The 18% was charged on the original ₹1,000, not on the total. The reliable method is to divide by 1.18, as above. For 5% GST, divide by 1.05, and for 40%, divide by 1.40.
| GST rate | Add GST: multiply by | Remove GST: divide by |
|---|---|---|
| 5% | 1.05 | 1.05 |
| 18% | 1.18 | 1.18 |
| 40% | 1.40 | 1.40 |
CGST, SGST and IGST
The total GST is the same, but how it is shown on an invoice depends on where the sale happens.
- Within one state: the GST is split equally into CGST (central) and SGST (state). With 18% GST on ₹1,000, that is ₹90 CGST and ₹90 SGST.
- Between two states: the full amount is charged as IGST. The same sale would show ₹180 IGST.
Working with several items
If an invoice has items at different rates, calculate each item separately and add the results. Applying one rate to the whole total will give a wrong figure.
Common mistakes
- Taking a percentage of the GST-inclusive total instead of dividing by (1 + rate).
- Using the wrong rate for the item. Always confirm the rate that applies.
- Mixing up the base price and the total when entering numbers.
- Rounding too early. Round the final amounts, not the steps in between.
Try it yourself
Use the GST Calculator to add or remove GST at any rate and see the CGST, SGST or IGST amounts. If the price also has a discount, work out the discounted price first with the Discount Calculator, then add GST.
This article is for general information and is not tax advice. GST rates and rules change, so confirm them on the official GST portal or with a qualified professional.