Whether you are writing an invoice, checking a bill or pricing a product, you will often need to add GST to a price or work out how much GST is already included. The maths is simple once you know two formulas. This guide explains both, with worked examples, and shows how GST is split into CGST and SGST in India.
Want the answer instantly? Use the free ToolWixo GST Calculator.
Formula 1: Adding GST to a price
Use this when you have a price before tax (the net or taxable value).
GST amount = Price × GST rate ÷ 100
Total price = Price + GST amount
Example at 18%: a service costs ₹1,000 before tax.
- GST = 1,000 × 18 ÷ 100 = ₹180
- Total = 1,000 + 180 = ₹1,180
Shortcut: multiply by 1.18 (for 18%) or 1.05 (for 5%).
Formula 2: Removing GST from an inclusive price
Use this when the price you have already includes GST, for example a shop price or a total on a bill.
Price before GST = Inclusive price ÷ (1 + rate ÷ 100)
GST amount = Inclusive price − Price before GST
Example at 5%: an item costs ₹2,100 including GST.
- Price before GST = 2,100 ÷ 1.05 = ₹2,000
- GST included = 2,100 − 2,000 = ₹100
Common mistake: do not simply take 5% of ₹2,100 (that gives ₹105, which is wrong). The tax was charged on the pre-tax price, so you must divide.
CGST, SGST and IGST explained
In India, GST on a sale within the same state is split equally between the central and state governments:
- CGST (Central GST) = half the rate
- SGST (State GST) = half the rate
So for the ₹1,000 example at 18%: CGST 9% = ₹90 and SGST 9% = ₹90, total ₹180.
For a sale between two different states, the full rate is charged as IGST (Integrated GST): ₹180 IGST in the same example. The total tax is the same; only the label on the invoice changes.
The GST Calculator shows the CGST and SGST split for you.
Which GST rate should I use?
Since India’s GST rate rationalisation in September 2025, most goods and services fall into two main slabs, 5% and 18%, with a higher 40% rate for a small list of luxury and sin goods. Some essentials are exempt, and a few items such as gold have special rates. Rates depend on the exact product or service, so always confirm the rate for your item from official GST sources or your tax adviser.
Does this work for VAT?
Yes. VAT and sales tax use exactly the same maths. Enter your local rate (for example 15% or 20%) and the formulas work the same way.
Frequently asked questions
How do I calculate 18% GST on ₹500?
500 × 0.18 = ₹90 GST, so the total is ₹590.
How do I find the price without GST from ₹1,180 at 18%?
1,180 ÷ 1.18 = ₹1,000 before GST, and ₹180 is the GST.
Is GST charged on the discounted price?
Generally, a discount shown on the invoice at the time of sale reduces the taxable value, so GST is charged on the price after discount. Use the Discount Calculator to get the sale price first.
Related calculators
This article explains the arithmetic only and is not tax advice.
