GST maths is simple once you know two formulas: one to add GST to a price, and one to take it out of a price that already includes GST. This guide covers both, with examples at the current rates.
Current GST slabs
From 22 September 2025, on the GST Council's recommendation, the old four-slab structure (5%, 12%, 18%, 28%) was replaced by two main rates:
- 5% merit rate for many everyday items
- 18% standard rate for most goods and services
- 40% special rate for a select few luxury and sin goods and services
Some items are exempt (nil rate) and a few have special rates, and the change for certain tobacco products was scheduled for a later date. Check the exact rate for your item or service on the official rate list before billing.
Formula 1: Add GST to a price
GST amount = Price × GST rate ÷ 100
Total price = Price + GST amount
Example (18%): A service costs ₹1,000 before tax.
GST = 1,000 × 18 ÷ 100 = ₹180
Total = 1,000 + 180 = ₹1,180
Example (5%): An item costs ₹500 before tax.
GST = 500 × 5 ÷ 100 = ₹25, total ₹525.
Formula 2: Remove GST from an inclusive price
If ₹1,180 already includes 18% GST, you can't just take 18% of ₹1,180. Use:
GST amount = Inclusive price × GST rate ÷ (100 + GST rate)
Price before GST = Inclusive price − GST amount
Example: GST = 1,180 × 18 ÷ 118 = ₹180. Price before GST = 1,180 − 180 = ₹1,000.
Example (40%): An item sells for ₹14,000 including 40% GST.
GST = 14,000 × 40 ÷ 140 = ₹4,000; price before GST = ₹10,000.
CGST, SGST and IGST
- Sale within the same state (intra-state): GST is split equally into CGST and SGST (or UTGST). At 18%, that is 9% CGST + 9% SGST. On ₹1,000: ₹90 CGST + ₹90 SGST.
- Sale to another state (inter-state): the full amount is IGST. At 18% on ₹1,000: ₹180 IGST.
Do it in one tap
Use the GST Calculator: enter the amount, pick 5%, 18%, 40% or a custom rate, and choose Add GST or Remove GST. It shows the CGST/SGST and IGST split. To bill a customer, make a proper tax invoice with the GST Invoice Generator.
Common mistakes
- Taking 18% of the inclusive price to "remove" GST. On ₹1,180 that gives ₹212.40, which is wrong.
- Charging CGST + SGST on an inter-state sale, where IGST applies.
- Using an old 12% or 28% rate for an item that moved to 5% or 18%.
This guide explains the arithmetic only and is not tax advice. GST rates depend on the exact item or service, so check the official notifications on cbic-gst.gov.in or gstcouncil.gov.in, or ask a tax professional.
Frequently asked questions
What is the formula to calculate GST?
GST amount = price × rate ÷ 100, and total = price + GST. For example, 18% on ₹1,000 is ₹180, so the total is ₹1,180.
How do I remove GST from a total amount?
GST = inclusive price × rate ÷ (100 + rate). For ₹1,180 at 18%, GST = 1,180 × 18 ÷ 118 = ₹180, so the price before GST is ₹1,000.
What are the new GST slabs?
From 22 September 2025 the main rates are 5% and 18%, with a special 40% rate for a select few luxury and sin goods and services. Some items are exempt or have special rates.
How is GST split into CGST and SGST?
Within a state, GST is split half and half: 18% becomes 9% CGST and 9% SGST. For sales to another state, the full 18% is charged as IGST.