GST (Goods and Services Tax) applies to most goods and services in India. Whether you are raising an invoice, checking a bill, or quoting a price to a customer, it helps to know how the maths works. This guide covers the two things people calculate most — adding GST and removing it — plus how the tax splits into CGST, SGST and IGST.
Adding GST to a price
If you have a base price (before tax) and want the final amount, the formula is simple:
GST amount = base price × rate ÷ 100
Total = base price + GST amount
Example: a product priced at ₹1,000 with 18% GST.
- GST amount = 1,000 × 18 ÷ 100 = ₹180
- Total = 1,000 + 180 = ₹1,180
Removing GST from an inclusive amount (reverse GST)
Sometimes you have the final, tax-included price and need to work backwards — for example, to see how much tax is inside a bill. This is called reverse GST:
Base price = inclusive amount × 100 ÷ (100 + rate)
GST amount = inclusive amount − base price
Example: a bill of ₹1,180 that includes 18% GST.
- Base price = 1,180 × 100 ÷ 118 = ₹1,000
- GST amount = 1,180 − 1,000 = ₹180
Doing this by hand is fiddly, so the GST Calculator has an "Add GST" and a "Remove GST" mode that handle both directions for you.
CGST, SGST and IGST — what's the difference?
The total GST is the same either way, but it is split differently depending on where the sale happens:
- Within the same state (intra-state): the tax is divided equally into CGST (central) and SGST (state). So 18% becomes 9% CGST + 9% SGST.
- Between two states (inter-state): the full rate is charged as a single IGST. So 18% is simply 18% IGST.
For our ₹1,000 example at 18%: an intra-state sale shows ₹90 CGST + ₹90 SGST, while an inter-state sale shows ₹180 IGST. Either way, the customer pays ₹180 in tax.
The main GST slabs
The common GST rates in India are 5%, 12%, 18% and 28%, with a special 3% rate for gold and some items taxed at 0%. The right rate depends on the product or service (its HSN/SAC code). Rates can change with GST Council decisions, so always confirm the current rate for your item before billing.
Quick reference
| You have | You want | Use |
|---|---|---|
| Base price | Final price | Add GST |
| Final (inclusive) price | Tax portion / base | Remove GST |
| Sale within a state | Tax breakup | CGST + SGST |
| Sale across states | Tax breakup | IGST |
Calculate it instantly
Instead of remembering formulas, enter your amount and rate into the free GST Calculator. It adds or removes GST, shows the CGST/SGST/IGST split automatically, and even handles a full bill with multiple items — so your invoices are correct every time.