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.
A quick GST calculation checklist
Before calculating, confirm whether your price is before GST or already inclusive of GST. The formula changes depending on that starting point. Also check the applicable rate for the specific goods or service instead of assuming that every invoice uses the same slab.
| Starting amount | What you need | Calculation direction |
|---|---|---|
| ₹1,000 before GST | Final bill | Add GST |
| ₹1,180 including 18% GST | Taxable value | Remove GST |
The GST Calculator supports both directions so you can check the arithmetic quickly.
Common mistakes
- Adding 18% to an amount that already includes 18% GST.
- Using a rate without confirming the applicable classification.
- Confusing the total tax rate with the CGST/SGST split for an intra-state supply.
Important: GST rates and applicability can change. Use this guide for understanding the maths and verify the current official treatment before filing or invoicing.