How to Calculate GST in India: CGST, SGST & IGST Explained (Free Calculator)

What Is GST and How Is It Calculated?

GST (Goods and Services Tax) is an indirect tax added to almost every product and service in India. You calculate it as a percentage of the base price, using one of five rates: 0%, 5%, 12%, 18%, or 28%.

The GST Calculator does the math instantly, but knowing the formula helps you understand your invoices and check whether you are being charged correctly.

The Two Ways GST Is Calculated

Forward calculation (add GST to a base amount)

GST = Base amount × GST rate

If a product costs ₹1,000 and attracts 18% GST:
- GST = 1,000 × 18% = ₹180
- Final price = ₹1,180

Reverse calculation (remove GST from a bill total)

Base amount = Total ÷ (1 + GST rate)

If you paid ₹1,180 total and the GST rate is 18%:
- Base = 1,180 ÷ 1.18 = ₹1,000
- GST inside the bill = ₹180

The reverse mode is useful when you are reimbursing expenses or working out taxable value from a receipt.

CGST, SGST, and IGST: The Split

GST is divided based on where the transaction happens:

Transaction	Tax split	Example
Within the same state	Half to Central (CGST), half to State (SGST)	Delhi to Delhi: 9% CGST + 9% SGST = 18%
Between different states	Full amount as IGST	Delhi to Mumbai: 18% IGST
Import / export	IGST applies	Imported goods: IGST

So at 18% within your own state, the tax is printed on the invoice as 9% CGST + 9% SGST, not one 18% line. The GST Invoice Generator fills in this split automatically for you.

GST Rates You Will Actually See

Category	GST rate
Essential food items	0%
Packaged foods, some services	5%
Processed foods, computers	12%
Electronics, furniture, most services	18%
Luxury goods, cars, tobacco	28%

Step-by-Step: Calculate GST on Any Product

1. Find the base price (before tax) or the bill total (after tax).
2. Pick the right GST rate for the product category.
3. Use forward or reverse math from above.
4. Split the central tax into CGST and SGST (half each) for same-state sales.
5. Cross-check with the free GST Calculator.

Common GST Mistakes to Avoid

• Adding 18% on an amount that already includes GST — you must divide, not multiply, to get the base.
• Forgetting the CGST/SGST split on invoices — it is mandatory on same-state invoices.
• Rounding each tax line separately — GST rounds per item, keep the GST amount as a single sum, then split.
• Using the wrong rate — hotels, cabs, and AC restaurants each have different effective rates.

For business owners, pair the calculator with the Invoice Generator to produce clean, GST-ready invoices and the Profit Margin Calculator to price products correctly before tax.

FAQ

Is GST always 18%? No. GST has five slabs — 0%, 5%, 12%, 18%, and 28% — depending on the product or service.

How do I remove GST from a total? Divide the total by (1 + rate). For 18%, divide by 1.18 to get the base amount.

What is the difference between CGST and SGST? CGST goes to the central government and SGST to the state government. Both are charged only on same-state (intra-state) sales, each at half the total rate.

Can I calculate GST without an online calculator? Yes. Use base × rate to add GST, or total ÷ (1 + rate) to remove it. But the free GST Calculator is faster and avoids rounding errors.

Use the GST Calculator for any amount, rate, or direction — it runs instantly in your browser and is completely free.