GST Calculator Guide: How to Add or Remove GST Correctly
Whether you run a small business, raise invoices, or just want to check a bill, knowing how GST is calculated saves you from costly mistakes. Here's how it actually works.
What is GST?
Goods and Services Tax is India's unified indirect tax on most goods and services. Common rates are 5%, 12%, 18% and 28%, depending on the item. GST is added on top of the base price of a product or service.
CGST and SGST: the split
For sales within the same state, GST is split into two equal halves: CGST (Central GST) and SGST (State GST). So 18% GST becomes 9% CGST + 9% SGST. For inter-state sales, it's a single IGST instead. Our calculator shows the CGST/SGST split automatically.
Adding GST to a price
If a product costs ₹1,000 before tax at 18% GST, the tax is ₹180, so the final price is ₹1,180. Simple: GST amount = base × rate ÷ 100.
Removing GST (the tricky part)
This is where people slip up. If a bill of ₹1,180 includes 18% GST and you want the base price, you cannot just subtract 18% of ₹1,180. You must divide: base = total × 100 ÷ (100 + rate). So ₹1,180 ÷ 1.18 = ₹1,000 base, and ₹180 GST. Getting this wrong overstates or understates your tax.
Do it instantly
Our free GST Calculator handles both directions — add GST to a base amount, or extract GST from a tax-inclusive total — and shows the CGST/SGST breakup. No signup, runs in your browser.
Why it matters for small businesses
Accurate GST maths keeps your invoices compliant and your input-tax-credit claims correct. When in doubt, confirm the applicable rate for your product category, and consult a chartered accountant for filing.
Also useful: our Discount Calculator for sale pricing and Percentage Calculator for quick maths.
General information only, not tax advice. GST rates and rules can change — verify current rates with official sources.