How to Calculate GST in India (With Examples)

GST math looks simple until you actually try it by hand — especially the "remove GST from a total" direction, which trips up almost everyone the first time. Here's exactly how both directions work.

Adding GST to a price

This is the easy direction. If you have a base price and want to know the final price including GST:

Final price = Base price + (Base price × GST rate)

Example: a product costs ₹1,000 before tax, and GST is 18%.

GST amount = ₹1,000 × 0.18 = ₹180
Final price = ₹1,000 + ₹180 = ₹1,180

Removing GST from a total (the part people get wrong)

This is where the common mistake happens. If a price of ₹1,180 already includes 18% GST, people often calculate 18% of ₹1,180 and subtract it — which gives the wrong answer. You can't apply the percentage to the final price, because the final price is already inflated by the tax.

The correct formula is:

Base price = Final price ÷ (1 + GST rate)

Using the same example: ₹1,180 ÷ 1.18 = ₹1,000 exactly.

If you'd incorrectly taken 18% of ₹1,180 (₹212.40) and subtracted it, you'd get ₹967.60 — about ₹32 off. It's a small-looking error, but it compounds fast on large invoices or when you're reconciling books at scale.

CGST and SGST split

For intra-state transactions, GST is split evenly into CGST (Central GST) and SGST (State GST) — so 18% GST becomes 9% CGST + 9% SGST, not an additional 18% on top. The total tax burden is still 18%, just divided between the two.

What about interstate transactions?

The CGST/SGST split only applies when the buyer and seller are in the same state. When goods or services cross state lines, the tax is charged as a single IGST (Integrated GST) instead — at the same total rate, just not divided into two components. So an 18% interstate transaction is 18% IGST, not 9%+9%. This distinction matters for how the invoice is structured, even though the total tax paid is identical either way.

Input Tax Credit, briefly

If you're GST-registered and buying goods or services for your business, you can usually claim back the GST you paid as Input Tax Credit (ITC) — effectively meaning you only pay GST on the value you add, not the full price at every stage. This is why GST is called a value-added tax. For everyday personal calculations (working out a final retail price, for instance), ITC doesn't come into play — it only matters if you're a registered business filing returns and offsetting tax already paid against tax collected.

Current GST slabs

Rate	Typical use
3%	Precious metals like gold, silver
5%	Essential goods, some food items
12%	Standard processed goods
18%	Most goods and services (the default rate)
28%	Luxury and sin goods

GST rates are set by the GST Council and do occasionally change for specific goods, so if you're unsure which slab applies to a particular product or service, check the latest official notification rather than assuming based on category alone.