How to Calculate GST: Adding and Removing Tax From a Price

GST calculations trip people up in one specific spot: adding tax to a price and extracting tax from a price that already includes it are two different formulas, not the same one run backward. Mixing them up gives a wrong number that still looks plausible.

Adding GST to a price (exclusive → inclusive)

If you have a price before tax and want the final price a customer pays:

GST amount = (Original price × GST rate) / 100
Final price = Original price + GST amount

Example: a ₹2,000 item at 18% GST → GST amount = (2000 × 18) / 100 = ₹360. Final price = ₹2,360.

Removing GST from a price (inclusive → exclusive)

This is where the mistake usually happens. You cannot just take 18% of the inclusive price and subtract it — that overstates the tax. Instead:

Original price = Inclusive price / (1 + GST rate / 100)
GST amount = Inclusive price − Original price

Example: a ₹2,360 price that already includes 18% GST → Original price = 2360 / 1.18 = ₹2,000. GST amount = ₹360 — not ₹2,360 × 18% = ₹424.80, which is the wrong-direction error.

CGST + SGST vs IGST

The GST amount itself is the same either way — what changes is how it's split on the invoice:

• Within the same state: the GST amount is split evenly into CGST and SGST (e.g. 18% becomes 9% CGST + 9% SGST)
• Across states: the full amount is charged as IGST, with no CGST/SGST split

Current GST slabs in India

Most goods and services fall into one of four slabs: 5%, 12%, 18%, or 28%, with a few categories exempt entirely. The applicable slab depends on the HSN/SAC code of the item — the same calculation formula applies regardless of which slab you're working with.

Our GST Calculator handles both directions — add GST to a price or extract it from an inclusive one — and shows the CGST/SGST split automatically. Paying GST late? The GST Interest Calculator works out the interest owed on a delayed payment.