How to Calculate GST on a Product (With Real Examples)

GST calculations trip people up not because the maths is hard, but because of one question: is the amount you have GST-inclusive or GST-exclusive? Get that wrong and every number after it is wrong too. Here's how both directions actually work, with real numbers.

Exclusive GST: adding tax to a base price

If you have a base price and need to add GST on top, the formula is: GST amount = base price × (GST rate / 100), and total = base price + GST amount. For example, a ₹1,000 product at 18% GST: GST = 1,000 × 0.18 = ₹180. Total price = ₹1,180.

Inclusive GST: extracting tax from a final price

If the price you have already includes GST and you need to find the base price and tax split, the formula is: base price = final price / (1 + GST rate/100), and GST amount = final price − base price. For a ₹1,180 final price at 18% GST: base = 1,180 / 1.18 = ₹1,000. GST amount = 1,180 − 1,000 = ₹180 — the same numbers as before, just worked backwards.

Splitting GST into CGST and SGST

For a sale within the same state, GST is split equally into Central GST (CGST) and State GST (SGST) — an 18% rate becomes 9% CGST + 9% SGST, shown as two separate lines on the invoice. For an inter-state sale, the full 18% is charged as a single IGST (Integrated GST) line instead. The total tax collected is identical either way; only how it's split and reported changes.

Step-by-step with a calculator

1. Open the GST Calculator and enter your amount.
2. Choose whether that amount is GST-inclusive or exclusive.
3. Select the applicable rate — 5%, 12%, 18%, 28%, or a custom rate.
4. Read off the base amount, GST amount, and total, already split into CGST/SGST.