The two directions, and why they're not symmetric
"GST exclusive" means you have a base price and want to add GST on top of it — you multiply the base price by the rate and add it. "GST inclusive" means you have a final price that already contains GST and want to work backward to find the base price and the GST portion within it — this needs division, not the same multiplication run in reverse, because the GST rate applies to the base price, not to the final price.
A common mistake is taking a GST-inclusive final price and simply multiplying it by the rate to find the "GST amount" — this overstates the tax, since the rate is meant to apply to the base price, which is smaller than the inclusive final price.
Exclusive — adding GST to a base price
Formula: GST amount = base price × rate. Total = base price + GST amount.
On a ₹10,000 base price at 18% GST: GST amount = ₹10,000 × 18% = ₹1,800. The final, GST-inclusive price the customer pays is ₹11,800.
Inclusive — extracting GST from a final price
Formula: Base price = final price ÷ (1 + rate). GST amount = final price − base price.
On a ₹11,800 final price at 18% GST: base price = ₹11,800 ÷ 1.18 = ₹10,000, and the GST amount within it is ₹11,800 − ₹10,000 = ₹1,800 — the same numbers as the exclusive example above, confirmed by working in the opposite direction. Note this is NOT ₹11,800 × 18% = ₹2,124, which is the mistake described earlier.
Splitting into CGST and SGST
For a sale within the same state, the GST amount splits equally into Central GST (CGST) and State GST (SGST) — an 18% rate becomes 9% CGST + 9% SGST, so the ₹1,800 GST in the example above splits into ₹900 CGST and ₹900 SGST. For a sale between different states, the full amount is instead charged as Integrated GST (IGST) — the total tax collected is the same either way, just structured differently for how it's shared between the central and state governments.
Which one do you need?
Use the exclusive calculation when you're quoting or invoicing from a known base price (common for B2B quotes). Use the inclusive calculation when you're given a final, all-in price (common for retail listings and receipts) and need to know the tax component — for filing, accounting, or claiming input tax credit.