GST (Goods and Services Tax) is a percentage added to the price of most goods and services. Two questions come up constantly: how much GST to add to a base price, and how to remove GST from a price that already includes it. Both are short formulas, shown below with worked examples, and the free GST calculator does either instantly.
What GST is
GST is charged as a percentage of a product's taxable value — common slabs are 5%, 12%, 18% and 28%. A GST-exclusive price is the base amount before tax; a GST-inclusive price already has the tax baked in. Knowing which one you're starting from decides which formula you need.
Adding GST to a price
GST amount = base price × (rate ÷ 100)
Final price = base price + GST amount
Example — a ₹1,000 item at 18% GST. GST = 1000 × 0.18 = ₹180, so the final price is 1000 + 180 = ₹1,180.
Removing GST from an inclusive price (reverse GST)
If you only know the final GST-inclusive price and want the base and tax, you can't just take 18% of the total — that overcounts. Divide instead:
Base price = inclusive price ÷ (1 + rate ÷ 100)
GST amount = inclusive price − base price
Example — ₹1,180 including 18% GST. Base = 1180 ÷ 1.18 = ₹1,000, so the GST portion is 1180 − 1000 = ₹180. Notice 18% of ₹1,180 would be ₹212.40 — the wrong answer, which is exactly why you divide rather than multiply.
The CGST and SGST split
For a sale within a state, GST is split equally into CGST (central) and SGST (state). So 18% GST is 9% CGST + 9% SGST — on ₹1,000 that's ₹90 + ₹90 = ₹180. For inter-state sales it's a single IGST of the full 18% instead. The total tax is the same either way; only the split on the invoice differs.
FAQ
How do I remove GST from a total? Divide the inclusive price by (1 + rate/100). At 18%, divide by 1.18 to get the base price.
Why can't I just subtract 18%? Because 18% of the inclusive total is larger than the actual tax — the tax was calculated on the smaller base, not the total.
What's the difference between CGST, SGST and IGST? Within a state, GST splits into equal CGST and SGST; across states it's a single IGST for the full rate.
Is my data private? Yes — the calculator runs in your browser and stores nothing.