Whether you are pricing a product, checking a bill, or filing returns, you need to calculate GST two ways: adding it to a base price, and extracting it from a total that already includes it. The formulas are simple once you see them. Here is exactly how to do both, with examples for every GST slab.
Open GST CalculatorThe Two GST Calculations
GST-exclusive means tax is added on top of the listed price (common in B2B quotes). GST-inclusive means the price already contains the tax and you need to work backwards to find it (common on retail bills). You will use both regularly.
Adding GST to a Price (Exclusive)
- GST amount = Price × (GST% ÷ 100)
- Final price = Price + GST amount
- Example: ₹1000 at 18% → GST = 1000 × 0.18 = ₹180 → Final = ₹1180
Extracting GST From a Total (Inclusive / Reverse GST)
- Base price = Total ÷ (1 + GST% ÷ 100)
- GST amount = Total − Base price
- Example: ₹1180 inclusive at 18% → Base = 1180 ÷ 1.18 = ₹1000 → GST = ₹180
Reverse GST trips people up most. Remember: divide by 1.18 for 18%, by 1.05 for 5%, by 1.12 for 12%, by 1.28 for 28%.
CGST + SGST vs IGST
The total GST is the same; only the split differs. For a sale within the same state, GST splits equally into CGST and SGST — so 18% becomes 9% CGST + 9% SGST. For a sale between different states, the full amount is charged as IGST (18%). The maths above is identical; only how you record it on the invoice changes.
Calculate GST Instantly
- 1Open the WowShortcuts GST Calculator and enter the amount.
- 2Choose the GST rate (5%, 12%, 18%, 28% or custom).
- 3Pick inclusive or exclusive — the tool shows the base, the GST amount, and the total, with the CGST/SGST split.
Conclusion
GST is simple once you know the two formulas: multiply to add it, divide by (1 + rate) to extract it. For quick, error-free numbers — including the CGST/SGST breakdown — use the free WowShortcuts GST Calculator, and create a matching bill with the Invoice Generator.