GST (Goods and Services Tax) replaced over a dozen indirect taxes in India on 1 July 2017. Understanding how to calculate GST is essential for business owners, accountants, and consumers alike. This guide covers every scenario you will encounter.
The four GST slabs
India uses four primary GST rate slabs:
- 5% — Essential goods: packaged food, household necessities, economy hotel rooms (under ₹1,000/night), economy class air travel
- 12% — Processed foods, business class air travel, non-AC restaurants, mobile phones
- 18% — Most services (IT, consulting, telecom), restaurants with AC, most manufactured goods, financial services
- 28% — Luxury goods: cars, motorcycles above 350cc, aerated drinks, cigarettes, 5-star hotels
There is also a 0% rate for exempted goods (fresh vegetables, milk, eggs, books) and a 3% rate for gold and jewellery.
How to calculate GST — the formula
Adding GST to a base price:
GST Amount = Base Price × GST Rate / 100
Final Price = Base Price + GST Amount
Example: Product costs ₹10,000 before GST. GST rate = 18%.
GST = ₹10,000 × 18/100 = ₹1,800
Final price = ₹10,000 + ₹1,800 = ₹11,800
Reverse GST calculation (when price includes GST):
Base Price = Inclusive Price × 100 / (100 + GST Rate)
GST Amount = Inclusive Price − Base Price
Example: You paid ₹11,800 inclusive of 18% GST.
Base = ₹11,800 × 100/118 = ₹10,000
GST = ₹11,800 − ₹10,000 = ₹1,800
CGST, SGST, and IGST — the split
When a transaction is intrastate (buyer and seller in the same state), GST splits equally between Central GST (CGST) and State GST (SGST). For an 18% GST transaction: CGST = 9%, SGST = 9%.
When a transaction is interstate (buyer and seller in different states), the entire GST is collected as Integrated GST (IGST) by the central government. For 18% GST: IGST = 18%.
Common GST calculation mistakes
- Calculating GST on an already GST-inclusive price (double counting)
- Using the wrong slab — many items changed slabs in the 2023 and 2024 GST Council meetings
- Forgetting the CGST/SGST split when issuing invoices
- Not charging GST on services because "it is just a small consultancy"
Use the free GST Calculator
Rather than calculating manually every time, use our free GST Calculator which handles all four slabs, intrastate/interstate split, and reverse calculations instantly.