GST Formula — How It's Calculated
GST (Goods and Services Tax) is India's unified indirect tax. Depending on whether your starting amount already includes GST or not, the calculation differs slightly:
To ADD GST (amount excludes tax):
GST Amount = (Original Amount × GST Rate) ÷ 100
Final Amount = Original Amount + GST Amount
To REMOVE GST (amount already includes tax):
Original Amount = (Final Amount × 100) ÷ (100 + GST Rate)
GST Amount = Final Amount − Original Amount
For transactions within the same state, GST is split equally into CGST (Central GST) and SGST (State GST) — each being half the total rate. For transactions between two different states, the full rate is charged as IGST (Integrated GST).
GST Slab Rates in India
| GST Rate | Applies To |
|---|
| 0% | Essential items — fresh vegetables, milk, eggs, unbranded food grains |
| 5% | Packaged food, medicines, economy class air travel, small restaurants |
| 12% | Computers, processed food items, business class air travel |
| 18% | Most services — IT, telecom, financial services, restaurants, electronics |
| 28% | Luxury items, automobiles, tobacco products, aerated drinks |
Example Calculation
Example 1: Adding GST (Intra-state)
A product costs ₹10,000 (excluding GST) and falls under the 18% GST slab, sold within the same state.
GST Amount = (10,000 × 18) ÷ 100 = ₹1,800
CGST (9%) = ₹900
SGST (9%) = ₹900
Final Amount = 10,000 + 1,800 = ₹11,800
Example 2: Removing GST
A product's final price is ₹11,800 (GST-inclusive) at 18% GST rate.
Original Amount = (11,800 × 100) ÷ (100 + 18) = ₹10,000
GST Amount = 11,800 − 10,000 = ₹1,800
Frequently Asked Questions
Q: What is the difference between CGST, SGST, and IGST?
CGST (Central GST) and SGST (State GST) apply to transactions within the same state — each is exactly half the total GST rate. IGST (Integrated GST) applies to transactions between two different states and is the full GST rate, collected by the central government and shared with the destination state.
Q: How do I calculate GST backwards from the final price?
Use the "Remove GST" option in the calculator above. The formula is: Original Amount = (Final Amount × 100) ÷ (100 + GST Rate). This tells you the pre-tax price when you only know the final amount.
Q: Is GST calculated on MRP or selling price?
GST is calculated on the actual transaction value (selling price), not necessarily the MRP. However, for most retail products, MRP is GST-inclusive, meaning the displayed price already has GST built in.
Q: What GST rate applies to my business or product?
GST rates vary by HSN/SAC code classification. Common rates are 5%, 12%, 18%, and 28%, with some items at 0%. Check the official GST portal (gst.gov.in) or consult a CA for your specific product or service classification.
Q: Do I need to register for GST to use this calculator?
No. This calculator is a free tool for anyone — business owners, freelancers, students, or shoppers — to calculate GST amounts. It does not require registration and does not connect to the GST portal.