What is a Reverse GST Calculation?
A reverse GST calculation works backwards from a GST-inclusive price to find the original base amount and the tax hidden inside it. It answers the question: "If I paid ₹11,800 including 18% GST, how much was the product and how much was GST?" This is the opposite of a normal GST calculator, which adds GST to a base price.
Reverse GST Formula
Base Amount = Total ÷ (1 + GST Rate / 100)
GST Amount = Total − Base Amount
GST Amount = Total − Base Amount
Example: ₹11,800 inclusive of 18% GST → Base = 11,800 ÷ 1.18 = ₹10,000, and GST = ₹1,800. For an intrastate sale that ₹1,800 splits into ₹900 CGST + ₹900 SGST; interstate, it is ₹1,800 IGST.
Reverse GST at Each Slab (on ₹1,000 base)
| Slab | Inclusive Price | Base | GST |
|---|---|---|---|
| 5% | ₹1,050 | ₹1,000 | ₹50 |
| 12% | ₹1,120 | ₹1,000 | ₹120 |
| 18% | ₹1,180 | ₹1,000 | ₹180 |
| 28% | ₹1,280 | ₹1,000 | ₹280 |
When Do You Need Reverse GST?
- Purchase invoices: separating GST for input tax credit (ITC) claims
- Retail receipts: verifying the GST charged on an MRP-inclusive bill
- E-commerce payouts: reconciling marketplace settlements net of GST
- Accounting: recording the correct CGST/SGST/IGST from inclusive totals