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Step-by-step reverse GST calculation: how to back-solve the base amount and GST portion from a GST-inclusive total, with worked examples at every GST 2.0 rate.
Checked against current rules for GST 2.0 rate slabs on 29 September 2026. Source: CBIC Notification No. 9/2025-Central Tax (Rate).
You have a total amount — maybe from a bank credit, a receipt, or a vendor bill — and you need to know how much of it was GST and how much was the actual price of the goods or service. This is reverse GST calculation, and it comes up constantly in bookkeeping, reconciliation and expense tracking.
Before reverse-calculating anything, confirm the amount you have actually includes GST. Retail prices, MRPs, e-commerce totals and money that's already landed in a bank account are typically GST-inclusive. A quoted rate or a line-item cost price on a purchase order is more often GST-exclusive — check the source document if you're unsure, since applying reverse GST to an already-exclusive amount will give you the wrong base figure.
Worked example
GST-inclusive amount = ₹1,180. GST rate = 18%. Base amount = 1,180 ÷ 1.18 = ₹1,000. GST = 1,180 − 1,000 = ₹180. This example assumes 18% is the applicable GST rate for the transaction — the calculation itself doesn't verify that; you need to know or confirm the rate first.
Once you have the total GST amount, split it if you need the CGST/SGST or IGST breakdown — for example, to reconcile a purchase against your input tax credit records by tax head.
| Supply type | GST amount split |
|---|---|
| Intra-state (buyer and seller in the same state) | CGST = GST ÷ 2, SGST = GST − CGST |
| Inter-state (buyer and seller in different states) | IGST = full GST amount |
Worked example, continued
Free GST Calculator
Calculate GST, CGST, SGST and IGST from a base amount, or reverse-calculate the base amount and GST from a GST-inclusive total — no signup required.
How do I know if an amount already includes GST?
Check the source: retail MRPs, e-commerce checkout totals, and amounts credited to a bank account are almost always GST-inclusive. A quotation, cost price, or line-item rate on a purchase order is more often GST-exclusive. When in doubt, check the source document — an invoice will state the base amount and GST separately even when the total is inclusive.
What is the reverse GST formula?
Base Amount = GST-Inclusive Amount ÷ (1 + GST Rate ÷ 100). The GST amount is then the inclusive amount minus the base amount.
Does reverse GST work the same way for CGST/SGST and IGST?
Yes — the base and total GST amount are calculated the same way regardless of tax type. Only the last step differs: an intra-state transaction splits the GST amount equally into CGST and SGST, while an inter-state transaction assigns the full amount to IGST.
GST & Tax
GST Inclusive vs GST Exclusive: What's the Difference?
What GST-inclusive and GST-exclusive prices mean, how to convert between them, and when each calculation is the right one to use — with worked examples.
GST & Tax
How to Calculate GST
GST-exclusive and GST-inclusive calculations, CGST/SGST vs IGST, and the formulas behind both — with worked examples at current GST 2.0 rates.
On the ₹180 GST from above, for an intra-state transaction: CGST = ₹90, SGST = ₹90. For an inter-state transaction: IGST = ₹180.
Using a ₹1,000 GST-inclusive amount as the starting point, here's the base amount, GST amount, and CGST/SGST split at each current GST 2.0 rate (effective 22 September 2025):
| GST rate | Base amount | GST amount | CGST | SGST |
|---|---|---|---|---|
| 0% | ₹1,000.00 | ₹0.00 | ₹0.00 | ₹0.00 |
| 5% | ₹952.38 | ₹47.62 | ₹23.81 | ₹23.81 |
| 18% | ₹847.46 | ₹152.54 | ₹76.27 | ₹76.27 |
| 40% | ₹714.29 | ₹285.71 | ₹142.86 | ₹142.85 |
Notice the 40% row's CGST and SGST split isn't perfectly even — ₹142.86 and ₹142.85 — because ₹285.71 doesn't divide exactly in half at the paisa level. One side absorbs the extra paisa so the two always add back up exactly to the total GST amount.
These examples assume the stated GST rate is the one that legally applies to the transaction. A GST calculator computes based on the rate you supply — it doesn't determine which rate applies to a given product or service. Under GST 2.0, the previous 12% and 28% slabs were phased out in favour of 5%, 18% and a 40% demerit rate; if you're reverse-calculating an older invoice dated before 22 September 2025, use the rate that was actually charged on that invoice, not the current slab list.
For the full formulas behind both directions of GST calculation, see our guide on how to calculate GST. For CGST/SGST/IGST math done automatically, use the reverse GST calculator below.
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Create a free Credit Note or Debit Note online with GST calculations, original invoice reference, professional A4 PDF and print support.