Reverse GST calculator: take the GST back out of a total
Most invoices you receive in India show a single total. This calculator splits that total into the taxable value and the GST inside it, at the slab that was in force on the document’s own date, so you can price correctly, reconcile a supplier bill, or record a purchase at its real cost. The slabs come from the same statutory catalogue the billing product uses, so a rate here is the rate that reaches your invoice.
Split a GST-inclusive total
Enter the total you were quoted or charged, set the date the document was raised, and pick the slab that date carries. Mark the supply inter-state if it is. Use it for planning and rough checking, then confirm the final breakup with a CA.
- Total paid / received
- ₹11,800
- Total GST at 18%
- ₹1,800
- CGST
- ₹900
- SGST
- ₹900
- Check: taxable + GST
- ₹11,800
Planning estimate only. The GST rate that applies depends on the HSN/SAC classification of the supply, and the place of supply decides CGST + SGST versus IGST. Discounts, freight, and rounded invoice totals also move the result, so confirm the final breakup with a CA.
Which slabs, and what happened to 0% and 28%
The chargeable goods slabs are 0.25%, 1.5%, 3%, 5%, 18% and 40%. There is no 12% entry. There is no 0% entry either, and that is a real point rather than an omission: zero is not a rate. An exempt supply, an export, and goods lying outside the GST Act are three different statements, and the calculator offers them as three named treatments instead of a “GST @ 0%” line.
The 28% slab is the fourth change. Notification No. 19/2025-Central Tax (Rate) omitted the previous 28% Schedule VII with effect from 1 February 2026, so a document dated 1 February 2026 or later cannot be charged at 28%. A bill raised in January 2026 still can, which is why the calculator filters the rate list on the date you enter rather than on today’s date, and flags a superseded slab when you pick one.
How this is calculated
Taxable value = Total ÷ (1 + Rate ÷ 100)
GST amount = Total − Taxable value
Intra-state: CGST = SGST = GST ÷ 2
Inter-state: IGST = GSTThe rate is applied to the taxable value, never to the total. That is why dividing the total by 1.18 is the correct way to remove 18% GST, and why subtracting 18% of the total under-states the GST every time. The result is shown to two decimals, which is where the rounding difference on a real invoice will land.
Worked example
A distributor sends a single-page bill totalling ₹11,800, GST included, for goods supplied within your state. At 18% the taxable value is ₹11,800 ÷ 1.18 = ₹10,000.00, and the GST inside that total is ₹1,800.00 — split as CGST ₹900.00 and SGST ₹900.00. The check row in the calculator adds those back to ₹11,800.00.
Now take a 5% slab. A bill of ₹10,500 inclusive works out to ₹10,000.00 taxable plus ₹500.00 GST, and a bill of ₹12,800 dated 20 January 2026 works out to ₹10,000.00 taxable plus ₹2,800.00 — the 28% slab that was still in force that day. Same taxable value each time, which is exactly why the rate has to come off the total rather than being estimated from it.
Where this gets used
A retailer buying stock at a distributor counter is usually quoted one total and rarely shown the breakup. If you record purchases at that gross number, your stock cost is inflated by tax you cannot claim back, and your margin reads lower than it is. Running the total through this calculator first gives you the cost that actually belongs on the stock record.
The same arithmetic shows up when a client sends a GST-inclusive budget for a project. A ₹5,90,000 budget at 18% is ₹5,00,000 of work and ₹90,000 of tax you will pass on. Quoting ₹5,90,000 of work against that budget is a margin problem you discover late, because the reverse calculation would have shown it on day one.
The other honest use is checking someone else's invoice. If a supplier's total does not reconcile against the rate they should have charged, the difference is worth asking about before the bill goes into your books.
What this tool does not do
- It does not pick the rate. The rate depends on the HSN or SAC classification of the supply.
- It does not claim input tax credit. Only the input component on eligible business supplies is claimable, and that is a CA question, not a calculator one.
- It does not handle discounts, freight, or round-off, which is why a live invoice total can differ by a rupee or two from a clean calculation.
Related tools
- GST Calculator — go the other way and add tax to a taxable amount.
- Markup vs Margin Calculator — price against cost once the cost is GST-exclusive.
- Profit Margin Calculator — see what a realisable revenue figure leaves after costs.
- Project Profitability Calculator — when a client’s budget arrives GST-inclusive.
- All free tools for small business operations.
Need the breakup on every invoice, not just in a calculator?
Nox-Billings raises GST-compliant invoices with a tax breakup against the client and the stock the sale drew from, and records payments, employee access, discount approvals, and a day-end review for owner-led counters. NoxOrigin keeps quotes, invoices, payments, and receivables on the same client record, so the cost you recorded and the money you collected can be read together.