Reverse GST Calculator
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How it is calculated
To remove GST from an inclusive amount, divide the total by (1 + rate/100) to get the base price; the difference is the GST. For example, ₹1,180 at 18% gives a base of ₹1,000 and ₹180 GST (₹90 CGST + ₹90 SGST for an intra-state supply).
Reverse calculation and reverse charge are unrelated
The two terms sound similar and mean completely different things, and confusing them leads to real errors on a return. Reverse calculation is arithmetic: you have a price that already includes GST and you want to know how much of it is tax. Reverse charge is a legal mechanism under which the liability to pay tax shifts from the supplier to the recipient — it applies to specified supplies such as certain services from a goods transport agency, legal services from an advocate, sponsorship, and imports of services, as well as to some purchases from unregistered suppliers. Under reverse charge you pay the tax directly to the government rather than to your supplier, report it in your return, and may then claim it as input credit subject to the usual conditions. This calculator does the first thing. If your question is about the second, the answer is a compliance question rather than an arithmetic one.
Why the arithmetic is not a simple subtraction
The mistake people make is subtracting the rate from the inclusive amount — taking 18% off ₹1,180 and arriving at ₹967.60, which is wrong. The reason is that the 18% was applied to the base, not to the total, so the total already represents 118% of the base. To work backwards you divide rather than subtract: the base is the inclusive amount multiplied by 100 and divided by 100 plus the rate. On ₹1,180 at 18% that gives ₹1,000 exactly, and the tax is ₹180. At 5% the divisor is 105, at 12% it was 112 before that slab was removed. The same logic applies to any inclusive figure, including MRP-based pricing where the printed price is inclusive by law. Getting this wrong understates the base and therefore understates the tax you owe, which is the direction that attracts interest.
When you actually need to work backwards
Several everyday situations produce an inclusive figure that has to be broken up. Retail sales at MRP are the obvious one, since the maximum retail price is inclusive of all taxes by law and the invoice has to show the base and the tax separately. Contracts that quote a single all-inclusive price need the same treatment, and it is worth settling in the contract itself whether the price is inclusive or exclusive, because the difference is the tax and disputes about it are common. Cash-and-carry and point-of-sale receipts, restaurant bills, and any composite quote given to a customer over the phone all end up needing the split. It also arises when correcting an error: if you charged a customer a round inclusive figure and now need to raise a proper tax invoice, the base has to be derived rather than assumed.
Split the tax into the right heads
Once you have the tax amount, it has to be allocated correctly, and the rule follows the place of supply rather than the location of your office. For a supply within the same state, the tax is split equally between CGST and SGST — so ₹180 becomes ₹90 and ₹90. For a supply between states, the whole amount is IGST and there is no split. For union territories without a legislature, UTGST takes the place of SGST. Getting this wrong is not a minor presentational issue: the wrong head means your customer's credit sits in a bucket they cannot use against the liability they have, and correcting it requires an amendment in a later period. Rounding is prescribed to the nearest rupee, which is why a half-paisa difference sometimes makes an invoice total differ by a rupee from a hand calculation — that is expected rather than an error.
The rates changed on 22 September 2025
Before working backwards from an inclusive price, make sure the rate you are dividing by is the current one. Following the 56th GST Council meeting, the general slabs became Nil, 5% and 18% with effect from 22 September 2025; the 12% and 28% slabs were removed, and a 40% rate applies to a short list of demerit goods. Individual life and health insurance became exempt in the same round, having previously carried 18%. That means an inclusive price agreed before that date may have been computed on a rate that no longer applies, and any rate you recall — or that older software or an AI tool suggests — may be the pre-reform one. Where a slab moved, invoices, item masters and marketplace listings all need checking, because the rate attaches to the supply rather than to when your system was updated. Verify on cbic-gst.gov.in before relying on a figure.
Rounding, and why a total differs by a rupee
Tax amounts are rounded to the nearest rupee, and this produces small differences that look like errors and are not. Working backwards from ₹1,000 at 18% gives ₹847.46 as the base and ₹152.54 as the tax; rounded for the invoice these become ₹847 and ₹153, and the two no longer sum exactly to the figure you started from. Different accounting systems resolve this differently — some round each tax head separately, some round the total and back-fit the split — which is why the same transaction can produce a one-rupee difference between your invoice and your customer's expectation. Neither is wrong, but the difference should be consistent within your own system rather than varying by document. Where a contract is priced inclusively and the amounts are large, it is worth stating in the contract how rounding will be handled so the point never becomes a dispute.
Inclusive or exclusive should be settled in writing
A surprising proportion of commercial disputes about GST are not about the rate at all but about whether the agreed price included it. The difference is the entire tax amount, so it is worth a sentence in the contract or the purchase order. Where a price is silent and the supplier is registered, the usual commercial assumption is that the price is exclusive and tax is added — but assumptions are not much use once money is in dispute. Retail is the exception: maximum retail price is inclusive of all taxes by law, so working backwards is the only correct approach there. For services quoted over the phone or by email, confirm the basis in the written quote. And if a rate change moves an item between slabs mid-contract, the contract should say who bears the difference, because otherwise both parties will reasonably assume the other does.
Frequently asked questions
How do I remove GST from an inclusive price?
Divide rather than subtract. The base is the inclusive amount multiplied by 100 and divided by 100 plus the rate — so Rs 1,180 at 18% gives a base of Rs 1,000 and tax of Rs 180. Subtracting 18% from the total is the common error and understates the base.
Is reverse calculation the same as reverse charge?
No. Reverse calculation is arithmetic on an inclusive price. Reverse charge is a legal mechanism shifting the liability to pay tax from the supplier to the recipient, applying to specified supplies such as goods transport agency and legal services, and to imports of services.
How do I remove GST from a total?
Base price = total ÷ (1 + GST rate/100). GST amount = total − base price. The calculator carries the current Nil, 5%, 18% and 40% rates, and keeps 12% and 28% available for reconciling invoices raised before 22 September 2025.
Is GST split into CGST and SGST?
For a sale within the same state, GST is split equally into CGST and SGST. For inter-state sales it is a single IGST at the same total rate.
Which GST rate applies to my product?
It depends on the HSN/SAC classification. Use our free HSN/SAC finder to check, and verify on the GST portal.
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India Law Simplified is an AI-assisted tool, not a substitute for a licensed CA or advocate. Tax rules and limits change with each Finance Act — verify before relying on any figure.