GST Calculator

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How it is calculated

To ADD GST: GST amount = price × rate%, and the total = price + GST. To REMOVE GST from a GST-inclusive amount: base = amount × 100 ÷ (100 + rate), and GST = amount − base. For an intra-state supply the GST splits equally into CGST and SGST (half each); for an inter-state supply it is a single IGST. The general rates are Nil, 5% and 18%, with 40% on a short list of demerit goods — the 12% and 28% slabs were withdrawn with effect from 22 September 2025.

Inclusive and exclusive are different questions

Adding GST to a base price and extracting it from an inclusive one are not the same operation, and the second is where errors happen. To add, multiply the base by the rate. To extract, divide rather than subtract: the base is the inclusive amount multiplied by 100 and divided by 100 plus the rate, so ₹1,180 at 18% yields a base of ₹1,000 and tax of ₹180. Subtracting 18% from ₹1,180 gives ₹967.60, which is wrong and understates the tax you owe. The distinction matters commercially as well as arithmetically, because a price quoted without saying which basis applies is a dispute waiting to happen — the difference between the two readings is the entire tax amount. Settle it in the contract or the purchase order. Retail is the exception: maximum retail price is inclusive of all taxes by law, so extraction is the only correct approach there.

CGST plus SGST, or IGST — the place of supply decides

Once the tax is computed it has to be allocated to the right heads, and the rule follows the place of supply rather than where your office happens to be. A supply within a state is split equally between CGST and SGST, so ₹180 becomes ₹90 each. A supply between states is entirely IGST with no split. Union territories without a legislature use UTGST in place of SGST. This is not a presentational detail: putting tax in the wrong head means your customer's input credit lands in a bucket they cannot use against the liability they have, and correcting it needs an amendment in a later period. Place of supply has its own rules that do not always follow intuition, particularly for services, where the location of the recipient or of performance can govern rather than where the invoice is raised.

The rate follows the classification, and classification is notified

The rate is not a matter of judgement or industry practice — it follows from where the goods or services fall in the notified schedules, read with the tariff and its explanatory notes. That means the starting point is always what the item actually is rather than what it is commonly called in your trade. The schedules changed materially on 22 September 2025: following the 56th GST Council meeting the general slabs became Nil, 5% and 18%, the 12% and 28% slabs were removed, and a 40% rate now applies to a short list of demerit goods. Individual life and health insurance became exempt in the same round, having previously carried 18%. Any rate recalled from before that date, or suggested by older software, may therefore be out of date. Verify against the current notification on cbic-gst.gov.in rather than against habit.

What the invoice must show

A tax invoice has prescribed contents, and missing elements can cost your customer their input credit rather than costing you directly — which makes it a commercial problem as much as a compliance one. It needs your name, address and GSTIN; a consecutive serial number unique for the financial year; the date; the recipient's name, address and GSTIN where registered; the HSN or SAC code at the digit level required for your turnover; description, quantity and value; the taxable value after any discount; the rate and amount of tax under each head separately; the place of supply for inter-state supplies; and a signature or digital signature. Where e-invoicing applies to you, an invoice without a valid IRN and QR code is not a valid tax invoice at all. Getting the recipient's GSTIN wrong by a single character sends the credit to somebody else entirely.

Charging GST requires registration, and registration starts obligations

You cannot lawfully collect GST from a customer without being registered, and collecting it without registration is an offence rather than an administrative lapse. Registration becomes compulsory once aggregate turnover crosses ₹40 lakh for goods or ₹20 lakh for services, with lower thresholds in special category states, and immediately regardless of turnover for supply through an e-commerce operator, inter-state supply of goods, and liability under reverse charge. Voluntary registration is available below the threshold and is often commercially necessary, since registered customers want the credit. What it also starts is a permanent cycle of returns that continues whether or not you trade — a nil period still requires a return, and returns file in sequence so one gap blocks everything after it. Weigh the credit and the customer requirement against that ongoing obligation before registering voluntarily.

Composite and mixed supplies change the rate

When a supply is not a single clean item the rate question shifts, and the two categories are treated very differently. A composite supply is one where goods and services are naturally bundled and supplied together in the ordinary course of business, with one of them principal — the entire supply takes the classification and rate of that principal supply. A mixed supply is two or more independent supplies sold together for a single price where the bundle is not natural, and there the whole attracts the highest rate among the components. On the same invoice that difference can be several percentage points, so the characterisation is worth deciding deliberately rather than by habit. Where a bundle can reasonably be split and priced separately, doing so usually produces a cleaner and more defensible position than arguing afterwards about the character of the package.

Rounding, and why totals differ by a rupee

Tax amounts are rounded to the nearest rupee, which produces small differences that look like errors and are not. Extracting tax from ₹1,000 at 18% gives a base of ₹847.46 and tax of ₹152.54; rounded for the invoice those become ₹847 and ₹153, which no longer sum exactly to the figure you started from. Systems resolve this differently — some round each tax head separately, some round the total and back-fit the split — so the same transaction can differ by a rupee between your invoice and your customer's expectation. Neither approach is wrong, but yours should be consistent across documents rather than varying. Where contracts are priced inclusively and the values are large, stating in the contract how rounding will be handled removes the point from ever becoming a dispute.

Frequently asked questions

Did GST rates change in 2025?

Yes. From 22 September 2025 the general slabs are Nil, 5% and 18%; 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. Verify any rate you recall against the current notification on cbic-gst.gov.in.

What must a valid tax invoice contain?

Your GSTIN, a unique consecutive serial number for the year, the date, the recipient's details and GSTIN where registered, the HSN or SAC at the required digit level, description and value, the taxable value, tax under each head separately, and place of supply for inter-state supplies. Where e-invoicing applies, a valid IRN and QR code are essential.

How do I remove GST from a total amount?

Divide the GST-inclusive amount by (100 + rate) and multiply by 100 to get the base price; the difference is the GST. For 18%, base = total × 100 ÷ 118.

What is the CGST and SGST split?

For a sale within the same state, GST is split equally into CGST and SGST (e.g. 18% = 9% CGST + 9% SGST). For an inter-state sale it is charged as a single IGST.

Is this GST calculator free?

Yes — it is completely free with no signup. It is for estimation; confirm the correct HSN/SAC rate for your goods or services before invoicing.

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