How GST is actually calculated, with real examples

The two directions

Every GST calculation is one of two things. Either you have a base price and you are adding tax to it, or you have a final price and you are working out how much tax is inside it. They need different arithmetic, and treating the second like the first is the single most common GST mistake in small businesses.

Adding GST to a price

You are selling a service for ₹10,000 and the rate is 18 percent.

• GST is 10,000 × 18 ÷ 100 = ₹1,800
• Total the customer pays is ₹11,800

Nothing surprising. This is the direction people get right.

Removing GST from an inclusive price

Now the other direction. A customer paid you ₹11,800 and that figure includes 18 percent GST. How much of it is tax?

The instinct is to take 18 percent of ₹11,800, which gives ₹2,124. That is wrong, and it is wrong in a way that always overstates your tax.

The reason is that the 18 percent was calculated on the base, not on the total. So the total is 118 percent of the base, and to get back to the base you divide rather than multiply.

• Base is 11,800 ÷ 1.18 = ₹10,000
• GST is 11,800 − 10,000 = ₹1,800

The wrong method overstated the tax by ₹324 on one invoice. On ₹50,00,000 of inclusive annual turnover, the same error is roughly ₹1,37,000 out. That is not a rounding difference, and it is exactly the kind of gap that makes a return refuse to reconcile.

The divisors, so you do not have to think

Rate	Divide the inclusive price by	GST inside ₹1,000
5%	1.05	₹47.62
12%	1.12	₹107.14
18%	1.18	₹152.54
28%	1.28	₹218.75

Notice that the GST inside a ₹1,000 inclusive price at 18 percent is ₹152.54, not ₹180. If you have ever wondered why the tax on your restaurant bill looks lower than the rate suggests, this is why.

Splitting into CGST and SGST

The total tax is one number. How it is split depends only on where the supply goes.

Within your own state. Half goes to the centre as CGST and half to the state as SGST. An 18 percent supply becomes 9 percent CGST and 9 percent SGST.

To another state. The whole thing is IGST at the full rate. The centre collects it and settles with the destination state afterwards, which is not your problem.

On ₹10,000 at 18 percent:

Supply	Split	Total tax
Gujarat to Gujarat	₹900 CGST + ₹900 SGST	₹1,800
Gujarat to Maharashtra	₹1,800 IGST	₹1,800

The customer pays the same either way. But the invoice must show the right one, because your buyer's input tax credit is claimed against the specific heads on the document. Put IGST on an intra state supply and their credit does not match, which means a query for them and then a corrected invoice from you.

Where the place of supply is decided

For goods it is normally where they are delivered. For services supplied to a registered business it is normally the recipient's location. There are specific rules for particular categories, including transport, events, telecom and immovable property, so anything unusual is worth checking rather than assuming.

The trap for anyone selling services remotely: your customer's registered address decides the tax, not where you are sitting. A developer in Surat billing a company in Bengaluru charges IGST, even though neither of them moved.

A worked example with mixed rates

You invoice a client in your own state for two things. Consulting at ₹40,000, which attracts 18 percent, and a printed manual at ₹5,000, which attracts 12 percent.

Line	Taxable value	Rate	Tax
Consulting	₹40,000	18%	₹7,200
Printed manual	₹5,000	12%	₹600

Taxable value ₹45,000. Tax ₹7,800, which shows as ₹3,900 CGST and ₹3,900 SGST. Invoice total ₹52,800.

The important detail: you group the tax by rate, not into one blended figure. An invoice showing "GST ₹7,800" without the rate breakdown is incomplete. Both rates have to appear separately, because the credit is claimed per rate.

Three errors that come up repeatedly

1. Multiplying an inclusive price by the rate. Covered above, and by some distance the most expensive habit. Always divide.
2. Applying GST after a discount inconsistently. A discount shown on the invoice reduces the taxable value, so tax is charged on the net. A discount given later, off invoice, generally does not reduce it. Get the sequence right on the document itself.
3. Blending rates into one figure. If your invoice has 18 percent and 12 percent lines, the tax must be shown against each rate separately.

Input tax credit, in one paragraph

If you are registered, the GST you paid on business purchases can generally be set off against the GST you collected on sales, and you remit the difference. Collect ₹1,80,000 and pay ₹60,000 on inputs, and you send ₹1,20,000 to the government. This is why GST is described as a tax on value added rather than on turnover, and it is why a correctly formatted invoice from your supplier matters to you directly. If their paperwork is wrong or their return is not filed, your credit is the thing at risk.

Doing it without arithmetic

The GST calculator handles both directions and shows the CGST and SGST split. The invoice generator produces a document with every required field and groups the tax by rate correctly.

A necessary note on rates. The GST Council revises rates and the classification of individual items from time to time. The rate structure described here is the familiar 5, 12, 18 and 28 percent arrangement, but before applying a rate to something unfamiliar, check the current notification or ask your accountant. Applying the wrong rate is your liability, not your customer's.

Common questions

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Written by Khanjan Kavani, who builds software in Surat and writes these to answer the questions clients keep asking. Found a mistake? Tell me at hello@khanjankavani.com and I will correct the article itself.

General information, not professional advice. Rules in India change. Check anything important against the current position or a qualified professional. See the disclaimer.