How to calculate GST: add, remove, CGST, SGST and the GST Calculator

Goods and Services Tax (GST) is added to most things you buy or sell in India: a restaurant bill, a phone, a freelance invoice. For a shopper it is a line at the bottom of a receipt. For a shop owner, a freelancer or an accountant it is a calculation that has to be right on every invoice, in both directions: adding tax to a price, and working backwards from a total that already includes it. The free GST Calculator does both, and shows the split into CGST and SGST or IGST. This guide explains the arithmetic, the one mistake that almost everyone makes, and how the tax is divided.

The two calculations

Adding GST to a price that does not include it:

• GST amount = price × rate ÷ 100
• Final price = price + GST amount

Removing GST from a price that already includes it:

• Price before tax = inclusive price ÷ (1 + rate ÷ 100)
• GST amount = inclusive price − price before tax

In the tool, you choose Add GST if your amount is the price before tax, or Remove GST if it already includes tax. Then you enter the amount and the rate.

Worked examples

Case	Calculation	Result
₹1,000 plus 18%	1,000 × 18% = 180	Pay ₹1,180
₹5,000 plus 5%	5,000 × 5% = 250	Pay ₹5,250
₹10,000 plus 40%	10,000 × 40% = 4,000	Pay ₹14,000
₹999 plus 18%	999 × 18% = 179.82	Pay ₹1,178.82
₹1,180 including 18%	1,180 ÷ 1.18 = 1,000	Tax ₹180
₹2,360 including 18%	2,360 ÷ 1.18 = 2,000	Tax ₹360
₹2,500 including 12%	2,500 ÷ 1.12 = 2,232.14	Tax ₹267.86

The classic mistake

Suppose a bill says ₹1,180 including 18% GST, and you want the original price. The tempting method is "take off 18%": 1,180 − 18% of 1,180 = 1,180 − 212.40 = ₹967.60. That is wrong. The correct pre-tax price is ₹1,000.

Why? The 18% was charged on the original price of ₹1,000, which gave ₹180. But 18% of the total is a larger number, because the total is bigger than the starting price. Subtracting it removes too much. The right way to reverse a percentage that was added is to divide by 1.18, as in the table. Every GST rate works the same way: divide by one plus the rate.

This matters in real money. If a shop owner treats ₹1,180 as ₹967.60 of sales, then the accounts are short by over ₹30 on every invoice of this size, and the GST paid to the government is under-reported.

CGST, SGST and IGST

GST has three parts, and which one applies depends on where the buyer and the seller are:

• Within one state (an intra-state sale): the tax is split equally into CGST (Central GST, goes to the central government) and SGST (State GST, goes to the state). For 18%, that is 9% CGST and 9% SGST.
• Between two states (an inter-state sale): the whole tax is IGST (Integrated GST), collected by the centre, which shares it with the destination state.
• Union territories without a legislature charge UTGST instead of SGST; the tax is split in the same way.

For a ₹1,000 sale at 18%:

Type of sale	Tax shown
Within a state	CGST ₹90 + SGST ₹90
Between states	IGST ₹180

The total tax is the same. Only the labels and the receiving governments differ. The calculator lets you choose within a state or between states and displays the correct split. On an invoice these appear as separate lines, and an invoice must show the right ones to be valid for claiming input tax credit.

Common GST rates

The tool has buttons for 0%, 3%, 5%, 18% and 40%, the common slabs after the rate changes in September 2025, and you can type any other rate between 0 and 100. This is the pattern in broad terms:

• 0% for items such as many basic foods and some essentials.
• 3% for items such as gold and silver jewellery.
• 5% for many everyday goods and some services.
• 18% for the standard rate on most goods and services, including many professional and IT services.
• 40% for a small group of luxury and sin goods.

The rate depends on the HSN code (for goods) or the SAC code (for services) of the item, not on its everyday name, and rates are revised from time to time. The buttons are shortcuts, not a rate directory. Always confirm the current rate for your item on the official GST portal or with your accountant before issuing an invoice. Older rates such as 12% and 28% were withdrawn for most goods in 2025, but you can still enter them, which is useful for checking old bills.

Using the calculator

1. Open the GST Calculator.
2. Choose Add GST or Remove GST.
3. Enter the amount. The label changes to "Price before GST" or "Price including GST" to remind you which one the tool expects.
4. Pick a common rate or type your own.
5. Choose within a state or between states.
6. Read the price before tax, the tax amount, the final price and the CGST/SGST or IGST split.

The calculation runs in your browser and is not stored.

Practical uses

• Checking a bill. Take the total on a restaurant or shop receipt, remove GST and see whether the pre-tax prices match the menu.
• Quoting a client. Freelancers and agencies add GST to a quote so that the client sees both the fee and the tax.
• Pricing a product. Work out the price to print on the label so that the customer pays a round amount including tax. To get a final price of ₹1,000 at 18%, the pre-tax price is 1,000 ÷ 1.18 = ₹847.46.
• Preparing invoices and returns. Split the tax into the right heads.
• Comparing prices between sellers who quote with and without tax.
• Teaching and study. Commerce students practise the formulas with real numbers.

Rounding

Invoices are normally rounded to the nearest rupee or paisa, and the rule depends on how you account. Tax on each line may be rounded separately or on the invoice total, which can give a difference of a paisa or two. The calculator shows the unrounded maths formatted for display, so keep one rounding method across a business and follow your accountant's advice.

What this tool does not do

It does the arithmetic correctly, but it cannot decide:

• which rate applies to your goods or services,
• whether you are registered, exempt or under the composition scheme,
• how much input tax credit you can claim,
• whether reverse charge applies, or whether an item is zero-rated for export.

Those questions depend on law and on your facts, so talk to a qualified chartered accountant or tax professional for them. Use the calculator for the number work only.

Related tools

Percentages are the heart of tax maths; the Percentage Calculator helps with the basics. To price a product with a margin and a discount before tax, use the Discount & Profit Margin Calculator. If you earn income and want an estimate of income tax rather than GST, see the Income Tax Calculator. Write the invoice total in words with the Number to Words Converter. And for loans on business equipment, the EMI Calculator shows what the financing costs.

Quick answers

How do I calculate 18% GST? Multiply the price by 0.18 to get the tax, and add it to the price. Or multiply the price by 1.18 for the total.

How do I remove 18% GST from a total? Divide by 1.18. Do not subtract 18% of the total.

What is the difference between CGST, SGST and IGST? CGST and SGST share the tax between the centre and the state for a sale within one state. IGST is charged on sales between states.

Which rate applies to my product? It depends on the item's HSN or SAC code; check the official GST portal or ask your accountant.

Is my data stored? No. The calculation happens in your browser.