GST Calculator: How to Calculate GST, CGST, SGST and IGST

GST looks complicated until you see the arithmetic, which is actually simple. Whether you are adding tax to a price, working out how much tax is hidden inside an all-in figure, or splitting the tax between the centre and the state, it comes down to a couple of formulas you can do on any calculator. This guide walks through each case with worked examples: how to add GST, how to extract it from an inclusive price, how CGST and SGST split for a local sale, when IGST applies, and how reverse GST works. A billing app does all of this for you, but knowing the maths lets you check any bill in seconds.

The basic idea

GST is a percentage added on top of the base price of goods or services. The base price is what the item costs before tax. The GST rate, such as 5, 18 or 40 percent under the GST 2.0 structure, is set for that item. The tax is the base multiplied by the rate, and the final price is the base plus the tax. Everything else is a variation on these two steps. The one thing to keep straight is whether a price you are given is exclusive of GST, meaning tax has not been added yet, or inclusive, meaning tax is already inside it. The method differs slightly for each.

How to add GST to a price

This is the common case: you know the base price and want the final amount. Multiply the base by the rate to get the tax, then add it back. For a product priced at 1,000 rupees before tax at 18 percent:

• Tax: 1,000 multiplied by 0.18 equals 180 rupees.
• Final price: 1,000 plus 180 equals 1,180 rupees.

The shortcut is to multiply the base by 1 plus the rate in one step: 1,000 times 1.18 gives 1,180 directly. The same logic works for any slab. At 5 percent the multiplier is 1.05, at 12 percent it is 1.12, and at 28 percent it is 1.28.

How to extract GST from an inclusive price

Sometimes you quote a single all-in figure and later need to show the tax portion on the invoice. Here the tax is already inside the price, so you divide rather than multiply. Divide the inclusive price by 1 plus the rate to get the base, then the difference is the GST. For an all-in price of 1,180 rupees at 18 percent:

• Base: 1,180 divided by 1.18 equals 1,000 rupees.
• GST inside: 1,180 minus 1,000 equals 180 rupees.

This is what people mean by a reverse GST calculation. It is the same relationship as adding GST, just worked backwards, and it matters whenever you sell at a round retail price and need to report the tax separately.

How CGST and SGST split

For a sale within your own state, the total GST is collected in two halves. One half is CGST, which goes to the central government, and the other half is SGST, which goes to your state government. The customer still pays the same total. Take that 1,000 rupee item at 18 percent sold within your state:

• CGST at 9 percent: 1,000 times 0.09 equals 90 rupees.
• SGST at 9 percent: 1,000 times 0.09 equals 90 rupees.
• Total GST: 90 plus 90 equals 180 rupees, the same as a single 18 percent charge.

So an 18 percent local sale is always 9 percent CGST plus 9 percent SGST. A 12 percent sale is 6 plus 6, and a 5 percent sale is 2.5 plus 2.5. The split is always the rate divided by two on each side.

When IGST applies

If your customer is in a different state, the sale is inter-state and the tax is charged as a single IGST instead of the CGST and SGST split. The rate and the total are identical, only the label and where the money goes change. That same 1,000 rupee item sold to another state is charged 18 percent IGST, which is 180 rupees, exactly what the customer would have paid locally. A billing app decides this for you the moment you enter the customer state, so you never have to remember whether a sale is local or inter-state.

Worked examples across the slabs

To make the pattern concrete, here is how a 1,000 rupee base price behaves at each common slab, both as a local sale and to another state:

The table shows the whole logic at a glance: the GST amount is the base times the rate, the final price adds it back, the local split halves it into CGST and SGST, and the inter-state figure is the same total as one IGST charge.

Which rate applies

The formulas never change, but the rate does depend on the item. GST in India uses a small set of slabs, with 0 percent on many essentials, 5 percent on necessities, 12 and 18 percent on most goods and services, and 28 percent on luxury and select categories. The correct slab is tied to the item and its HSN or SAC code, and slabs are revised from time to time, so check the current rate for your specific goods rather than assuming. Using the wrong slab means correcting your return later. For more on how the tax works overall, see our explainer on GST, and for the invoice rules see the GST bill format guide.

Letting the app do the maths

Knowing the calculation is useful for checking a bill or quoting a price, but you should not be doing it by hand on every sale. A GST billing app stores the rate against each item, applies it automatically, decides between the CGST and SGST split or IGST based on the customer state, and shows the full breakup on the invoice. That removes the two most common errors, using the wrong rate and getting the split wrong, and it means your GST return matches exactly what you charged. You keep the understanding, the app keeps the consistency.

The bottom line

GST maths is just two moves: multiply the base by the rate to add tax, or divide the inclusive price by 1 plus the rate to extract it. CGST and SGST split a local sale in half, while IGST charges the same total to another state. Once you see the pattern you can check any bill, but for daily billing let an app apply the rates and the split for you. IndiaCRM calculates GST on every invoice, splits CGST and SGST or charges IGST automatically, and keeps your records clean, all in one free app. See GST billing software, read the GST explainer, or get the mobile app and bill with correct tax today.