GST Calculator
Two jobs, one tool. Add GST to a base price, or work backwards from a price that already includes it. Both give you the CGST and SGST split you need for an invoice.
Adding GST is easy. Removing it is where people slip.
Adding is what everyone expects. Take the base price and multiply.
Total = base + GST
₹10,000 at 18 percent gives ₹1,800 of tax and a total of ₹11,800. Nothing surprising.
Removing it is where the common mistake lives. If a customer paid you ₹11,800 including 18 percent GST, the tax inside that is not 18 percent of ₹11,800. That would be ₹2,124, and it is wrong. You have to divide, not multiply.
GST = total − base
₹11,800 ÷ 1.18 = ₹10,000. So the GST inside is ₹1,800. The wrong method overstates the tax by ₹324 on a single ₹11,800 invoice. Repeat that across a year of sales and your return does not reconcile.
CGST, SGST and IGST
Which one you charge depends only on where the supply goes, not on what you are selling.
| Supply | What you charge | On ₹10,000 at 18% |
|---|---|---|
| Within your own state | CGST + SGST, half each | ₹900 CGST and ₹900 SGST |
| To another state | IGST, the full rate | ₹1,800 IGST |
The total tax is identical either way. Only the split changes, because CGST goes to the centre and SGST to the state where the supply happens. Get this wrong on an invoice and your buyer cannot claim the credit cleanly, which is the kind of error that gets you a phone call rather than a notice.
The rate slabs
Most goods and services fall into 5, 12, 18 or 28 percent, with a set of exempt and zero rated items and some special rates outside this structure. Essentials sit low, standard services mostly sit at 18, and luxury and demerit goods sit at 28 with cess on top in some categories.
Rates and the classification of individual items are revised by the GST Council from time to time. Before you apply a rate to something unfamiliar, check the current notification or ask your accountant. Applying the wrong rate is your liability, not your customer's.
Input tax credit, briefly
If you are registered, the GST you pay on business purchases can usually be set off against the GST you collect on sales. You pay the government the difference. This only works if your supplier actually files their return and the invoice carries every required field, which is why a correctly formatted invoice matters more than it looks. The invoice generator produces one with the fields in place.
Common questions
How do I remove GST from a price that already includes it?
Divide by 1 plus the rate as a decimal. For 18 percent, divide by 1.18. For 12 percent, divide by 1.12. Do not multiply the inclusive price by the rate, which is the usual error and always overstates the tax.
Is the total the same for CGST plus SGST as for IGST?
Yes. An 18 percent supply carries ₹1,800 of tax on ₹10,000 either way. Within a state that is ₹900 CGST and ₹900 SGST. Across states it is ₹1,800 IGST. Only the split and the destination of the money differ.
When do I need to register for GST?
Registration becomes mandatory once turnover crosses the threshold that applies to your state and to the type of supply you make, and it is compulsory from the first rupee for certain categories such as inter state supply of goods and most e-commerce sellers. The thresholds differ for goods and services and for special category states, so check your specific position rather than assuming a single number.
What is the reverse charge mechanism?
For certain notified supplies the buyer pays the GST directly to the government instead of the seller collecting it. It applies to specific categories, including some purchases from unregistered suppliers. If you think it might apply to you, confirm it with your accountant, because getting it wrong creates a liability you did not budget for.
Does this calculator handle GST cess?
No. Compensation cess applies to a narrow set of goods such as tobacco, aerated drinks and certain vehicles, at rates that vary by item. It sits on top of GST. If you deal in those categories, add the cess separately.