What is the formula to remove GST from a total?

Divide the total by one plus the rate as a decimal. At 18%, divide by 1.18 to get the taxable value, then subtract that from the total to get the tax. A common mistake is taking 18% of the inclusive amount, which overstates the tax: 18% of 1,180 is 212.40, but the tax actually inside 1,180 is 180.

When is GST split into CGST and SGST instead of IGST?

When the supplier and the place of supply are in the same state, the rate is split in half between CGST and SGST. When they are in different states, the whole rate is charged as IGST. The total tax is identical either way; only the split changes.

Which GST rate applies to what I sell?

Rate depends on the HSN or SAC classification of the specific goods or service, and rates are revised by notification from time to time. This calculator does the arithmetic once you know your rate. Confirm the rate itself against the CBIC rate finder or with your accountant.

Why does the calculator include rates like 0.1% and 1.5%?

Alongside the main slabs, a few special rates exist for specific supplies such as certain exports against advance authorisation, rough diamonds and some construction services. They are included so the arithmetic works if one of them applies to you.

GST calculator

Enter an amount, pick a rate, and choose whether GST is already included. You get the taxable value, the tax, the total, and the CGST/SGST or IGST split.

Taxable value	10,000.00
CGST 9%	900.00
SGST/UTGST 9%	900.00
IGST 18%	1,800.00
Total	11,800.00

Adding GST versus taking it out

These are two different sums and mixing them up is the most common invoicing error at small scale. If your price is 10,000 before tax at 18%, the tax is 1,800 and the customer pays 11,800. If your price is 10,000 including tax at 18%, the taxable value is 8,474.58 and the tax inside it is 1,525.42.

The inclusive sum is the one people get wrong. You cannot take 18% of an inclusive total, because the 18% was calculated on the smaller taxable value, not on the total. Divide by 1.18 instead. The calculator switches between the two when you change the radio button, so you can see the difference on your own numbers.

Why the split matters when the total does not change

A customer pays the same 1,800 whether it arrives as 900 CGST plus 900 SGST or as 1,800 IGST. The split matters for your returns and for your customer's input tax credit, which is why the invoice has to show it correctly rather than just showing a single tax line.

Get it wrong in the wrong direction and the fix is not simply an amended invoice. If you charged CGST and SGST on what was actually an inter-state supply, the tax has to be paid again correctly and the wrongly-paid amount claimed as a refund. Section 77 of the CGST Act and section 19 of the IGST Act deal with this. It is recoverable, but it is a lot of paperwork for a dropdown you set once.

Rounding

Tax amounts are worked out to two decimals here. Invoices are conventionally rounded to the nearest rupee at the total, with the difference shown as a separate rounding line. The invoice generator does that for you and prints the rounding line on the document.

Common questions

What is the formula to remove GST from a total?: Divide the total by one plus the rate as a decimal. At 18%, divide by 1.18 to get the taxable value, then subtract that from the total to get the tax. A common mistake is taking 18% of the inclusive amount, which overstates the tax: 18% of 1,180 is 212.40, but the tax actually inside 1,180 is 180.
When is GST split into CGST and SGST instead of IGST?: When the supplier and the place of supply are in the same state, the rate is split in half between CGST and SGST. When they are in different states, the whole rate is charged as IGST. The total tax is identical either way; only the split changes.
Which GST rate applies to what I sell?: Rate depends on the HSN or SAC classification of the specific goods or service, and rates are revised by notification from time to time. This calculator does the arithmetic once you know your rate. Confirm the rate itself against the CBIC rate finder or with your accountant.
Why does the calculator include rates like 0.1% and 1.5%?: Alongside the main slabs, a few special rates exist for specific supplies such as certain exports against advance authorisation, rough diamonds and some construction services. They are included so the arithmetic works if one of them applies to you.

Last reviewed 2026-09-10. Written and maintained by Kartikeya Mishra.