Free tool

GST Calculator

Add GST to a price, or find out what's really in one that already includes it. Pick a rate, see the CGST/SGST or IGST split, done.

By Mohit Sehgal, founder of BahiSathi · Updated 22 Aug 2026

I have

Amount (₹)

GST rate

Supplier and buyer are

Not sure which applies? See how place of supply decides.

Result

Taxable value: ₹10,000
CGST 9.00%: ₹900
SGST 9.00%: ₹900
Total payable: ₹11,800

The two formulas

Both directions use the same rate; the difference is which amount you already have.

You have	You want	Formula
The amount before GST	GST amount and total	GST = base × rate ÷ 100
A GST-inclusive amount	The base amount and GST	base = total ÷ (1 + rate ÷ 100)

The second one is where people go wrong most often. Taking 18% straight off an inclusive total overstates the GST, because that 18% was worked out on the smaller base amount, not on the total itself.

CGST + SGST, or IGST?

The rate and the total tax are the same either way; only the heads on the invoice change. Same state as the place of supply: the tax splits evenly into CGST and SGST. Different states: the whole amount is IGST. Read the full rule, with the cases that catch people out.

Questions

Straight answers.

What is the formula to add GST to an amount?

Multiply the base amount by the rate, then add it back: GST amount = base × (rate ÷ 100), and the total is base + GST amount. For ₹10,000 at 18%, that is ₹1,800 GST on a total of ₹11,800.

What is the formula to remove GST from an inclusive amount?

Divide the inclusive amount by (1 + rate ÷ 100) to get the base: base = total ÷ (1 + rate ÷ 100). For ₹11,800 inclusive at 18%, the base is ₹10,000 and the GST is ₹1,800. That comes from dividing by 1.18, not subtracting 18% off the total.

How do I know whether to charge CGST + SGST or IGST?

Compare your state against the place of supply on the transaction: same state splits the tax into CGST and SGST at half the rate each, different states means the whole amount is IGST. See our full guide on how place of supply decides for the cases that trip people up.

Does this calculator file or report anything?

No. It only does the arithmetic, in your browser, for whatever number you type in. To put a result on an actual numbered tax invoice with your GSTIN, your customer's details and the fields GST law requires, use BahiSathi.

More free tools

Put this on an actual tax invoice.

BahiSathi runs this same split automatically on every line of every document, and hands you a PDF ready to send.

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