How do I remove GST from a total that already includes it?

Divide by (1 + rate/100), not subtract the rate. At 18%, a ₹1,180 inclusive price has a taxable value of 1180 ÷ 1.18 = ₹1,000 and GST of ₹180. Subtracting 18% of 1,180 instead gives ₹212.40, which is wrong by ₹32.40 on this one bill and by more on every larger one.

When is it CGST and SGST, and when is it IGST?

It follows the place of supply. Supplier and place of supply in the same state means CGST and SGST, each half the rate. Different states means IGST at the full rate. It is not a choice - it is decided by where the supply happens.

Is CGST always exactly half?

Yes. An 18% intra-state supply is 9% CGST and 9% SGST; they always split the rate evenly.

Which GST rate applies to my product?

That depends on its HSN code and the rate notification in force, and it changes. Check the current rate for your HSN on the CBIC site rather than assuming - this calculator works out the tax once you know the rate, it does not decide the rate.

Is anything I type here stored?

No. The whole calculation runs in your browser. Nothing is sent to a server, so there is nothing to store.

Free tool

GST calculator

Add GST to an amount, or work it back out of a price that already includes it, and see how the tax splits into CGST and SGST — or into IGST when the supply crosses a state border.

Amount (₹)

GST rate (%)

The amount you entered is

Inter-state supply (IGST instead of CGST + SGST)

Enter an amount to see the split.

Worked out in your browser — nothing you type here is sent anywhere. Which rate applies to your goods depends on their HSN code and the current notification; this works out the tax once you know the rate, it does not decide the rate for you.

Working GST backwards is where it goes wrong

Adding GST is easy — take the rate, apply it, done. Taking it back out of an inclusive price is where the common mistake lives, because the instinct is to subtract the same percentage. It does not work that way: the tax was calculated on the smaller number, so it has to be divided out, not deducted.

At 18%, ₹1,180 inclusive is ₹1,000 plus ₹180. Subtracting 18% of ₹1,180 gives ₹967.60, and every figure after it — your taxable value, your output tax, your return — is wrong. Switch the calculator above to “Including GST” and it divides properly.

CGST and SGST, or IGST

Which one applies is not a preference. If the supplier and the place of supply are in the same state, the rate splits evenly into CGST and SGST — 18% becomes 9% and 9%. If they are in different states, the whole 18% is IGST instead.

This is the single most common error in hand-kept GST, and it is a quiet one: an inter-state sale billed as intra-state looks perfectly normal on the invoice and only surfaces when the return will not reconcile.

Questions

How do I remove GST from a total that already includes it?: Divide by (1 + rate/100), not subtract the rate. At 18%, a ₹1,180 inclusive price has a taxable value of 1180 ÷ 1.18 = ₹1,000 and GST of ₹180. Subtracting 18% of 1,180 instead gives ₹212.40, which is wrong by ₹32.40 on this one bill and by more on every larger one.
When is it CGST and SGST, and when is it IGST?: It follows the place of supply. Supplier and place of supply in the same state means CGST and SGST, each half the rate. Different states means IGST at the full rate. It is not a choice - it is decided by where the supply happens.
Is CGST always exactly half?: Yes. An 18% intra-state supply is 9% CGST and 9% SGST; they always split the rate evenly.
Which GST rate applies to my product?: That depends on its HSN code and the rate notification in force, and it changes. Check the current rate for your HSN on the CBIC site rather than assuming - this calculator works out the tax once you know the rate, it does not decide the rate.
Is anything I type here stored?: No. The whole calculation runs in your browser. Nothing is sent to a server, so there is nothing to store.

Doing this on every invoice?

In Zembook the CGST/SGST versus IGST decision is made from the place of supply and your own state code rather than chosen on each bill, HSN codes sit on the item, and GSTR-1 and GSTR-3B come out of the same vouchers as portal-ready JSON. Free for 90 days on Windows, free on Android.