GST calculations trip people up in one specific way: working backwards from a tax-inclusive price is not the same as taking the tax percentage off it. Getting that wrong produces invoices that do not reconcile.
How to use the GST Calculator
- Enter the amount
- Choose whether it already includes GST or not
- Select the applicable GST rate
- Read the base amount, tax and the CGST/SGST or IGST breakdown
Adding GST versus removing it
Adding is straightforward: ₹1,000 plus 18% GST is ₹1,180.
Removing is where the mistake happens. If ₹1,180 is the GST-inclusive price, the base is not ₹1,180 minus 18%, which would give ₹967.60. The correct calculation divides:
Base = Inclusive ÷ (1 + rate/100)
= 1180 ÷ 1.18 = ₹1,000The subtraction method is wrong because the 18% was calculated on the base, not on the inclusive total. The error grows with the rate — at 28% it is substantial enough to fail an audit.
CGST, SGST and IGST
Which applies depends on where the supply crosses state lines, not on the rate.
- Intra-state (buyer and seller in the same state) — the total splits evenly into CGST (central) and SGST (state). 18% GST becomes 9% CGST + 9% SGST.
- Inter-state (different states) — the whole amount is IGST, collected centrally and apportioned later.
The total tax is identical either way. The split determines which government collects it, and it must be shown correctly on the invoice for input tax credit to be claimable.
Frequently asked questions
What are the GST rate slabs in India?
0%, 5%, 12%, 18% and 28%. Essentials are mostly 0% or 5%, most goods and services sit at 18%, and luxury and 'sin' goods are at 28%, sometimes with an additional cess.
Why can't I just subtract 18% to remove GST?
Because the 18% was calculated on the base, not the inclusive total. You must divide by 1.18, not subtract 18%. Subtracting understates the base and the error grows with the rate.
When is IGST charged instead of CGST and SGST?
On inter-state supplies, and on imports. For transactions within one state the tax splits into CGST and SGST instead. The total is the same; only the split differs.
Is a cess included in these figures?
No. The compensation cess on certain 28% goods — tobacco, aerated drinks, some vehicles — is levied separately and varies by product. Add it on top of the GST calculated here.