GST calculator
Split any invoice value into base and tax, either way round.
Figures are indicative. Thresholds, surcharge and current-year notifications are not fully modelled — confirm edge cases with your CA.
What this calculates
GST is charged on the taxable value of a supply, not on the total the customer pays. The two questions that come up daily are the two directions of the same sum: adding tax to a quoted price, and stripping tax out of a price that already includes it. Getting the direction wrong is the single most common invoicing error, because the two answers differ — 18% added to ₹100 is ₹118, but ₹118 inclusive of 18% has a base of ₹100, not ₹96.72.
How it is worked out
- 1Exclusive: the amount you enter is the taxable value. Tax is that amount times the rate, and the invoice total is the two added together.
- 2Inclusive: the amount you enter is the total. Divide it by one plus the rate to recover the taxable value, then take the difference as tax.
- 3Within a state the tax splits equally into CGST and SGST — 18% becomes 9% and 9%. Across states it stays whole as IGST.
- 4The split is presentational only. The total tax is the same either way; what changes is which government it is credited to.
Worked example
A ₹1,00,000 invoice at 18%, within the same state, exclusive of tax.
- Taxable value
- ₹1,00,000
- CGST @ 9%
- ₹9,000
- SGST @ 9%
- ₹9,000
- Invoice total
- ₹1,18,000
Common questions
How do I remove GST from an amount that already includes it?
Divide the total by one plus the rate. At 18%, divide by 1.18 — so ₹1,18,000 inclusive of GST has a taxable value of ₹1,00,000 and tax of ₹18,000. Subtracting 18% of the total instead is the common mistake, and it gives the wrong answer.
When is it CGST and SGST, and when is it IGST?
A supply within a state attracts CGST and SGST, each at half the rate. A supply from one state to another attracts IGST at the full rate. The total tax is identical; only the split differs.
What are the GST rate slabs?
The main slabs are 5%, 12%, 18% and 28%, with 0.25% and 3% applying to specified goods such as precious stones and gold. The rate depends on the classification of the goods or services supplied.
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