The useful bit: For an inclusive amount, recover the base by dividing by 1 plus the rate. The tax is the difference, not the rate times the inclusive total.
First identify what the number includes
An amount before GST and a final amount including GST are different starting points. Adding tax starts with the base. Removing tax starts with the total, where the tax has already been applied to a smaller number. Label your starting amount before doing the arithmetic.
CalculateKro offers separate add and remove modes so that the formula matches the question. The GST rate is entered by the user. The calculator does not select a legal classification or determine which rate applies to a transaction.
Work through a purely illustrative rate
Take a hypothetical base of ₹1,000 and an assumed 18% rate. Adding tax gives ₹180 of tax and a total of ₹1,180. To reverse this, divide ₹1,180 by 1.18. That recovers the ₹1,000 base and the ₹180 tax portion.
Subtracting 18% of ₹1,180 would remove ₹212.40 and produce ₹967.60. That is not the original base because 18% was originally applied to ₹1,000, not to the final total. The direction of the calculation changes the denominator.
Separate arithmetic from tax treatment
A split illustration can show equal CGST and SGST portions or a single IGST amount. It does not decide place of supply, registration status, exemptions, credits or cess. Those are legal and transaction-specific questions beyond a percentage tool.
The example rate is not a statement that a particular product or service attracts that rate. Check current CBIC information for the relevant classification and date, or ask a qualified professional.
Keep a clear calculation trail
Record the original amount, whether it was inclusive, the rate you applied and the resulting base and tax. Preserve the invoice or other evidence separately. A calculator output is not itself an invoice or filing record.
When a seller’s total differs slightly, check rounding and whether additional charges enter the tax base. This tool rounds the display, retains precision in the model and does not infer those extra conditions.
Context checked 30 September 2026. External sources do not endorse this tool.