GST-inclusive and GST-exclusive arithmetic
See why tax is added to a net amount but removed from a gross amount using division, with a worked 18% example.
Draft prepared: 2026-10-02. Publisher: GXA Technologies. No named expert review is claimed. AI-assisted draft; human editorial review pending
Identify what the amount already contains
A net amount excludes the tax being calculated. A gross amount includes it. The rate applies to the net base, so adding and removing the same percentage are not symmetric percentage subtractions.
This page explains arithmetic only. It does not identify current rates, exemptions, cess, eligibility for credit or place-of-supply treatment for a real transaction.
Add GST to a net value
At an assumed rate t percent, tax = net × t / 100 and gross = net + tax. For ₹1,000 and an assumed 18%, tax is ₹180 and gross is ₹1,180.
The GXA calculator also displays equal CGST and SGST halves. That presentation assumes a split; it does not determine whether the actual invoice should use IGST.
Remove GST from an inclusive value
Use net = gross / (1 + t/100), then tax = gross − net. With ₹1,180 at the assumed 18%, dividing by 1.18 gives ₹1,000 net and ₹180 tax.
Subtracting 18% of ₹1,180 would instead subtract ₹212.40. That is wrong for this reversal because it applies the percentage to gross rather than the original base. CBIC’s inclusive-value rule describes the corresponding tax extraction relationship.
Check the invoice assumptions separately
Confirm the applicable rate and tax components from authoritative current guidance or a qualified adviser. A rate being selectable in a calculator is not proof that it applies to your product or service.
Check rounding at the level the receiving invoice system uses. Net plus tax should reconcile with gross, allowing for its documented rounding convention. Mixed-rate invoices need separate line-level treatment rather than one assumed percentage.
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