How to Calculate GST: Formulas with Examples

Last updated: 2 July 2026 · 6 min read

Calculating GST is simple once you know two formulas — one to add GST to a base price, and one to extract GST from a price that already includes it. This guide covers both, plus how the tax splits into CGST, SGST and IGST.

Adding GST to a base amount

When your price is exclusive of tax:

GST amount = Base × Rate ÷ 100 and Total = Base + GST.

Example: ₹1,000 at 18% → GST = 1,000 × 18 ÷ 100 = ₹180; total = ₹1,180. Try it in the GST calculator.

Removing GST from an inclusive price

When the price already includes tax:

Base = Inclusive × 100 ÷ (100 + Rate); the GST is the difference.

Example: ₹1,180 including 18% → base = 1,180 × 100 ÷ 118 = ₹1,000; GST = ₹180. See the reverse GST calculator.

CGST, SGST and IGST

For a sale within the same state (intra-state), GST is split equally: half CGST and half SGST. For a sale to another state (inter-state), the entire GST is charged as a single IGST. So ₹180 GST on an intra-state sale is CGST ₹90 + SGST ₹90; on an inter-state sale it is IGST ₹180.

Related: reverse GST calculation, GST rates explained, and the GST bill generator.