GST (Goods and Services Tax) applies to most goods and services in India, and getting the maths right matters for invoicing, pricing and filing. The two mistakes people make most often are confusing GST-inclusive with GST-exclusive amounts, and misapplying the CGST/SGST vs IGST split. This guide clears both up.
Exclusive vs inclusive — an exclusive price is the base amount before GST, so you add GST on top. An inclusive price already contains GST, so to find the base you remove it. For a rate r, GST on an exclusive amount A is A × r; the base inside an inclusive amount T is T ÷ (1 + r).
A quick example at 18%: on an exclusive ₹1,000 the GST is ₹180 and the total is ₹1,180. If instead ₹1,180 is inclusive, the base is 1,180 ÷ 1.18 = ₹1,000 and the GST component is ₹180. Same numbers, opposite direction — mixing them up is where invoices go wrong.
CGST/SGST vs IGST — for a sale within the same state, GST splits equally into CGST (central) and SGST (state); an 18% rate becomes 9% CGST + 9% SGST. For a sale between states it is a single IGST at the full 18%. The total tax is identical; only the split and reporting differ.
Common slabs are 0%, 5%, 12%, 18% and 28%, with different goods and services falling into each. Always confirm the correct slab for your specific product or service.
Rather than doing this by hand on every invoice, use Farvixo's GST Calculator: enter the amount, choose the rate, and toggle inclusive or exclusive to see the base, the GST amount, the CGST/SGST/IGST breakdown and the final total instantly.
It runs free in your browser with nothing to install. Calculate GST accurately every time with the Farvixo GST Calculator.