What is a GST Calculator (India)?
Indian GST works both directions: quoting a price before tax (add GST) and figuring out the tax hidden inside an inclusive price (remove GST): the second one is where the mental math goes wrong, because you divide by 1.18, not subtract 18%.
Pick the rate, choose the direction, and read base, tax, total and the CGST/SGST split.
For businesses, the direction of the calculation decides compliance: quotes and invoices to consumers must show GST-inclusive prices under Indian rules, while B2B invoices itemize base and tax so buyers can claim input credit. Getting the reverse calculation wrong (subtracting 18% instead of dividing by 1.18) understates the base and overstates the tax on every line.
Where the paise go missing
Adding GST and then removing it does not always return the number you started from, and neither does the tool being wrong. Tax on an odd base produces a fraction of a paisa, which has to be rounded somewhere, and once it is rounded the reverse calculation cannot recover what was discarded.
The same effect scales up on a multi-line invoice. Tax calculated per line and then summed will differ from tax calculated once on the invoice total, usually by a rupee or two, because each line rounds independently. Accounting systems pick one convention and apply it everywhere; problems only appear when a quote produced one way is reconciled against an invoice produced the other way. If a total is out by a rupee, this is almost always the reason, and the fix is to agree on where the rounding happens rather than to hunt for an arithmetic error.
Frequently asked questions
What is input tax credit?
Registered businesses offset GST paid on purchases against GST collected on sales, remitting only the difference: the mechanism that makes GST a value-added tax rather than a cascading one.
When do I need GST registration?
Broadly: turnover above Rs 40L for goods (Rs 20L for services, lower in special-category states), or any interstate B2B supply. Thresholds change: confirm current rules with a CA.
How do I remove GST from an inclusive price?
Divide by (1 + rate/100): ₹1,180 inclusive at 18% → 1180 ÷ 1.18 = ₹1,000 base, ₹180 GST. Subtracting 18% gives the wrong answer.
What are CGST, SGST and IGST?
Within a state, GST splits equally into central (CGST) + state (SGST). Across states, the full rate is charged as IGST.
Which slab applies to my product?
Since 22 September 2025 most supplies are at 5% (merit), 18% (standard) or 40% (special, for select goods), set per HSN/SAC code. The old 12% and 28% rates no longer apply to current supplies, except the tobacco items that stay on the old structure for now. Check the rate finder on cbic-gst.gov.in or ask your CA.
Can I still use 12% or 28%?
Yes, through the custom rate: choose Custom Rate and type 12 or 28. This helps with invoices dated before 22 September 2025 and with the tobacco items that have not moved yet. Source: PIB, Ministry of Finance, 3 September 2025 (pib.gov.in, PRID 2163555 and FAQ PRID 2163560).
What about 3% and 0.25%?
These special rates (gold, silver and jewellery at 3%, rough diamonds at 0.25%) were part of the pre-2025 structure. The September 2025 releases do not mention them, so the tool keeps them as special options. Confirm the current entry for your HSN code before relying on them.
Is this a substitute for tax advice?
No: it is arithmetic. Classification and compliance questions belong with a professional.