Free GST Calculator: CGST, SGST and IGST
Calculate GST inclusive and exclusive amounts free. Understand the CGST/SGST split and when IGST applies instead.
GST arithmetic is simple until you have to do it backwards. Adding 18% to ₹1,000 is easy. Working out how much of a ₹1,180 invoice is tax is where people reliably make mistakes — and the mistake propagates into your books, your returns and eventually a mismatch notice.
Why a free GST calculator matters more in India
Every registered business in India deals with this daily, and the reverse calculation is where errors concentrate. The other frequent confusion is the split. For a transaction within a state, GST divides equally into CGST and SGST — 18% becomes 9% plus 9%. For a transaction across state lines, the whole 18% is IGST instead. Getting this wrong on an invoice is not a formatting issue; it creates a genuine reconciliation problem for both parties.
The best free options compared
At a glance
Of the 4 options below, 4 are fully free with no paid tier to upgrade to, and 0 offer a free tier alongside paid plans. That distinction matters more than it first appears: a fully free tool cannot restrict you later, whereas a free tier is a commercial decision that can be revised.
| Tool | Cost | Best for |
|---|---|---|
| ClearTax GST calculator | Fully free | Handles both directions — tax-inclusive and tax-exclusive — and splits CGST/SGST… |
| A spreadsheet formula | Fully free | For anyone invoicing regularly this beats any web tool |
| Free invoicing software | Fully free | Zoho Invoice and similar free tiers compute GST automatically as you build the invoice,… |
| GST portal | Fully free | Not a calculator as such, but the authoritative source for current rates by HSN code |
Each option in detail
ClearTax GST calculator
✓ FREEHandles both directions — tax-inclusive and tax-exclusive — and splits CGST/SGST automatically. Clean and quick for one-off checks, and it presents the split clearly enough to copy straight onto an invoice.
A spreadsheet formula
✓ FREEFor anyone invoicing regularly this beats any web tool. Exclusive: amount × rate. Inclusive: amount × rate ÷ (100 + rate). Build it once into your invoice template and the calculation stops being a task at all.
Free invoicing software
✓ FREEZoho Invoice and similar free tiers compute GST automatically as you build the invoice, apply the right split based on the place of supply, and produce a compliant document. If you are calculating GST manually more than a few times a week, you are using the wrong tool.
GST portal
✓ FREENot a calculator as such, but the authoritative source for current rates by HSN code. Since rates change and vary by category, verify the rate here before calculating with it.
Note on free tiers: limits and features change often. Everything here was checked in July 2026, but confirm current terms on the provider's own site before committing to a workflow. Official guidance: Income Tax Department GST Portal.
How to choose the right one
For occasional checks, any web calculator is fine — the arithmetic is not in dispute. For regular invoicing, stop calculating manually and use free invoicing software that applies GST automatically and records the place of supply, because that is where the CGST/SGST versus IGST decision is actually made. Always confirm the applicable rate against the GST portal rather than assuming.
If you are assembling a wider free toolkit, these related guides cover adjacent problems: Free SIP Calculator: Project Mutual Fund Returns, Free Income Tax Calculator India (Old vs New), Free EMI Calculator: Home, Car and Personal Loan.
Verdict
ClearTax's calculator is the quickest free option for a one-off figure. But if you are running a business, the real answer is free invoicing software: it removes the calculation, applies the correct split, and leaves an audit trail that a calculator never will.
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See all Finance & Tax Calculators →Frequently asked questions
How do I calculate GST backwards from an inclusive amount?
Divide the inclusive amount by (100 + rate) and multiply by the rate. For ₹1,180 at 18%: 1180 ÷ 118 × 18 = ₹180 GST, leaving ₹1,000 base. The common error is taking 18% of the inclusive figure, which overstates tax.
When is IGST charged instead of CGST and SGST?
IGST applies to inter-state supplies and imports. CGST plus SGST applies to intra-state supplies. The determining factor is the place of supply relative to the supplier's state, not where the customer happens to be at the time.
What are the current GST rates in India?
The main slabs are 5%, 12%, 18% and 28%, with some goods at nil or special rates. Rates are revised by the GST Council periodically, so verify against the official GST portal for your specific HSN code rather than relying on memory.
Do I need to register for GST?
Registration thresholds depend on turnover and differ for goods, services and special category states, and some categories require registration regardless of turnover. Check current thresholds on the GST portal or consult a professional.