How to use the gst calculator
- Choose whether your amount is before GST (add GST) or already includes GST (remove GST).
- Enter the amount and pick a rate, or type a custom one.
- Select within a state for CGST + SGST or between states for IGST.
- Read the base amount, GST, split and total. Copy the summary for an invoice or message.
Features
- GST-exclusive and GST-inclusive (reverse) calculations
- Presets for 0%, 0.25%, 3%, 5%, 12%, 18% and 28% plus custom rates
- CGST/SGST split for intra-state supply, IGST for inter-state supply
- Paise-accurate rounding
- Formula displayed for every result
How GST is calculated
When a price excludes GST: GST = price × rate, and the customer pays price + GST. When a price already includes GST: base = price ÷ (1 + rate), and GST = price − base. Getting the second case wrong is the classic mistake: 18% of a ₹1,180 inclusive price is ₹212.40, but the GST inside it is ₹180, because the tax was charged on ₹1,000.
CGST, SGST and IGST
For a supply within one state or union territory, the GST rate is split equally between the Centre (CGST) and the state (SGST, or UTGST in union territories): an 18% rate means 9% + 9%. For a supply from one state to another, the whole rate is charged as IGST and later apportioned between governments. The customer pays the same total either way; the split only matters for invoicing and returns.
Rates and assumptions
The presets cover the main slabs in force in September 2026, including the 0.25% rate for rough diamonds and the 3% rate for gold, silver and jewellery. Rates change with GST Council decisions, and many goods carry a cess on top; this calculator applies only the rate you enter and does not include compensation cess, TDS/TCS under GST or composition-scheme rules. Check the current rate for your HSN or SAC code before invoicing.