Reviewed: 16 September 2026. This guide explains general calculations and workflows; regulated, tax, financial or legal decisions should be checked against the current official rule or provider terms.
Start with the taxable value
Most GST mistakes start before the multiplication. Decide whether the price you have is before tax or already includes tax. If ₹1,00,000 is the taxable value and the applicable rate is 18%, GST is ₹18,000 and the invoice total is ₹1,18,000. If ₹1,18,000 is an all-inclusive selling price, the GST portion is not 18% of ₹1,18,000. It is extracted using 18/118, which returns ₹18,000.
CGST and SGST
For a typical intra-state supply, GST is divided equally between central and state components. An 18% rate appears as 9% CGST and 9% SGST. For a ₹1,00,000 taxable value, that is ₹9,000 CGST plus ₹9,000 SGST. The arithmetic is simple; the important part is deciding whether the transaction is actually intra-state under the place-of-supply rules.
IGST
Inter-state supplies generally use IGST instead of the CGST/SGST split. On the same ₹1,00,000 example at 18%, IGST is ₹18,000. A calculator can compute either presentation, but it cannot determine the legal place of supply from an amount alone.
Rounding and line items
For multi-item invoices, calculate consistently. Systems may round tax at the line level or after aggregation, which can create paise-level differences. Keep sufficient precision during the calculation and round only according to the invoicing method you actually use. When comparing an ERP total with a manual calculation, check whether both systems round at the same stage.
Quotation workflow
- Confirm whether each price is taxable or tax-inclusive.
- Confirm the HSN/SAC and current rate from an official or professional source.
- Determine whether CGST/SGST or IGST applies.
- Calculate taxable value, tax and grand total.
- Show exclusions such as transport, installation or stamping separately when they are not included.
What a calculator cannot decide
Classification, exemption, reverse charge, place of supply, composition rules and notification-specific treatment depend on facts outside the price itself. For material transactions, use official GST/CBIC material or professional tax advice rather than treating a generic calculator as a legal opinion.
Why this guide exists
Divya Tools publishes utilities for quick practical work. The purpose of this guide is to expose the assumptions and common failure points that a calculator or converter cannot communicate through input fields alone. If you spot a factual or functional error, use the contact page so the page can be reviewed and corrected.