The arithmetic of adding tax differs from extracting tax already included in a total. Decide which amount you hold before calculating. The examples below use an assumed rate for arithmetic; they do not assign a current legal rate to a product or service.
Add tax to a base amount
Tax = base × rate / 100. Total = base + tax. For an illustrative base of 1,000 and an assumed 10% rate, tax is 100 and the total is 1,100. Use the exclusive mode in the GST Calculator.
Extract tax from an inclusive total
Base = total / (1 + rate / 100). Tax = total − base. At the same assumed rate, an inclusive total of 1,100 gives a base of 1,000 and tax of 100. Multiplying the inclusive total by 10% would incorrectly produce 110.
Keep a rounding record
Line-by-line rounding can differ from rounding once on an invoice total. Record which method your accounting workflow requires and compare the output with the actual invoice. The GST Invoice Formatter is a draft formatting aid; it does not validate compliance.
Choose the rate separately
The rate, supply classification, component treatment, taxable base and any conditions must come from applicable rules and transaction facts. A calculator cannot determine these from an amount. Use the official GST portal and qualified advice for current requirements. Our GST rate-checking guide explains what to record; it is not a verified current-rate table.