How to Calculate GST-Inclusive and GST-Exclusive Amounts

Updated 8 October 2026

Before calculating, decide whether the amount already includes tax. A base amount and an inclusive total are different inputs. These tools use the rate you enter; they do not determine the legally applicable rate or tax treatment.

Add GST to a base amount

Use the GST or GST Exclusive Calculator for an amount before tax. Tax = base × rate ÷ 100. Total = base + tax.

Open the GST Exclusive Calculator.
Enter a base amount of 1,000 and an example combined rate of 18%.
Choose Run. The result shows a base of 1,000, tax of 180 and total of 1,180.

Extract GST from an inclusive total

Use GST Inclusive Calculator when tax is already included. Base = total ÷ (1 + rate ÷ 100); included tax = total − base.

For an inclusive total of 1,180 and an entered 18% rate, the base is 1,000 and GST is 180. Multiplying 1,180 by 18% would calculate tax on the inclusive total and produce the wrong extraction.

CGST and SGST input convention

The current CGST and SGST tools expect the combined rate, not an already-halved component rate. Each returns half the tax calculated from the base. With a 1,000 base and 18% combined rate, each component is 90 and combined tax is 180. This is an implementation convention, not a determination that a transaction requires CGST/SGST rather than IGST.

Common mistakes to check

Do not add tax again to an amount that already includes it.
Check whether you entered a combined or component rate.
Use consistent currency units for the amount.
The example rate is illustrative; confirm the correct rate independently before using a result in an invoice.

Why might my accounting result differ?

Invoice-level rounding, line-item rounding, discounts and transaction-specific rules can affect final billing. These arithmetic tools do not replace an accounting system or determine tax liability.