How to Reverse Calculate GST

Instructions

1

Add one to the current GST. This allows you to take into account the original cost with the tax. For example, the GST is 5 percent so 1 plus 0.05 equals 1.05.
2

Divide the bill for the goods or services by one plus the GST. In the example, if your bill including GST was $229, then $229 divided by 1.05 equals $218.10. This is your bill without GST.
3

Subtract your bill without GST from Step 2 from the bill for the goods or services with the GST to find the GST paid. In the example, $229 minus $218.10 equals GST of $10.81.