GST is a tax in Canada on sales of goods and services. The GST rate as of December 15, 2010 is 5%. All tax calculations are similar, where you multiply the cost of the goods by 1 plus the tax rate to find the total bill. Therefore, in order to reverse calculate the GST, you need to know how much your total bill was and the tax rate. This allows you to calculate how much the bill was without GST. The difference between the bill with and without the GST is the GST.
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.
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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.
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.