The formula for calculating GST and QST used in 2017
The calculation of the GST and the QST is normally done in two steps, you must calculate the GST (at 5%) on the sale price, then add the QST (at the rate of 9.975%) at the same price of the sale. The calculation of the GST and QST can also be done in one step, you must use the rate of 14.975% to calculate both taxes. The resulting amount includes GST and QST.
For the reverse calculation GST and QST, you must take the amount with taxes and divided by the combined rate of 1.14975 to obtain the original amount without the two taxes. You simply multiply the amount before taxes obtained by 5% for the amount of GST and multiply by 9.975% for the amount of QST.
Note: An error margin of $ 0.01 is possible due to rounding of amounts in the calculation.