If you have an amount of GST (@15%) from a transaction and need to calculate the original price here’s the formula…
For the GST Exclusive Price: Multiply the GST amount by 6.66666666
For the GST Inclusive Price: Multiply the GST amount by 7.66666666
Note: The numbers above should actually be 6.6 recurring and 7.6 recurring respectively, so some minor rounding will be required. There’s a calculator for this formula here.
Update 08/09/2010: For accuracy IRD recommend using 8 decimal places for the above calculations. We’ve updated the above figures to reflect this.