Whats the GST? Australia, adds and takes away 10% Goods and Services Tax. Use the Add 10% GST Calculator to work out what the pricing is after-GST has been added, an equation formula will reveal the value of the tax and the final calculated price. OR alternatively use the Minus 10% GST Calculator to work out what the value of the product was before-GST was added. Custom after-tax and before-tax calculators included.