To determine the GST, simply type in the purchase price in the appropriate box, then click on the final purchase price. Note these numbers are the same for both the Homeowner and the Investor Rebate but keep in mind that for certain developers the grant may be claimed on closing. Be careful with this, as not all developers allow the Buyer to claim the rebate, resulting in the Buyer requiring all of the funds, then applying and waiting several months for the rebate to arrive. The Investor Rebate is never given on closing, so the Buyer will need to pay all the GST and then apply for a refund.
For the reverse calculator (when the GST in included in the price) simply type the final purchase price into the box and then click in the Purchase Price box.