How do I get expenses to calculate GST on receipts?

Hi danielstilingaut, 

GST is automatically calculated on both sales and expenses once you've enabled the GST feature. Let me show you how to enable GST in QuickBooks Online.

1. Select GST on left navigation pane.
2. Select Get started.
3. Select taxes that needs to be enabled. 
4. Then select Save and finish. 

Once done, you will see a GST column when you create sales and purchase transactions where you can select the correct tax code. If you would like to have the tax code to be automatically populated when you create a transaction, you can set a default tax code on your products and services lists.

Feel free to drop a comment below if you have any other questions. Have a nice day!