Do I have to put my HST number on every invoice I send in Ontario?

Yes. Once you are registered for HST, you are required to show your GST/HST registration number on invoices for taxable supplies. The CRA's rules specify what must appear on an invoice depending on the total amount.

For sales over $30 (plus tax), you need: your business name (or trading name), the date of the invoice, a description of the goods or services, the total HST charged (or the rate applied), and your 9-digit GST/HST registration number. For sales over $150, you also need the buyer's name or trading name and payment terms. For sales over $1,000, you need a sequential invoice number as well.

Failing to include your HST number on invoices is a problem for your customers, not just for you — your customers need the number to claim input tax credits on what they paid you. If you omit it, they may ask you to reissue the invoice or you may face CRA scrutiny. Unregistered businesses must not put an HST number on their invoices and must not charge HST.