To avoid paying interest on your credit card purchases, you'll need to pay off the balance each month. That is the total amount owing at the end of the month, in full by the 21st of the following month. This gives you up to 52 days interest-free for purchases made from the 1st of the previous month to the 21st of the current month. You can find the balance owing on your monthly statement.
How the interest free period works:
