You have to register for VAT once your business passes R2.3 million in sales in any 12-month period. Once you cross that line, you must register within 21 business days.
Below that, registering is optional. Most businesses can register voluntarily once they've passed R120,000 in sales in the past 12 months, and some can register on less — see how much you need to make to register for VAT voluntarily.
Not sure it's worth it? Weigh up the pros and cons of registering for VAT.