What penalty does CRA charge for over-contributing to my TFSA?

CRA charges a monthly penalty tax calculated on the amount of your excess TFSA contribution, for every month, or part of a month, that the excess remains in your account. Because this is a monthly-accruing tax rather than a one-time charge, the total cost of an over-contribution grows the longer it sits uncorrected - a small excess left in place for several months can end up costing considerably more than the same excess caught and fixed quickly.

The specific percentage rate CRA applies is set out in the Income Tax Act and is worth confirming currently, since it's exactly the kind of figure that can be easy to find outdated information about online. What matters more immediately if you discover an over-contribution is the timing: because the penalty accrues month by month, the single most effective thing you can do is withdraw the excess amount as soon as you become aware of it, which is covered in more detail in a related question, rather than waiting to sort out exactly how it happened first. Understanding that this penalty is ongoing rather than fixed is the key thing to take away if you're trying to figure out how seriously to treat a TFSA over-contribution.