Withholding is not your tax rate. How the annualized withholding formula inflates a bonus cheque, what the bonus actually costs at your marginal rate, and what settles at filing time.
A $10,000 bonus lands and $5,000 of it seems to vanish. Your group chat says bonuses are taxed at 50% in Canada. They are not — there is no bonus tax, no special bracket, and no separate rate. What you saw was withholding: an estimate your employer is required to send to the government on the spot, calculated by a formula that assumes the bonus cheque is what you earn every pay period. Your actual tax on the bonus is settled months later, when you file, at your real marginal rate — and the difference comes back as a refund or a smaller balance owing. The gap between those two numbers is the entire story.
For a regular paycheque, payroll software annualizes your pay: it takes this period's taxable pay, multiplies it out to a year, computes the tax on that annual figure, and divides back down. That's reasonable for salary. Now drop a $10,000 bonus into one bi-weekly period on top of a $100,000 salary. This period's pay is $3,846 of salary plus $10,000 of bonus — $13,846. Annualized, the software sees a $360,000 income and withholds accordingly, deep into the top brackets, with CPP and EI piled on if they haven't maxed out yet. The withholding on the bonus portion alone can approach 45–50% for a higher earner. None of that is your tax rate; it's the formula doing exactly what it's designed to do — over-collect rather than under-collect, and let the return sort it out.
There is a second method you may meet: for certain lump sums paid outside regular payroll — retiring allowances, some severance, and similar payments — the CRA prescribes flat withholding rates instead (10% up to $5,000, 20% to $15,000, 30% above, with different rates in Quebec). Those are also just withholding. A $30,000 retiring allowance withheld at a blended flat rate still gets taxed on your return at your real marginal rates, with the withholding credited against the bill.
Your real cost is your marginal rate on the bonus dollars, full stop. Take the $100,000 Ontario salary from our $100,000 Ontario breakdown: the marginal rate is 31.48% (20.5% federal, plus 9.15% Ontario grossed up by the first surtax layer). A $10,000 bonus on that salary costs $3,148 of income tax — and, because CPP and EI are already maxed at that salary, nothing more. If $5,000 was withheld, roughly $1,850 of the difference returns at filing time (assuming the rest of the return is neutral). At a $60,000 Ontario salary, the same bonus costs less in rate — about 29.65% in income tax — but CPP and EI are still running at that income, adding up to 7.58 points until the maximums; the all-in marginal hit is in the mid-30s, and again the annualized withholding on the bonus cheque will look worse than the truth.
A few legitimate levers exist. RRSP: if your employer lets you direct the bonus straight into a group RRSP, it can be paid without tax withholding at source (the contribution is a deduction the employer can see), and you get the same deduction a personal contribution would give. Timing: a bonus paid in January instead of December lands in a fresh CPP/EI year — same tax, but the payroll deductions restart, which changes the cheque, not the return. A bonus that straddles a genuinely lower-income year (a parental leave year, a sabbatical) is taxed at that year's lower marginal rate — real savings, when the timing is real. Form T1213: if you have large, predictable deductions, you can ask the CRA for a letter authorizing your employer to withhold less year-round. And the simplest framing of all: a bonus withheld at 50% and taxed at 31% is an interest-free loan to the government for a few months — annoying, worth optimizing lightly, and never a reason to turn the bonus down. Run your salary plus the bonus through the income tax calculator twice — with and without — and the difference is your true tax on the bonus, to the dollar.