How bonuses are taxed
The IRS treats bonuses as 'supplemental wages.' Employers most commonly withhold a flat 22% federal rate (for supplemental pay under $1 million). Social Security (6.2%) and Medicare (1.45%) also apply, plus any state tax.
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Withholding vs. actual tax
The 22% flat rate is withholding, not your final tax. If your marginal rate is lower, you may get some of it back at tax time; if higher, you may owe more.