Bonus Tax Withholding
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Many tax systems allow employers to withhold tax on bonuses and other supplemental wages using a separate flat percentage rather than the regular payroll withholding formula, since bonuses are irregular and don't fit neatly into a per-paycheck withholding table.

Not necessarily — the flat rate applies only to how much is withheld upfront from the bonus payment, not necessarily your final tax liability on that income. When you file your annual tax return, the bonus is combined with all your other income and taxed according to your actual overall tax situation, which can result in a refund or additional amount owed related to the bonus.

This is a common perception because the flat supplemental withholding rate on bonuses is often higher than what's withheld from an equivalent amount of regular salary, even though the bonus isn't necessarily taxed at a fundamentally different final rate once your full year's income and taxes are settled at filing time.

If your total withholding for the year (including from the bonus) ends up higher than your actual tax liability, the excess is typically returned to you as part of your tax refund when you file your return.

It generally assumes the flat-percentage supplemental method — some employers instead combine the bonus with your regular pay for that period and apply standard withholding tables to the total, which can produce a different withholding amount than the flat-rate method.