How bonuses are taxed
A bonus is "supplemental wages". Many employers withhold federal income tax on it at a flat 22% (37% on the part of supplemental pay above $1 million in a year). Others add it to your regular paycheck and withhold using the "aggregate" method, which can be a different percentage. Social Security, Medicare and state tax also apply.
Withholding is not your final tax. Your bonus is added to your income for the year, so it is really taxed at your normal bracket rates when you file. This calculator shows both numbers, so you can see whether you may get money back or owe a little more. It assumes the flat 22% method and does not include local taxes.