Form W-4
The federal employee withholding certificate that determines how much federal income tax to withhold each pay period.
Form W-4 is the federal Employee's Withholding Certificate, which employees complete at hire (and any time their tax situation changes) to specify how much federal income tax should be withheld from each paycheck. The 2020 redesign of Form W-4 eliminated the long-standing "withholding allowances" concept in favor of dollar-amount adjustments; employees can claim multiple jobs adjustments, dependent credits, other income adjustments, and additional voluntary withholding directly.
The employer uses the W-4 plus the employee's pay frequency and gross wages to compute federal withholding from IRS Publication 15-T's percentage method or wage bracket tables. The withholding amount is recomputed each pay period based on that period's wages — there's no annual "smoothing" — which is why a one-time bonus paycheck often produces a much higher withholding rate than the employee's regular paycheck.
W-4s are not filed with the IRS; the employer keeps them on file. State withholding usually requires a separate state-specific form analogous to the W-4.