The IRS tax withholding calculator for the current tax year is free to use, although previous versions are not available. The calculator uses input information to estimate the appropriate amount of federal income tax for a worker to withhold on a W-4 form.
Information required by the calculator include the worker's filing status, number of jobs, number of dependents and tax credits. The calculations use the amount of a worker's wages, bonuses, tips and other forms of taxable income, along with deductions, such as those relating to medical expenses and charitable donations. The final page of the calculator provides an estimate and instructions for modifying withholding.