Online withholding calculators are available on many payroll and tax service websites, including ADP, TurboTax and Kiplinger. The Internal Revenue Service also provides a withholding calculator on the official IRS website, as of 2015. These calculators assist taxpayers when filling out the Form W-4, Employee's Withholding Allowance Certificate.
Each of these sites provides a slightly different set of input options and results. Kiplinger uses a simple 3-question form and recommends changes to the number of withholdings to maximize take-home pay.
TurboTax, ADP and the IRS require detailed information from previous tax returns and paycheck stubs. They inform the taxpayer how to properly fill out the W4 form to achieve desired income. Tax information changes each year, so taxpayers need to be sure the withholding calculator they use is for the correct tax year.