The Tax Withholding Estimator works for most employees by helping them determine whether they need to give their employer a new Form W-4. They can use their results from the estimator to help fill out the form and adjust their income tax withholding. If they receive pension income, they can use the results from the estimator to complete a Form W-4P, Withholding Certificate for Pension and Annuity PaymentsPDF, and give it to their payer.