Introduction The federal income tax is a pay-as-you-go tax. You must pay the tax as you earn or receive income during the year. There are two ways to pay as you go.• Withholding. If you are an employee, your employer probably withholds income tax from your pay. In addition, tax may be withheld from certain other income, such as pensions, bonuses, commissions, and gambling winnings. The amount withheld is paid to the IRS in your name.• Estimated tax. If you don’t pay your tax through withholding, or don’t pay enough tax that way, you might have to pay estimated tax. People who are in business for themselves will generally have to pay their tax this way. You may have to pay estimated tax if you receive income such as dividends, interest, capital gains, rents, and royalties. Estimated tax is used to pay not only income tax, but other taxes such as self-employment tax and alternative minimum taxThis publication explains both of these methods. It also explains how to take credit on your return for the tax that was withheld and for your estimated tax payments. If you didn’t pay enough tax during the year, either through withholding or by making estimated tax payments, you may have to pay a penalty. Generally, the IRS can figure this penalty for you.Nonresident aliens. Before completing Form W-4, Employee's Withholding Certificate, nonresident alien employees should see Notice 1392, Supplemental Form W-4 Instructions for Nonresident Aliens (Rev. January 2020), which provides nonresident aliens who are not exempt from withholding instructions for completing Form W-4, and the Instructions for Form 8233, Exemption From Withholding on Compensation for Independent (and Certain Dependent) Personal Services of a Nonresident Alien Individual. Also, see chapter 8 of Pub. 519, U.S. Tax Guide for Aliens, for important information on withholding.