Once upon a time, Social Security income was not taxed. Today, millions receiving monthly benefits must file social security tax. If you use tax software to prepare your return or file taxes online, you can simply input the amount of your Social Security benefits, and the taxable amount is automatically calculated. Otherwise, you'll need IRS Form 1040. Use the figure in Box 3 shown on your SSA-1099 statement. Be sure to subtract any Medicare premiums you paid to reduce your total reportable income. Individuals or married couples with IRA contributions must use IRS Pub. 590 worksheets. None of your benefits may be taxable if you repaid more in the previous year that you received. For other scenarios, use the worksheet found on page 27 of Form 1040 to determine your tax liability.