If Social Security benefits are your only income, they're probably not taxable. Even if you have other income, you still might not owe tax on your benefits. IRS Publication 915 explains how to calculate if there's a tax on your benefits, and how to file the tax payment if there is. You can also find a worksheet in the instructions for Form 1040.
If you received Social Security benefits last year, whether retirement or disability, the Social Security Administration will send you an SSA-1099 form. This shows your total Social Security income for the year, though it might not match the amount you received. The Social Security Administration can adjust the amount it pays out if, for example, you want your Medicare premiums deducted from your Social Security check, or you think you'll owe taxes and request tax withholding on your benefits. The 1099 notes these adjustments.
When Benefits Are Taxable
The instructions for Form 1040 include a worksheet for figuring whether your benefits are taxable. To start, take the net benefits in Box 5 of the SSA-1099, enter it on the worksheet, then write half that figure on the next line. If you're filing a joint return, include your spouse's benefits too.
Report your other taxable income from the front of the 1040. Include your tax-exempt interest, but not any money you made from qualified dividends. Add the total of your other income to half your Social Security benefits.
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Add up all the income adjustments you entered on Form 1040, lines 23 through 32, plus anything from line 36. If the total adjustments are more than the result from Step Two, your benefits aren't taxable. If the adjustments are less, subtract them from your Step Two income.
If you're married, filing jointly, write down $32,000. If you file under any other status, write down $25,000. If the total of your adjusted income is under this figure — what the IRS calls your base amount — your benefits aren't taxable.
Subtract your base amount from your income. Enter this on line 9 of the worksheet
Enter $12,000 on line 10 if you're married and filing a joint return. Otherwise, enter $9,000.
Subtract line 10 from line 9 and enter the result on line 11.
Enter the smaller amount from line 9 or line 10 on line 12. Enter half that figure on line 13. On line 14, enter the number from line 13 or one-half your net benefits, whichever is smaller.
Multiply line 11 by 0.85. Add the result to line 14 and enter on line 16.
Multiply your total net benefits by 0.85 percent and enter on line 17. Whichever line, 16 or 17, is the smaller, enter that on Form 1040, line 20b, as your taxable Social Security benefits. You pay tax on the benefits as the same rate as your other income.