Turbotax will calculate that after you enter the SSA-1099 in Federal > Wages and Income > Retirement > Social Security; and all of your other income in the appropriate sections of the program.
Social Security benefits are reported on the SSA-1099.
The total benefits are reported as you note on line 6a on the 1040.
However, up to 85% of the benefits may be taxable based on how much SS you get and how much other income you have. TurboTax will calculate the amount of SS that is taxable and put that amount in line 6b.
If you have the CD/download software or are using the Online software and have paid for it, you can see the "Social Security Benefits Worksheet" which shows the calculation.
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To see the Social Security Benefits Calculation Worksheet in Turbo Tax Online version you would have to save your return with all the worksheets to your computer. Or if you are using the Desktop CD/Download Software you can switch to Forms Mode (click Forms in the upper right) and click on SS in the list on the left side.
Is your Social Security taxable?
Up to 85% of Social Security becomes taxable when all your other income plus 1/2 your social security, reaches:
Married Filing Jointly: $32,000
Single or head of household: $25,000
Married Filing Separately: 0