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
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 right side.
The portion of your social security benefits that is taxable varies with your income level. If your total income is between $25,000-$34,000 for an individual or $32,000-$44,000 for a married couple filing jointly, you must pay up to 50% of federal income taxes on your Social Security benefits. If your total income is higher than $34,000 for an individual or $44,000 for married filing jointly, then up to 85% of your benefits may be taxed.
The IRS has on online tool that can help you calculate how much of your social security is subject to taxation.