How is The taxable part of my social security determined? Why is there a tax on it at all. My income is very small.

• Combined income under $25,000 (single) or $32,000 (couple): Benefits are not taxed.
• Combined income of $25,000 to $34,000 (single) or $32,000 to $44,000 (couple): Up to 50 percent of benefits can be taxed.
• Combined income above $34,000 (single) or $44,000 (couple): Up to 85 percent of benefits can be taxed.

If you are married filing separately, your social security is automatically taxed (85% of your benefit is subject to tax) without regard to the usual limits.

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.  @olliedare 

Here is the Turbo Tax SS Worksheet.  I couldn't fit it all on one screen shot so it's 2 parts.