there are multiple items that affect the calculation of taxable social security benefits. I suggest you review the worksheet contained in the 1040 instructions on page 32, Then if you have further questions post then in this thread so we can follow the conversation. there is also a similar worksheet in Turbotax. With desktop you can review it in forms mode. With online you'll have to pay any fees to be able to review it.
https://www.irs.gov/pub/irs-pdf/i1040gi.pdf
