The amount of your SS benefit that is taxable is calculated in accordance with the details in IRS Pub. 915, which you can find here:
https://www.irs.gov/pub/irs-pdf/p915.pdfIf you are using the worksheet on the back of your SSA-1099 and getting a different result than Turbotax gets, then you have a mistake in your calculations. For whatever reason, most people who use that worksheet get the wrong answer.
