There is no formula. You have to use the Social Security Benefits Worksheet to calculate the taxable amount of Social Security benefits. You can find the Social Security Benefits Worksheet in the IRS Instructions for Form 1040. For 2024 the worksheet is on page 32 of the instructions.
TurboTax uses its own version of the Social Security Benefits Worksheet. It has some slight differences from the IRS worksheet, but the result is the same. You can find the worksheet in TurboTax as VolvoGirl described above.
UPDATE: Corrected link for 2024 IRS Instructions for Form 1040.
