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.
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.
When I first posted my reply, the link for the IRS Instructions for Form 1040 was incorrect. I have corrected the link in my reply above, and the link in this post is correct.