@mrbillyo
The Interactive Tax Assistant is not part of a tax return. The taxable amount of Social Security benefits on Form 1040 line 6b has to be calculated using the Social Security Benefits Worksheet - either the one in TurboTax or the one in the IRS instructions. You started off comparing the two versions of the Social Security Benefits Worksheet, but you have gone off on tangents.
If your manual calculations on the Social Security Benefits Worksheet in the IRS instructions come out with an amount on line 18 of the worksheet that is different from the amount on line 18 of the TurboTax worksheet, we can explore why there is a difference. But you have to be clear and precise, and focus on the Social Security Benefits Worksheet.
When you refer to a line number, please make sure it's clear what form or worksheet you are referring to. Your reference to "my entry into line 2a" is not clear because a lot of forms have a line 2a, and the Social Security Benefits Worksheet is not one of them. And why do you say "my entry"? Are you talking about a form where you key an amount into TurboTax from a form that you received, such as box 2a on a 1099-DIV or box 2a on a 1099-R?
Also, you still have not said whether you are using TurboTax Online or the CD/Download TurboTax software.
As I said earlier, there are differences between the TurboTax worksheet and the IRS worksheet because TurboTax has modified the worksheet to handle some situations that the IRS worksheet does not handle. Among those differences, TurboTax has added lines 19 and 20 to handle the lump-sum election. What is on line 19 of the Social Security Benefits Worksheet in TurboTax? If there is an amount on that line it could account for the difference in the amount on Form 1040 line 6b. If you have an amount on line 19, what are the exact amounts on lines 18, 19, and 20 of the TurboTax worksheet, what is the exact amount on line 18 of your manually calculated IRS worksheet, and what is the exact amount on Form 1040 line 6b?
In a previous post you said that you think the root of the problem is a difference in line 3 of the Social Security Benefits Worksheet. Another of the differences between the TurboTax worksheet and the IRS worksheet is that line 3 is not the same. But if you think that's where the problem lies, what is the exact amount on line 3 of the Social Security Benefits Worksheet in TurboTax, and what is the exact amount on line 3 of your manually calculated IRS worksheet? What are the exact amounts on each of the following lines on the Form 1040 that you filed: 1z, 2a, 2b, 3b, 4b, 5b, 7, and 8?
Finally, let me say that it is extremely unlikely that the calculation of taxable Social Security benefits in TurboTax is wrong. The calculation and the worksheet haven't changed in years, so there's nothing new about it in this year's TurboTax. By now, any bugs that might have previously existed would have been found and fixed years ago.
