No, please do not add the amount of taxable Social Security benefits by hand (in Forms mode, I assume is what you meant). This invalidates the Tax Accuracy Calculation Guarantee and may block you from being able to e-file.

When you enter the lump sum amounts as described in this FAQ, you are not guaranteed that your taxes will go down, only that they might. It depends on your answers to the interview. Your answers between 2018 and 2019 will likely be different.

As you may know, the Free Edition of TurboTax does not provide phone support. If you have a question, you can ask it here on the Community with more details and we will try to help you.

If, however, you are using a paid version of TurboTax, please see this TurboTax FAQ for phone support.

to take the option to apply all benefits to 2019 enter the total amount and tell TurboTax it is all a 2019 benefit.

To use the prior years option, you must have a copy of your tax returns for all those years. some filers don't have access to that.

Yes, I own TT desktop and already spoke to three Advisors. One threw in the towel, the other one was incompetent (thought I should amend my returns for the prior years), the third one disconnected but never called back. Nobody seems to understand how TT calculates total taxable SS disability benefits, even when going line by line. The problem is still there -- entering lump sums for prior years makes no difference.

After entering the hefty amount of Box 5 from SSA-1099 which contains payments for 2019, 2018, 2017, 2016, and 2015, the whole amount shows on 1040 as if it were all for 2019 which puts me in a different tax bracket. This doesn't look right. I wonder if I should disregard what Box 5 says and enter the 2019 portion of the lump sum payment in my 2019 return.

You have the two methods to handle multiple years. As pointed out there's no guarantee that one method will save you money over the other, rather than giving the same result.

OR, treating it all for one year (2019) may be more favorable.

I worked with another poster a couple of years ago who tested the two methods rigorously and came away convinced TurboTax was doing it right. Those posts were lost when AnwerXchange went away.

Without details, no reason to think it is broken now.

"Nobody seems to understand how TT calculates total taxable SS disability benefits"

the worksheets and instructions are available from the IRS.

What would be another method?

"the worksheets and instructions are available from the IRS."

Yes, feel free to do the whole return by hand.