@bob_cutler I felt like I was totally missing your question, so I shared it with some of my co-Champions.
One of them believes that you are, rather than entering your income and estimated tax payments for your 2020 tax return, asking about calculating estimated tax payments for 2021. I'm SO sorry that I totally misunderstood what you were looking for.
According to @rjs , a co-Champion, the TurboTax calculator isn't well prepared to properly calculate your estimated taxes. You'll just have to estimate how much of your SS benefits will be taxable for 2021, then do you own calculations from there. According to @rjs , you can dig deeper into the "Help" portion of the TT tax estimator and possibly figure it out.
I'm linking you to a couple of IRS publications that may assist you. Both of the publications are the most current revision, but each is for 2020. The first one is the Publication Form 1040-ES. You'll find a worksheet there, on page 8, that may be helpful.
2020 Form 1040-ES (irs.gov)
Then there's the "big" publication, Publication 505, that goes into much more detail about calculating estimated tax payments.
2020 Publication 505 (irs.gov)
I'm sorry that the TurboTax calculator isn't more concise. I also apologize for the misunderstanding at the outset. Best of luck to you!
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*** I am NOT a tax expert. I am a seasoned TurboTax user, and volunteer to provide assistance to TT users. Nothing I post is to be considered TAX ADVICE; I bear no legal liability for responses.***
