What?  No, Turbo Tax can't decide Single or Joint.  If you were married on December 31; 2018 you can only file as Married Joint or Married Filing Separate or Head of Household if you qualify.

Did you mean between Joint and Separate.  No Turbo Tax doesn't decide that either.  How did you lose by filing Joint?

Turbo Tax only decides to give you the Standard Deduction or your Itemized Deductions whichever is higher.

And you can choose to not follow the program's deduction choice ... it is ultimately your decision.  The program does what you tell it to do ... it is a DIY program that is totally dependent on your data entries.  You cannot blame the paper for what the pencil wrote.

Yes, I meant between joint and separate. According to TT, I lost > $10K by filing joint (I just compared the amount of state and federal tax/refunds due using both methods.

I asked this question because the TT website says, "If you use TurboTax to prepare your return, we’ll do the calculation for you, and recommend the filing status that gives you the biggest tax savings.” I interpret that to mean literally what it says: that TT will calculate the FILING STATUS (NOT itemized vs. standard deduction, which I know it does).

Ok .. TT will not compute whether MFJ or MFS is better for you ... if you want to check that out you must complete the returns both ways to see which is better ... the downloaded version has a tool in the FORMS mode that you can use but the calculation is not automatic.

It is highly unlikely that Joint would cause you to lose $10k. Most likely you did MFS incorrectly. Did either of you have Social Security income?

Many people think they come out better when filing Married Filing Separate but they are probably doing it wrong.  If one person itemizes deductions then the other one must itemize too, even if it's less than the standard deduction, even if it is ZERO!  
 
And there are several credits you can't take when filing separately, like the
EITC Earned Income Tax Credit
Child Care Credit
Educational Deductions and Credits

And contributions to IRA and ROTH IRA are limited when you file MFS.

Also if you file Married Filing Separately up to 85`% of your Social Security becomes taxable right away even with zero other income.

See …….
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Thanks to all who responded. I did indeed calculate incorrectly. Using the "What-If" worksheet (which doesn't account for state tax), I determined that what I had thought was a recalculation of taxes for both my wife and me, I would have had to do a separate return for each of us to get that. The What-If calculations showed that we still come out ahead MFJ. That's comforting.

You can't file an amended return if you're going from joint to separate. Only the other way around.

That's somewhat correct. You can amend from MFJ to MFS, but not after April 15. That's a special rule.