How does TurboTax calculate federal tax amount? For a 2018 single taxpayer with taxable income $55,133, TurboTax says tax amount is $8,132, using formula is $8,069!

...........hard to know unless you are looking at the final 1040 form itself and all of it's lines and schedules too. 

There may be some penalty added in.  Or if it contains some minor Self-Employment income, there may be some SS/Medicare taxes added in too.

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Of course, you are not indicating whether you are looking at the tax form for your actual 2018 tax return

...or are just commenting on the current 2019 software, and just using the 2018 tax tables as an example.

 

(For the current 2019 software, it is unlikely that  the calculations will be correct until mi-late-January. The IRS has not released all their forms and calculations yet either. so Many Many software updates remain and will be needed before you file in Feb-March) 

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for 2018, the tax tables indicate $8,067 tax assessed

for 2019 the "draft" tax tables indicate 7,936 tax assessed.

....But these are only if ALL your income is ordinary income,....i.e, no stock or bond Cap Gains, no Mutual fund distributions, no Qualified dividends, no Self-employment (1099-MISC) income.

 

Actually the chart says 8,067  https://www.irs.gov/pub/irs-pdf/i1040tt.pdf   so review the form 1040  & Sch 4 to see if any other taxes are being added. 

In additon to Schedule 4 $$, If looking at 2018 forms, Schedule 2 $$ can add in too.

 

If by "tax amount" you are referring to seeing a $8,132 Federal Tax Due, you could also have $8,069 on Form 1040 line 15 plus a $63 underpayment penalty on line 23.