No it is not based on AGI. Tax brackets are determined by taxable income, not by gross income or adjusted gross income. ... Taxable income can be reduced by deductions and credits, so your total taxable income is usually less than your gross income or even your adjusted gross income. It is your taxable income that determines your tax bracket.

No....the bracket tables use the taxable income after certain  deductions from the AGI value.

On the 2018 form 1040,

AGI was on line 7

Then Std Deduction or Itemized Ded on line 8

Then certain business Ded on line 9

Then taxable income on line 10...and line 10 is used with the tax brackets

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Line numbers may change for 2019