Updated October 18, 2026. Quick answer: Line 1 is where Form 6251 starts. Line 1a is a subtraction that uses line 14 of your Form 1040 and line 37 of Schedule 1-A. Line 1b then compares that result with line 11b of your Form 1040, and the result feeds the alternative minimum taxable income on line 4.
What line 1 asks for
Part I of the form is headed “See instructions for how to complete each line.” Line 1 has two entry lines. Line 1a reads “Subtract Schedule 1-A (Form 1040), line 37, from Form 1040, 1040-SR, or 1040-NR, line 14”. Line 1b reads “Subtract line 1a from Form 1040, 1040-SR, or 1040-NR, line 11b (if less than zero, enter as a negative amount)”. This page is part of the Form 6251 line-by-line walk-through, and the lines that follow are on the lines 2a to 2c page.
Line 1a: the first subtraction
The instructions repeat the caption in their own words: “Enter on Form 6251, line 1a, the difference between line 14 of your Form 1040, 1040-SR, or 1040-NR, and line 37 of your Schedule 1-A (Form 1040).” Both numbers come from forms you have already completed, so have the finished Form 1040 and Schedule 1-A in front of you before you start. The Form 1040 lines are explained on the Form 1040 line by line page.
Line 1b: the second subtraction
The instructions say “Enter on Form 6251, line 1b, the difference between line 1a of Form 6251 and line 11b of your Form 1040, 1040-SR, or 1040-NR.” They add “If less than zero, enter as a negative amount.” The form’s own caption tells you which number to take away from which, so follow the caption wording on the form you are filling in.
Where the number comes from
Everything on line 1 comes from your Form 1040 and your Schedule 1-A. Nothing on this line is figured from a second AMT version of a form. The instructions note elsewhere that for the AMT, “certain items of income, deductions, etc., receive different tax treatment than for the regular tax.” Those differences are entered on the lines after line 1, not here. The standard or itemized deduction that sits behind Form 1040 is covered on the Form 1040 line 12 page.
Where it goes next
Line 1b is the first number that line 4 adds up. The form’s line 4 says “Combine lines 1b through 3.” The adjustments between them begin with lines 2a to 2c, and the later part of Part I is on the lines 3 and 4 page. The final AMT result is carried to the return, as the form says: “Enter here and on Schedule 2 (Form 1040), line 2”. That Schedule 2 line is explained at Schedule 2 lines 2 and 3.
Edition and line numbers
This page reads the 2025 Form 6251 and its 2025 instructions, the current files irs.gov served on 2026-10-18. They are the files for the tax year 2025 return, filed in 2026. The Form 6251 header says “Attach to Form 1040, 1040-SR, or 1040-NR.” Check the tax year printed on the form you are filling in and use the line numbers on that year’s form, because a different year’s form can number or word its lines differently.
Common mistakes
- Using the wrong Form 1040 line for line 1a. The caption is “Subtract Schedule 1-A (Form 1040), line 37, from Form 1040, 1040-SR, or 1040-NR, line 14”.
- Dropping the sign on line 1b. The instructions say “If less than zero, enter as a negative amount.”
- Skipping line 1a because it looks like a worksheet number. Line 1b depends on it: “Subtract line 1a from Form 1040, 1040-SR, or 1040-NR, line 11b”.
Related: Form 6251, Form 1040 line by line, Form 1040 line 12, Schedule 2 lines 2 and 3, and Form 6251 lines 2a to 2c.
Sources
- Instructions for Form 6251, Alternative Minimum Tax, Individuals (2025): https://www.irs.gov/pub/irs-pdf/i6251.pdf
- Form 6251, Alternative Minimum Tax, Individuals (2025): https://www.irs.gov/pub/irs-pdf/f6251.pdf
Read from the IRS documents linked above on October 18, 2026. General information, not tax advice. Your own facts decide the outcome, and a preparer, the account provider or the IRS is the right place to confirm anything consequential.