Updated October 18, 2026. Quick answer: Line 2a takes your taxes from Schedule A, or the standard deduction if you did not file Schedule A. Line 2b takes certain tax refunds and is entered as a negative amount. Line 2c is the difference between regular tax and AMT investment interest expense, figured on a second Form 4952.
Line 2a: taxes
The form’s caption reads “If filing Schedule A (Form 1040), enter the taxes from Schedule A, line 7; otherwise, enter the amount from Form 1040 or 1040-SR, line 12e”. The instructions say “Enter the amount of all taxes from Schedule A (Form 1040), line 7, except any generation-skipping transfer taxes on income distributions.” And if you are not filing Schedule A: “If you aren’t filing Schedule A (Form 1040), then enter the standard deduction amount that you reported on Form 1040 or 1040-SR, line 12e.”
The instructions explain why the standard deduction shows up here: “the standard deduction isn’t allowed for the AMT”. One exception is spelled out: “If you filed Schedule A just to claim an increased standard deduction on Form 1040 or 1040-SR due to a loss you suffered related to property in a federally declared disaster area, then enter zero on line 2a and go to line 2b.” The itemized deduction itself is explained at Form 1040 line 12.
Line 2b: refunds of taxes
The caption is “Tax refund from Schedule 1 (Form 1040), line 1 or line 8z”. The instructions say “Include any refund from Schedule 1 (Form 1040), line 1, that is attributable to state or local income taxes.” On the amount: “Enter the total as a negative amount.” If you use line 8z, “you must enter a description and the amount next to the entry space for line 2b.”
Line 2c: investment interest
The caption is “Investment interest expense (difference between regular tax and AMT)”. The instructions start with “If you filled out Form 4952, Investment Interest Expense Deduction, for your regular tax, you will need to fill out a second Form 4952 for the AMT as follows.” They then give four steps. Step 2 says “Enter your AMT disallowed investment interest expense from 2024 on line 2.” The step list also covers refiguring the Part II items and finishing Part III of that second form, so work through the steps in the instructions in order.
The last step ends with the entry for this line: “Enter on Form 6251, line 2c, the difference between line 8 of your AMT Form 4952 and line 8 of your regular tax Form 4952.” And on the sign: “If your AMT expense is greater, enter the difference as a negative amount.”
If you did not itemize
The instructions cover a person who did not itemize but had investment interest expense: “don’t enter an amount on Form 6251, line 2c, unless you reported investment interest expense on Schedule E (Form 1040)”. If you did, they send you back to the second Form 4952 steps and tell you to compare the amount allowed on Schedule E for the regular tax with the amount allowed for the AMT.
Where the number comes from
Line 2a comes from your Schedule A or from line 12e of your Form 1040, line 2b from Schedule 1, and line 2c from a second Form 4952 that you complete for the AMT. The instructions tell you not to attach such a second form unless asked: “If you do complete an AMT version of a form or worksheet, don’t attach it to your tax return unless instructed to do so.” Keep it with your records. The overall layout of Form 6251 is on the Form 6251 page.
Where it goes next
All three lines are added into the line 4 total, which says “Combine lines 1b through 3.” The start of Part I is on the line 1 page, and the next group of adjustments is on the lines 2d to 2h page.
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
- Including generation-skipping transfer taxes in line 2a. The instructions exclude “any generation-skipping transfer taxes on income distributions.”
- Entering a refund on line 2b as a positive number. The instructions say “Enter the total as a negative amount.”
- Skipping the second Form 4952 for line 2c. The instructions say “you will need to fill out a second Form 4952 for the AMT as follows.”
Related: Form 6251, Form 1040 line 12, Schedule 2 lines 2 and 3, and Form 6251 line 1.
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.