Updated October 18, 2026. Quick answer: Line 3 is a catch-all for AMT adjustments that have no line of their own, such as pollution control facilities, certain mortgage interest and related adjustments. Line 4 combines lines 1b through 3 into alternative minimum taxable income. The instructions add extra steps on line 4 for married filing separately and for holders of a REMIC residual interest.
Line 3: what goes on it
The form’s caption is “Other adjustments, including income-based related adjustments”. The instructions say “Enter on line 3 the total of any other adjustments that apply to you, including the following.” Each topic under that sentence has its own heading in the instructions. The headings are “Depreciation Figured Using Pre-1987 Rules”, “Pollution Control Facilities”, “Tax Shelter Farm Activities”, “Charitable Contributions of Certain Property”, “Business Interest Limitation”, “Mortgage Interest”, “Net Qualified Disaster Loss”, and “Related Adjustments”.
The adjustments, one at a time
Pre-1987 depreciation: “This preference generally applies only to property placed in service after 1987 but depreciated using pre-1987 rules due to transitional provisions of the Tax Reform Act of 1986.” “Figure the excess of the regular tax depreciation over the AMT depreciation separately for each property and include on line 3 only positive amounts.”
Pollution control facilities: “The section 169 election to amortize the basis of a certified pollution control facility over a 60-month or 84-month period isn’t available for the AMT.”
Tax shelter farm activities: “Figure this adjustment only if you have a gain or loss from a tax shelter farm activity (as defined in section 58(a)(2)) that isn’t a passive activity.” “If the activity is passive, you must include it with your other passive activities on line 2m.” That line is on the lines 2m to 2t page.
Charitable property: “If you made a charitable contribution of property to which section 170(e) applies and you had a different basis for AMT purposes, you may have to make an adjustment.”
Business interest: “Complete an AMT Form 8990 using amounts adjusted for AMT.” “Enter the difference between the AMT and regular tax allowable interest expense.”
Mortgage interest: “If you deducted home mortgage interest on Schedule A for a dwelling that isn’t a principal residence (within the meaning of section 121) or qualified dwelling for AMT, include that deducted interest on line 3.” The standard deduction and Schedule A choice is explained on the Form 1040 line 12 page.
Disaster loss: “include on line 3 the standard deduction amount you listed on the dotted line next to Schedule A, line 16”. The instructions add “If you filed Schedule A to itemize your deductions, then don’t make this adjustment.”
Related adjustments
“Affected items include the following.” The first item listed is “Section 179 expense deduction (Form 4562, line 12).” The Form 4562 return is covered on the Form 4562 page.
“Combine the amounts for all your related adjustments and include the total on line 3.” “Keep a copy of all computations for your records, including any AMT carryover and basis amounts.” “Don’t include on line 3 any adjustment for an item you refigured on another line of this form”.
Line 4: alternative minimum taxable income
The caption reads “Alternative minimum taxable income. Combine lines 1b through 3.” It also carries the note “(If married filing separately and line 4 is more than $900,350, see instructions.)”. The instructions explain it: “If your filing status is married filing separately and line 4 is more than $900,350, you must include an additional amount on line 4.” “If line 4 is $1,174,350 or more, include an additional $68,500. Otherwise, include 25% of the excess of the amount on line 4 over $900,350.”
The instructions give their own example: “For example, if the amount on line 4 is $920,350, enter $925,350 instead”. The extra amount in that example is 25% of the excess over the starting figure, which is a computed illustration using the instructions’ numbers: $920,350 minus $900,350 is $20,000, and $20,000 x 25% is $5,000.
REMIC holders: “If you held a residual interest in a REMIC in 2025, the amount you enter on line 4 may not be less than the amount on Schedule E, line 38, column (c).” For married filing separately: “be sure to include the additional amount that must be added to line 4” before you make that comparison.
Where the number comes from
Lines 1b through 2t come from the earlier pages in this series, starting at Form 6251. Line 3 is the total of whichever adjustments above apply to you, and line 4 is the combined total of everything on lines 1b through 3.
Where it goes next
Line 4 is the starting point for the exemption. The form’s line 6 caption says “Subtract line 5 from line 4.” The exemption, subtraction and tax rates are on the lines 5 to 7 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’s own 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
- Entering the same item twice. The instructions say “Don’t include on line 3 any adjustment for an item you refigured on another line of this form”.
- Treating a passive tax shelter farm as a line 3 item. The instructions say “If the activity is passive, you must include it with your other passive activities on line 2m.”
- Leaving out the extra amount when married filing separately. The instructions say “If your filing status is married filing separately and line 4 is more than $900,350, you must include an additional amount on line 4.”
Related: Form 6251, Form 4562, Form 1040 line 12, Schedule 2 lines 2 and 3, and AMT exemption.
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.