U.S. · Form 6251 line 4
Form 6251 line 4 alternative minimum taxable income.
Which lines feed line 4, the married-filing-separately add-back, and what line 4 does on lines 5 and 6.
Line 4 is alternative minimum taxable income (AMTI): you combine lines 1b through 3 of Form 6251. If you are married filing separately and line 4 is more than $900,350, the instructions add an amount: 25% of the excess over $900,350, or $68,500 once line 4 reaches $1,174,350. Line 4 is then compared with the exemption table on line 5.
Compute line 4
Result
What goes into line 4
| Item | What the 2025 form and instructions say |
|---|---|
| Line 1b | Subtract line 1a from Form 1040, 1040-SR or 1040-NR line 11b; if less than zero, enter it as a negative amount. |
| Lines 2a to 2t | The adjustments and preferences, each with its own instructions. The form prints a parenthesised entry box on lines 2b, 2f and 2s, so follow those line instructions for the sign. |
| Line 3 | Other adjustments, including income-based related adjustments. |
| Married filing separately above $900,350 | Add 25% of the excess of line 4 over $900,350; if line 4 is $1,174,350 or more, add $68,500. |
| REMIC residual interest | Line 4 may not be less than Schedule E line 38, column (c). If column (c) is larger, enter it and write "Sch. Q" next to line 4. |
| Line 5 exemption (single or head of household) | Line 4 not over $626,350: exemption $88,100. |
| Line 5 exemption (married filing jointly or qualifying surviving spouse) | Line 4 not over $1,252,700: exemption $137,000. |
| Line 5 exemption (married filing separately) | Line 4 not over $626,350: exemption $68,500. |
Why it matters
Line 4 is the starting point for the rest of Form 6251. Line 5 is the exemption: the form lists the amount for your filing status next to a "not over" ceiling on line 4, and if line 4 is over that ceiling you use the Exemption Worksheet in the instructions. Line 6 subtracts line 5 from line 4, and if the result is zero or less you enter -0- on lines 7, 9 and 11 and go to line 10.
Worked example
The instructions give this married-filing-separately case: if line 4 is $920,350, you enter $925,350 instead, because the extra $5,000 is 25% of $20,000 ($920,350 minus $900,350). At $1,174,350 or more the add-back stops growing at $68,500. The calculator above applies the same arithmetic to the total you enter.
What this page does not do
It does not figure lines 1b to 3 for you, since each depends on your return and its own instructions, and it does not apply the Exemption Worksheet. It enters nothing for REMIC residual interests. Use it to check the final combination only.
Sources
- Instructions for Form 6251 (2025): Line 4 text, the married-filing-separately add-back, the REMIC rule and the line 5 chart. Read November 2, 2026.
- Form 6251 (2025): The exact wording of lines 1b to 6. Read November 2, 2026.
These are the tax year 2025 edition instructions and forms (published for returns filed in 2026). The IRS revises them each year, so thresholds, line numbers and limits can change; check the current edition before relying on a figure.
Frequently asked questions
What is line 4 on Form 6251?
Alternative minimum taxable income. You combine lines 1b through 3.
Which lines are combined on line 4?
Line 1b, the adjustments and preferences on lines 2a through 2t, and line 3 (other adjustments). Follow each line's instructions for its sign.
What changes on line 4 if I am married filing separately?
If line 4 is more than $900,350, the instructions add 25% of the excess over $900,350, or $68,500 if line 4 is $1,174,350 or more.
What happens after line 4?
Line 5 is the exemption for your filing status, based on a ceiling on line 4. Line 6 is line 4 minus line 5, and if that is zero or less you enter -0- on lines 7, 9 and 11.
Is there a special rule for REMIC residual interests?
Yes. Line 4 cannot be less than Schedule E line 38, column (c). If column (c) is larger, enter that amount and write "Sch. Q" next to line 4.
Not tax or legal advice
This page summarizes IRS instructions and is not tax, legal or financial advice. A CPA, enrolled agent or the IRS can confirm what applies to your situation. Everything runs in your browser; nothing you enter is sent anywhere.