Updated October 18, 2026. Quick answer: Line 4a is the qualified business income deduction and line 4b is the deductions for tips, overtime, car loan interest and seniors from Schedule 1-A. Line 5 subtracts both from line 3 to give taxable income. The instructions send you to the forms for your return year for the amounts.
What lines 4a to 5 ask for
The captions are “Qualified business income deduction” for line 4a, “Deductions for tips, overtime, car loan interest, and seniors from Schedule 1-A (Form 1040). Attach your Schedule 1-A” for line 4b, and “Subtract the total of lines 4a and 4b from line 3.” for line 5. The instructions’ What’s New note says “discusses the new deductions for tips, overtime, car loan interest, and seniors on new Schedule 1-A (Form 1040).” This page is part of the Form 1040-X walk-through.
Line 4a: qualified business income deduction
The instructions give a pointer rather than a calculation: “See the instructions for the year of the form you are amending.” They continue “Also see the instructions for Forms 8995 and 8995-A for more information for figuring and reporting your qualified business income deduction.” The form itself is explained at Form 8995, and the return line at Form 1040 line 13.
Line 4b: tips, overtime, car loan interest and seniors
The instruction is short: “If claiming any of these deductions, attach your Schedule 1-A.” The rest reads “See the instructions for Schedule 1-A (Form 1040) found in the Instructions for Form 1040 for the year of the form you are amending.” On timing they say “Schedule 1-A and these deductions were introduced in 2025.” The instructions give no amounts for these deductions on this page, so use the Schedule 1-A instructions for the return year you are amending.
Line 5: taxable income
If the original return showed no taxable income: “If the taxable income on the return you are amending is $0 and you have made changes on Form 1040-X, line 1, 2, or 4b, enter on line 5, column A, the actual taxable income instead of $0.” “Enclose a negative amount in parentheses.” Also “The amounts in columns A and B can be less than zero.” Column C is different: “if the result for column C is zero or less, you must enter” zero in column C.
Computed illustration, not an IRS example: if column C of line 3 is $31,200 and line 4a is $4,000 and line 4b is $2,500 then the total of lines 4a and 4b is $6,500 ($4,000 plus $2,500) and line 5 is $31,200 minus $6,500 and the result is $24,700.
Which lines to complete
The instructions say “Complete the lines shown in the charts under Columns A Through C below according to what you are changing.” The chart under Which Lines To Complete has separate rows for the qualified business income deduction and for the Schedule 1-A deductions. Read the row for your change there.
Where it goes next
Line 5, column C, is the taxable income the next section works from. The instructions say “Figure the tax on your taxable income shown on line 5, column C.” That step is on the lines 6 to 11 page. The return-side line is explained at Form 1040 lines 14 and 15.
Edition and line numbers
This page reads the December 2025 revision of Form 1040-X, Amended U.S. Individual Income Tax Return, and its December 2025 instructions, the current files irs.gov served on 2026-10-18. Form 1040-X is not tied to one tax year: it amends the return year you enter at the top. The form’s own header says “Attach a completed Form 1040, 1040-SR, or 1040-NR, with your changes, for the return year entered below”. The instructions add “These instructions can be used with the December 2025 and later revisions of Form 1040-X, unless later instructions state otherwise.” Check the revision date printed on the form you are filling in and the return year you are amending, and use the line numbers on that form.
Common mistakes
- Entering zero for an original return that really had a loss. The instructions say “enter on line 5, column A, the actual taxable income instead of $0.”
- Claiming a Schedule 1-A deduction without the schedule. The instructions say “If claiming any of these deductions, attach your Schedule 1-A.”
- Showing a negative amount in column C. The instructions say “if the result for column C is zero or less, you must enter” zero.
Related: Form 8995, Form 1040 line 13, Form 1040 lines 14 and 15, and Form 1040.
Sources
- Instructions for Form 1040-X, Amended U.S. Individual Income Tax Return (Rev. December 2025): https://www.irs.gov/pub/irs-pdf/i1040x.pdf
- Form 1040-X, Amended U.S. Individual Income Tax Return (Rev. December 2025): https://www.irs.gov/pub/irs-pdf/f1040x.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.