Updated October 17, 2026. Quick answer: Line 17 of Form 1040 takes the amount from Schedule 2, line 3, and line 18 adds line 16 and line 17 together. Nothing on line 18 is looked up or chosen: it is the tax from line 16 plus whatever Schedule 2 contributed. The Schedule 2 figure comes from that schedule, and the line 18 total feeds the credits step further down the page.
More IRS forms and schedules for individual filers, read line by line: Schedule H instructions, Schedule H Part II FUTA tax and Form 8839 Part II adoption credit.
Business use of your home (Form 8829), the saver’s credit (Form 8880), clean vehicle credits (Form 8936) and amended returns (Form 1040-X), read line by line: Form 8936, Form 8936 line 13 and Form 8936 line 18.
IRS worksheets from the 2025 Form 1040 instructions, read line by line: Tax Computation Worksheet and Foreign Earned Income Tax Worksheet.
Foreign tax credit (Form 1116) and alternative minimum tax (Form 6251) for 2025, read line by line: Form 1116 line 7, Form 1116 line 8 and Form 6251 line 11.
Schedule 3 (Form 1040) for 2025, read line by line: Schedule 3 lines 7 and 8.
Schedule 2 (Form 1040) for 2025, read line by line: Schedule 2 lines 1a to 1c, Schedule 2 lines 1d to 1z and Schedule 2 lines 19 to 21.
What lines 17 and 18 ask for
The form’s caption for line 17 reads “Amount from Schedule 2, line 3”. The caption for line 18 reads “Add lines 16 and 17”. So line 17 is a transfer from another schedule, and line 18 is a plain addition of the tax figure on line 16 and the line 17 figure. If you have nothing on Schedule 2, line 3, there is no amount to carry to line 17, and line 18 then rests on line 16 alone.
The lines on either side of these two are explained in Form 1040 line by line. This page covers only the pair that sits between the tax figure on line 16 and the credits on lines 19 to 21.
Where the number comes from
The line 17 amount is not worked out on Form 1040. It comes from Schedule 2. The instructions describe when that schedule is used: “Use Schedule 2 if you have additional taxes that can’t be entered directly on Form 1040, 1040-SR, or 1040-NR.” The same instructions say where the schedule’s line 3 lands: “Include the amount on Schedule 2, line 3, in the total on Form 1040, 1040-SR, or 1040-NR, line 17.” You will find the schedule itself explained on the Schedule 2 page.
The line 16 figure that line 18 adds is worked out in the tax part of the instructions, and the taxable income behind it is covered in the 2025 tax brackets page. For line 18 you only need the finished number from line 16, as it stands on your return.
Doing the addition on line 18
Line 18 follows the caption exactly: take the line 16 figure, take the line 17 figure, and add them. A labelled illustration, not an IRS example, shows the arithmetic with made up numbers: if line 16 shows $9,000 and line 17 shows $1,200 then line 18 is $9,000 plus $1,200 which is $10,200. If line 17 is blank because there is no Schedule 2 amount, line 18 is simply the same as line 16.
Because line 18 is only a sum, an error here almost always started earlier. A wrong line 16 or a wrong Schedule 2, line 3 figure carries straight through to line 18 and onward. Before you finalize the addition, check that the line 16 figure on your return is the finished one and that the Schedule 2 line 3 figure you copied matches the schedule you are attaching. Matching the two takes a minute and saves a correction later.
Where it goes next
Line 18 is the starting point for the credits step. The form’s caption for line 22 reads “Subtract line 21 from line 18.” Lines 19 to 21 gather the credits that line 22 takes away, and they are explained in lines 19 to 21. Line 22 and the total tax lines after it are covered in lines 22 to 24.
The amount on line 17 therefore does two jobs: it raises line 18, and through line 18 it raises the figure from which credits are subtracted.
Edition and line numbers
This page reads the 2025 Form 1040 and its 2025 instructions, the current files irs.gov served on 2026-10-17. They are the forms for the tax year 2025 return, filed in 2026. The instructions say “The lines on Forms 1040 and 1040-SR are the same.” so everything here applies to Form 1040 and to Form 1040-SR alike. 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 a figure from the wrong line of Schedule 2. The instructions say “Include the amount on Schedule 2, line 3, in the total on Form 1040, 1040-SR, or 1040-NR, line 17.”
- Skipping the Schedule 2 amount when line 18 is added. The form’s caption is “Add lines 16 and 17” and line 17 is part of that sum.
- Treating Schedule 2 as optional extra tax. The instructions say “Use Schedule 2 if you have additional taxes that can’t be entered directly on Form 1040, 1040-SR, or 1040-NR.”
Related: Schedule 2 (Form 1040), 2025 tax brackets, Self-employment tax explained, and Form 1040.
Sources
- Instructions for Form 1040 and Form 1040-SR (2025): https://www.irs.gov/pub/irs-pdf/i1040gi.pdf
- Form 1040, U.S. Individual Income Tax Return (2025): https://www.irs.gov/pub/irs-pdf/f1040.pdf
Read from the IRS documents linked above on October 17, 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.