What this line means
Additional taxes beyond the basic income tax, reported in three columns. This includes self-employment tax, the 10% early distribution penalty on retirement accounts, net investment income tax (3.8%), additional Medicare tax (0.9%), household employment tax, and repayment of first-time homebuyer credit. These come from Schedule 2 of the original or corrected return.
Does this apply to you?
- You are correcting self-employment income that changes self-employment tax
- You took an early retirement distribution and the penalty was calculated incorrectly
- Your amendment changes investment income that affects net investment income tax
- You are adding or removing household employment tax
- You owe additional Medicare tax due to changed wage or self-employment income
Easy to overlook
Self-employment tax recalculates when Schedule C income changes If your amendment changes self-employment income (adding a missing 1099-NEC, correcting Schedule C expenses), self-employment tax on Schedule SE changes too. Self-employment tax is 15.3% on the first $176,100 of combined earnings (2025) and 2.9% above that. A $10,000 increase in self-employment income adds approximately $1,530 in self-employment tax. 1 IRS Form 1040-X Instructions — Line 9
Net investment income tax has its own AGI threshold The 3.8% net investment income tax applies to the lesser of net investment income or the amount your MAGI exceeds $200,000 (single) or $250,000 (married filing jointly). If your amendment increases AGI above these thresholds, the NIIT applies to investment income that was previously untaxed. Recalculate Form 8960. 2 IRS Schedule 2 Instructions — Additional Taxes
Watch out for this
Forgetting to amend Schedule 2 when other taxes change. Line 9 pulls from Schedule 2. If you change self-employment income, early distribution amounts, or investment income, you must recalculate and attach a corrected Schedule 2 (and the specific supporting form — Schedule SE, Form 5329, or Form 8960).
Footnotes
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IRS Form 1040-X Instructions, Line 9. https://www.irs.gov/instructions/i1040x ↩
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IRS Schedule 2 (Form 1040) Instructions, Additional Taxes. https://www.irs.gov/instructions/i1040s2 ↩