What this line means
Line 1 minus line 2 in each column. Column A is the original income after deductions. Column B is the net change. Column C is the corrected amount. This is a straight subtraction — if your AGI increased and your deductions also increased, Column B reflects the net effect of both changes.
Does this apply to you?
Easy to overlook
Column B must be internally consistent across lines 1 through 3 Column B on line 3 must equal Column B on line 1 minus Column B on line 2. If you increased AGI by $5,000 (line 1 Column B) and increased deductions by $2,000 (line 2 Column B), line 3 Column B is $3,000. Arithmetic errors in Column B are the most common reason the IRS returns amended returns for correction. 1 IRS Form 1040-X Instructions — Line 3
A negative Column B means your taxable income decreased If the net effect of your changes reduces taxable income, Column B on line 3 is a negative number. Write the amount in parentheses or with a minus sign. A decrease in income after deductions flows through to lower tax and a potential refund. 2 General filing pattern — arithmetic errors on amended returns
Watch out for this
Skipping Column B and entering only Column A and Column C. The IRS uses Column B to verify the amendment. Column C minus Column A must equal Column B on every line. If Column B is blank or inconsistent, the return is flagged for manual review and processing slows.
Footnotes
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IRS Form 1040-X Instructions, Line 3. https://www.irs.gov/instructions/i1040x ↩
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IRS Form 1040-X Instructions, How To Complete Column B. https://www.irs.gov/instructions/i1040x ↩