How to properly complete Columns A, B, and C on Form 1040-X

If you are filing an amended return using Form 1040-X, the single most common mistake that causes rejection, delays, and processing confusion is filling out the three-column section incorrectly.

This portion — Columns A, B, and C — is the financial heart of the amendment.
If it’s wrong, the entire return stops.

Let’s walk through exactly how these columns must be completed and why Column B is where most taxpayers go wrong.

Understanding the Three Columns

Each column has a specific purpose:

Column A — Original amount
This should match exactly what you reported on your original return (or as the IRS adjusted it).

Column B — Net change
This is the adjustment — the difference. It must show the increase (+) or decrease (-), not the final number.

Column C — Corrected amount
This is the new total after the change has been applied.

The Most Common Error: Mixing Up Columns B and C

Many taxpayers mistakenly put the corrected number into Column B.

Example of incorrect entry:

Column A: 40,000
Column B: 41,000  (incorrect)
Column C: 41,000

This is WRONG because Column B is not for the new amount.

Column B should show ONLY the amount of change.

Correct version:

Column A: 40,000
Column B: +1,000
Column C: 41,000

Column B is always the difference, never the final amount.

How to Fill Out Columns Properly (Step-by-Step)

Step 1: Start with Column A

Enter the values exactly as originally filed.

Step 2: Enter the change in Column B

• If your income increases → use a positive number
• If your deduction increases → use a negative number
• If your withholding changes → use the net difference

Column B should always represent only the delta — the adjustment.

Step 3: Calculate Column C

Column C = Column A + Column B

This must mathematically reconcile.

Real Example: Tip Deduction Adjustment

Let’s say you forgot to claim $1,500 in deductible tip income adjustment.

Correct entries:

Column A: 50,000   (original taxable income)
Column B: -1,500   (reduction from tip deduction)
Column C: 48,500   (new corrected taxable income)

Column B shows the change only — not the end result.

If You Received a CP Notice Before the Amendment

If the IRS made adjustments to your original return (for example via a CP12 or CP2000), Column A should reflect the IRS-adjusted numbers — not the ones you originally filed.

This is crucial.

If you ignore IRS adjustments and put the wrong amount in Column A, your 1040-X will be rejected.

Why the IRS Rejects Incorrect Columns

When Columns A, B, and C are wrong:

• math doesn’t reconcile
• IRS cannot confirm change reason
• manual review gets triggered
• the amended return stalls
• refund adjustments freeze
• refund processing time increases

This is one of the top causes of Form 1040-X delay.

Additional Best Practices When Filing Form 1040-X

• attach supporting documentation for all changes
• include corrected W-2 or 1099 if applicable
• ensure explanation section is detailed (Part III)
• avoid handwritten corrections in margins
• keep numbers consistent across all fields

And most importantly:

Do not skip explaining Column B changes in the explanation section.

The IRS is not rejecting amended returns because they’re skeptical — they’re rejecting them because the numbers don’t mathematically connect.

Column A: original numbers
Column B: amount of change
Column C: corrected totals

If you get this right, the rest of the return flows smoothly.

If you get this wrong, your amendment enters the slow lane of manual review.