What Form 1120-X Is For

Form 1120-X is the IRS's official form that allows corporations to correct mistakes made on their original Form 1120 (U.S. Corporation Income Tax Return) or Form 1120-A (the simplified short form that some smaller corporations used). Think of it as a "do-over" form—but one that comes with specific rules and requirements.

Corporations use Form 1120-X to accomplish several important tasks. First and foremost, it corrects errors on the original return, whether those errors involve income reporting, deductions, credits, or tax calculations. For example, if your corporation accidentally omitted revenue from a contract, overstated expenses, or miscalculated depreciation, Form 1120-X is how you set the record straight.

Beyond simple error correction, Form 1120-X also allows corporations to make certain tax elections after the normal deadline has passed. Tax law sometimes permits "late elections" for specific provisions, and this form is the vehicle for making those elections official. Additionally, if the IRS previously adjusted your return through an audit or examination, and you need to make further corrections, Form 1120-X is how you report those additional changes.

The form is designed to show three columns of information: what you originally reported, the net changes you're making, and the corrected amounts. This side-by-side comparison helps the IRS quickly understand what you're changing and why.

When You’d Use Form 1120-X

Superseding Returns (Before the Deadline)

Superseding Returns are filed after your original return but before the filing deadline (including extensions). For example, if your corporation filed its 2013 Form 1120 on February 1, 2014, but the deadline was March 17, 2014 (the 15th day of the 3rd month after the tax year ends, adjusted for weekends), you could file a superseding return anytime before that March deadline. A superseding return completely replaces the original return—it's as if the first filing never happened. This is the preferred method if you catch errors early because it's simpler and avoids some complications.

Amended Returns (After the Deadline)

Amended Returns using Form 1120-X come into play after the filing deadline has passed. Once March 17, 2014, came and went for 2013 returns, any corrections would require Form 1120-X. The critical deadline to remember is the statute of limitations: generally, you must file Form 1120-X within three years from the date you filed your original return, or within two years from the date you actually paid the tax, whichever is later.

For 2013 returns filed on time by March 17, 2014, this means the deadline to amend would typically be March 17, 2017—three years later. If you're seeking a refund, timing matters significantly. Miss this window, and you forfeit your right to claim that refund, even if the IRS owes you money.

Common Scenarios That Trigger Form 1120-X

Common situations that trigger the need for Form 1120-X include: discovering missed deductions after reviewing year-end records, receiving corrected information returns (like a revised 1099), identifying mathematical or computational errors, claiming research and development credits that were overlooked, correcting inventory valuation mistakes, or responding to changes made to related tax returns (such as partnership or S-corporation returns that flow through to the corporate return).

Key Rules or Details for 2013

Filing Deadlines

For calendar-year corporations, the original 2013 Form 1120 was due by March 17, 2014 (the 15th day of the third month after year-end, adjusted because March 15 fell on a Saturday). Fiscal-year corporations had deadlines based on their specific year-end dates, following the same "15th day of the 3rd month" rule.

Statute of Limitations

The three-year window is sacred in tax law. For refund claims, you must file within three years of filing the original return or two years of paying the tax—whichever gives you more time. If you owe additional tax from an amendment, the IRS has three years from when you file the amended return to assess that tax, though the original statute continues running for other assessment purposes.

Electronic Filing

In 2013, the IRS was transitioning to mandatory electronic filing for larger corporations. Those with $10 million or more in total assets and filing at least 250 returns annually were required to e-file their original returns. If a corporation was required to e-file the original return, it must also e-file the amended return. However, tax years that were no longer available on the Modernized e-File (MeF) system could be paper-filed without requesting a waiver.

Payment Requirements

If your amended return shows additional tax owed, you must pay it when filing Form 1120-X. The IRS charges interest on unpaid tax from the original due date, even if you filed on time initially. Penalties may also apply, including the failure-to-pay penalty (0.5% per month) and potential accuracy-related penalties (20% of the underpayment) if the IRS determines the error was due to negligence or substantial understatement.

Recordkeeping

Corporations must maintain supporting documentation for at least three years from the date they file Form 1120-X. This includes amended schedules, explanatory statements, and all documentation supporting the changes made. Since audits can reach back into earlier years, maintaining thorough records is critical.

Step-by-Step (High Level)

Step 1: Gather Documentation

Before starting the form, collect all relevant documents: your original 2013 Form 1120 as filed, all supporting schedules and attachments, documentation supporting the changes you're making (corrected 1099s, receipts, contracts, etc.), and your corporation's books and records for 2013.

Step 2: Complete Form 1120-X

The form has several critical parts. Part I shows the income and deductions section with three columns: Column A (as originally reported or previously adjusted), Column B (net change—increase or decrease), and Column C (correct amount). You'll enter figures for gross receipts, cost of goods sold, deductions, taxable income, and tax liability. Part II requires you to explain each change. For every line item you're amending, you must provide the line number from the original Form 1120, a clear description of the change, and a detailed explanation of why you're making it. This narrative is crucial—the IRS needs to understand your reasoning.

Step 3: Attach Supporting Documents

Include any forms or schedules that changed as a result of your amendments. If you're claiming a new credit, attach the relevant credit form. If deductions changed, attach updated schedules. Also attach a complete amended Form 1120 showing the corrected figures.

Step 4: Calculate Tax Impact

Determine whether the amendment results in a refund, additional tax owed, or no change to your tax liability. If you owe money, you must pay it when filing to minimize interest charges. If you're due a refund, clearly indicate on the form how you want to receive it (direct deposit information or check).

Step 5: Sign and Date

An authorized corporate officer must sign the form under penalties of perjury. For 2013, acceptable signers included the president, vice president, treasurer, assistant treasurer, or chief accounting officer.

Step 6: File at the Correct Address

The mailing address depends on your corporation's location and whether you're enclosing payment. The IRS has different processing centers for different regions. For corporations required to e-file, submit the amended return electronically through approved software.

Step 7: Track Your Amendment

Unlike individual amended returns (Form 1040-X) which have an online tracking system, corporate amended returns don't have a "Where's My Amended Return?" tool. You may need to call the IRS business tax line at 1-800-829-4933 to check on processing status.

Common Mistakes and How to Avoid Them

Mistake #1: Missing the Filing Deadline

The three-year statute of limitations is unforgiving. Calculate your deadline carefully from the date you actually filed (not the due date), and don't wait until the last minute. Filing at least 30-60 days before the deadline allows for processing issues or mail delays.

Mistake #2: Incomplete or Vague Explanations

The IRS frequently denies amended returns or sends them back because the Part II explanation is inadequate. Don't write "Math error" or "Incorrect amount." Instead, explain specifically: "Depreciation on Equipment A was originally calculated using a 7-year recovery period. The equipment should have used a 5-year period per IRS Publication 946, resulting in additional depreciation expense of $12,500."

Mistake #3: Forgetting to Amend Related Returns

If your corporation is part of a controlled group or consolidated return, changes to one return may require amendments to others. Similarly, if your corporation is a shareholder in an S-corporation or partner in a partnership, changes may flow through and require amendments at multiple levels.

Mistake #4: Not Paying Additional Tax Owed

Some corporations file Form 1120-X without payment, thinking they can pay later. This triggers immediate interest and penalties. Always include payment for any additional tax shown on the amendment.

Mistake #5: Amending Too Quickly After IRS Changes

If the IRS already adjusted your return through a notice, wait to receive the complete IRS documentation before filing Form 1120-X. You need to reference the IRS's adjustments and explain your additional corrections clearly to avoid confusion.

Mistake #6: Using the Wrong Year's Form

While the 2013 Form 1120 can be amended with a later revision of Form 1120-X (the form itself doesn't have year-specific versions like Form 1120 does), you must clearly indicate you're amending a 2013 return. Using current software without properly identifying the tax year leads to processing errors.

Mistake #7: Failing to Attach Complete Documentation

The IRS needs to see the complete picture. Attach all changed schedules, Forms 1125-A (Cost of Goods Sold), Schedule D (Capital Gains), Schedule K (Other Information), and any applicable credits forms. Incomplete attachments result in processing delays or rejections.

What Happens After You File

Processing Timeline

Amended corporate returns typically take significantly longer to process than original returns—often 6 months to a year, or even longer if the amendment is complex or triggers a review. Unlike individual returns, there's no online tracking system for corporate amendments, so patience is essential.

IRS Review

Every Form 1120-X receives some level of review. Simple amendments with clear explanations and proper documentation may be accepted as filed. More complex amendments, especially those claiming substantial refunds or involving technical tax issues, will receive closer scrutiny. The IRS may request additional documentation or schedule an examination (audit).

Outcome Scenarios

Several outcomes are possible. The IRS may accept your amendment as filed and process your refund or adjustments. They may partially accept it, agreeing with some changes but not others. They may deny the amendment entirely if they disagree with your position or find the documentation insufficient. Or they may propose additional changes beyond what you amended, if their review uncovers other issues.

Refunds

If your amendment results in an overpayment, the IRS will issue a refund with interest. Interest accrues from the original payment date or the due date of the return, whichever is later, until the refund is issued. This interest is taxable income in the year received.

Additional Assessments

If the amendment shows additional tax owed, the IRS will assess that tax plus interest from the original due date. If you paid when filing Form 1120-X, the interest stops accruing at that payment date.

Audit Risk

Filing Form 1120-X may increase audit risk, particularly for large changes. The amendment draws IRS attention to the return, and they may examine the entire return, not just the amended items. However, this shouldn't deter you from filing a necessary amendment—failing to correct known errors poses greater risk.

FAQs

Q1: Can I amend my 2013 corporate return in 2025?

No. For a 2013 return filed by the March 17, 2014 deadline, the three-year statute of limitations expired on March 17, 2017. As of 2025, it's too late to amend and claim a refund. However, if the amendment would increase your tax liability (you owe more), you can voluntarily file an amended return at any time, though penalties and interest will apply from the original due date.

Q2: What's the difference between Form 1120-X and just filing a new Form 1120?

Form 1120-X is specifically designed for amendments and requires specific information that a regular Form 1120 doesn't request—particularly the explanation of changes in Part II. For amendments filed after the deadline, you must use Form 1120-X. For superseding returns filed before the deadline, you can file a complete new Form 1120 marked "Superseding Return."

Q3: Do I need to amend my state return if I amend my federal return?

Most states require corporations to file amended state returns when federal returns are amended, typically within 90-120 days of filing the federal amendment. Each state has its own forms and rules. Failing to amend state returns can result in state penalties and interest.

Q4: Can I e-file Form 1120-X?

For tax year 2013, e-filing availability depended on when you filed the amendment. The IRS expanded e-filing for corporate amended returns over time. Generally, if you were required to e-file your original 2013 return (because of the $10 million asset threshold), you would also need to e-file the amendment. Check with your tax software provider for specific availability.

Q5: What if I discover another error after filing Form 1120-X?

You can file a second amended return for the same tax year. Use another Form 1120-X, but this time Column A should reflect the figures from your first amended return (or the IRS's adjustment to that return), not the original return. Clearly indicate this is a second amendment. Multiple amendments on the same return increase IRS scrutiny, so ensure thorough accuracy.

Q6: Will filing an amended return trigger an audit?

Amended returns do receive closer review than original returns, which may increase audit likelihood. However, the IRS examines only a small percentage of corporate returns overall. Large changes, claims for substantial refunds, or amendments involving complex technical issues have higher audit risk. Proper documentation and clear explanations minimize this risk.

Q7: Can I use Form 1120-X to change my accounting method?

Generally, no. Changing accounting methods requires Form 3115 (Application for Change in Accounting Method) filed separately, though you would reference that change on your amended return if it affects the tax year you're amending. Form 1120-X is for correcting errors or claiming missed deductions, not for systematic accounting method changes, which follow their own complex procedures.

Sources

All information in this guide comes from authoritative IRS sources:

• About Form 1120-X, Amended U.S. Corporation Income Tax Return
• 2013 Instructions for Form 1120
• Amended and Superseding Corporate Returns

This guide is for informational purposes only and does not constitute legal or tax advice. Consult a qualified tax professional for guidance specific to your corporation's situation.