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Official form guide
Form CT1X: Adjusted Employer's Annual Railroad Retirement Tax Return or Claim for Refund
IRS Form CT1X is used to correct errors on a previously filed Form CT-1 regarding employee tax types, such as Tier 1 Employee Medicare Tax or Additional Medicare Tax. Calculations for adjustments require multiplying the amount in column 3 by 0.062 (6.2% tax rate).
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IRS Form CT1X - Adjusted Employer's Annual Railroad Retirement Tax Return or Claim for Refund
IRS Form CT1X is used to correct errors on a previously filed Form CT-1 regarding employee tax types, such as Tier 1 Employee Medicare Tax or Additional Medicare Tax. Calculations for adjustments require multiplying the amount in column 3 by 0.062 (6.2% tax rate).
Risk Radar
Scan points- 1Failure to pay the amount shown on line 26 by the time of filing risks triggering failure-to-pay or failure-to-deposit penalties.
- 2Attempting to file Form CT-1 X for Tier 1 Employee Additional Medicare Tax withheld in a prior year is not allowed.
- 3If errors relate only to RRTA tax liabilities reported on Form 945-A, File Form 945-A instead of using Form CT-1X.
- 4Failing to enter the Employer Identification Number (EIN) and RRB number on all required pages and attachments.
- 5Omitting the calendar year being corrected (Return Year) from the top of the form and its attachments.
Plain English
Use this form if you found mistakes when reporting certain employee taxes that were already filed on your original Form CT-1. It allows employers to adjust those amounts, either by paying what is owed or claiming back an overpayment. This process corrects errors related to various types of employee payroll taxes.
Submission Date
- Filing date: 2025-04-16 22:10:17
- Preparation window: collect IDs, supporting records, and signatures in advance.
- Final review: verify names, dates, and required fields before submission.
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What this form is for
- Use this form when correcting errors on a previously filed Form CT-1 for specific employee tax types, including Tier 1 Employee tax, Medicare tax, Additional Medicare Tax, or Tier 2 Employee tax.
- Do not use it when filing to correct Tier 1 Employee Additional Medicare Tax that was actually withheld from an employee in a prior year.
- Check Form CT-1 instead when the filer did not file Forms CT-1 for one or more years.
Deadline or filing window
The due date for Form CT-1X depends on whether the tax was underreported or overreported. If a calculated due date falls on a Saturday, Sunday, or legal holiday in the District of Columbia, filing may occur on the next business day.
Checklist
What you need before filling it out
Filing Purpose
Correcting errors on a previously filed Form CT-1 · General Instructions
Underreporting Tax Payment
Additional amount owed · Line 26 instructions
Summary of Corrections (Line 43)
Amounts for employees with and without written statements · Instructions p.7
Record Keeping Period
All employment tax records · General Instructions
Before you submit
- 1Complete all five pages of Form CT-1 X and sign on page 5.
- 2If underreporting the tax, check the box on line 1 and pay any additional amount owed by filing Form CT-1 X.
- 3Use Courier font for all typed or computer-generated entries, if possible.
- 4Enter dollar amounts to the left of the preprinted decimal point and must show an amount for cents, even if it is zero.
- 5If filing late, attach an amended Form 945-A to Form CT-1 X.
- 6In Part 2 (Certifications), if all corrections relate to underreported tax amounts, complete line 3 only and skip lines 4 and 5.
- 7Staple multiple sheets in the upper-left corner of the form.
How to file this form
- 1Determine if the return is an adjusted railroad retirement tax return or a claim for refund/abatement.
- 2If underreporting the tax, check the box on line 1 and pay any additional amount owed using instructions provided for line 26.
- 3Complete all required correction details on Form CT-1X, including providing a summary of corrections on line 43.
- 4Submit the completed Form CT-1X separately from the original Form CT-1.
- 5Retain copies of the filed return and supporting documentation for at least 4 years.
Known limitations
- 1Form CT-1 X cannot be used to correct Tier 1 Employee Additional Medicare Tax that was actually withheld from an employee in a prior year.
- 2If errors relate only to RRTA tax liabilities reported in Part II of Form CT-1 or on Form 945-A, do not file Form CT-1 X; instead, file Form 945-A and enter only the monthly totals.
- 3Form CT-1 X cannot be used to correct errors found on Forms 941, 941-SS, 943, 944, or 945; use the corresponding 'X' form for those documents instead.
Field map
Compact field-by-field guide
Employer Info
1 items
Full legal name and Employer Identification Number.
Quarter
1 items
Select the calendar quarter this return covers.
Wages
1 items
Sum of all wages, tips, and other compensation paid to employees during the quarter.
Withholding
1 items
Total federal income tax withheld from employee paychecks.
SS & Medicare
1 items
Wages subject to Social Security tax (capped) and Medicare tax (uncapped).
Deposit
1 items
Indicate your deposit schedule: monthly, semiweekly, or next-day rule if over $100k.
Signatures
1 items
Must be signed by an authorized officer or agent of the employer.
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Fillable formOpen in Editor->The current edition is revised April 2025. For the latest information about developments related to Form CT-1X and its instructions, visit IRS.gov/CT1X.
What changed or needs a fresh check
- Edition date — confirm the revision date reads April 2025.
- Fee — Not stated in the official source.
- Mailing address — Not stated in the official source.
- Signature — State plainly when not in source.
Quick Facts
Downloads
After you file
- 1Keep a copy of Form CT-1 X for records.
- 2If receiving a notice about a penalty after filing, reply to the notice with an explanation; do not attach this explanation when submitting the return.
- 3To potentially avoid penalties or interest, ensure documentation exists detailing the date discovered and the grounds/facts relied on to support the correction.
- 4The IRS will review whether all criteria for interest-free treatment were met, especially if the underreported amounts relate to an issue raised in a prior examination.
Sources
- SRCInstructions p.2 — Use Form CT-1 X to correct Tier 1 Employee tax, Medicare tax, Additional Medicare Tax, and Tier 2 Employee tax on a previously filed Form CT-1.
- SRCInstructions p.3 — If errors relate only to RRTA tax liabilities reported in Part II of Form CT-1 or on Form 945-A, file Form 945-A instead of Form CT-1 X.
- SRCInstructions p.2 — The form cannot be used to correct Tier 1 Employee Additional Medicare Tax actually withheld from an employee in a prior year.
- SRCInstructions p.3 — For errors on other forms (like Form 941), use the specific 'X' form corresponding to that document (e.g., Form 941-X).
- SRCInstructions p.7 — Line 3 must be checked on Form CT-1 X to certify filing or future filing of Forms W-2 or Forms W-2c with the SSA.
- SRCInstructions p.4 — If correcting overreported tax amounts in the last 90 days, the claim process must be used instead of the adjustment process.
Common confusion points
What is the purpose of filing this form?
Use Form CT-1 X to correct errors on a previously filed Form CT-1 regarding employee tax types, including Tier 1 Employee Medicare Tax and Additional Medicare Tax.
Can I use this form if my error relates to other forms (like Form 941)?
No; use the specific 'X' form that corresponds to the original document being corrected (e.g., Form 941-X).
What happens if I am correcting a tax liability reported on Form 945-A?
File Form 945-A and only enter the monthly totals; do not use Form CT-1 X.
Do I need to certify W-2 filing after submitting this form?
Yes, check box on line 3 of Form CT-1 X to confirm that Forms W-2 or Forms W-2c were filed or will be filed with the SSA.
What if I am correcting overreported taxes recently?
If correcting overreported tax amounts in the last 90 days of a period of limitations, use the claim process and cannot use the adjustment process.
Where should I send my completed Form CT-1 X?
Send the form to the specific mailing address provided in the official instructions.
Workflow map
Related forms and next steps
Current
Questions about IRS Form CT1X
What is IRS Form CT1X used for?
Use this form if you found mistakes when reporting certain employee taxes that were already filed on your original Form CT-1. It allows employers to adjust those amounts, either by paying what is owed or claiming back an overpayment. This process corrects errors related to various types of employee payroll taxes.
Who must file IRS Form CT1X?
Employers must file IRS Form CT1X when they discover errors on a previously filed Form CT-1 that relate to Tier 1 Employee tax, Medicare tax, Additional Medicare Tax, or Tier 2 Employee tax.
What information does IRS Form CT1X require?
The form requires entering identifying information including the Employer Identification Number (EIN), RRB Number, Name, and Address. Additionally, it collects the calendar year being corrected (Return Year) and the date errors were discovered. Adjustments are calculated using amounts from column 3 to determine results reported in column 4.
When is IRS Form CT1X due?
If correcting underreported tax, Form CT-1X must be filed by the due date of the return for the period in which the error was found. If overreporting is corrected, the filer may choose an interest-free adjustment or file a claim for refund or abatement.
Where do I file IRS Form CT1X?
The form must be sent via the U.S. Postal Service (USPS) or an IRS-designated private delivery service (PDS) on or before the due date. If mailing is late, the envelope must be properly addressed and contain sufficient postage to be considered timely.
How do I complete IRS Form CT1X?
First, provide the required identifying information, including EIN and RRB Number, in the designated spaces. Next, enter the calendar year being corrected (Return Year) and the date errors were discovered on pages 2 through 5. Finally, calculate tax adjustments by multiplying the amount in column 3 by 0.062 to report the result in column 4.
What happens if IRS Form CT1X is filed incorrectly?
If correcting underreported tax amounts, filing late requires attaching an amended Form 945-A; otherwise, the IRS may assess an “averaged” FTD penalty. Filing and paying on time generally ensures that the correction is interest free and avoids failure-to-pay or failure-to-deposit penalties.
What is the purpose of filing this form?
Use Form CT-1 X to correct errors on a previously filed Form CT-1 regarding employee tax types, including Tier 1 Employee Medicare Tax and Additional Medicare Tax.
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