How to Complete and File California Form 100X: Amended Corporation Tax Return

California Form 100X is the amended franchise or income tax return corporations file with the Franchise Tax Board to correct a previously filed Form 100, 100W, or 100S. You use it whether you found the error yourself, the IRS adjusted your federal return, or you are carrying a net operating loss back to an earlier year. The deadlines and the paperwork differ depending on which of those applies, so start with the trigger.¹

When Form 100X Is Required

Three situations typically call for an amended return.

The first is a mistake you found on your own: a missed deduction, a miscalculated credit, revenue reported in the wrong period. If the correction means the corporation overpaid, Form 100X is how you claim the refund. If it means you underpaid, filing before the FTB finds the error keeps penalties from stacking on top of the tax.

The second is a federal change. When the IRS adjusts your federal return through audit or accepts an amended Form 1120-X, California requires you to report those changes to the FTB. The obligation applies to all corrections identified — gross income, deductions, credits, penalties, supplemental income — including items the IRS did not specifically examine. Even if the federal adjustments do not increase your California liability, you still have to notify the FTB, though in that case an amended return itself is not required.²

The third is a net operating loss carryback. If your corporation is carrying a loss back to recover taxes paid in an earlier year, the amended return for that earlier year is how the FTB processes the carryback.³

Filing Deadlines

Errors You Found Yourself

For a refund claim, you generally have four years from the date the original return was filed.⁴ If the overpayment came after the return was filed — for example, an estimated payment applied to the wrong year — you may have one year from the date of that later overpayment, if that gives you more time.

If you owe additional tax, there is no fixed deadline, but interest keeps accruing from the original due date until you pay. File promptly.

Amendments Triggered by Federal Changes

Two very different clocks run from the date the IRS finalizes its changes:

The six-month window is the one that catches corporations off guard. Miss it and you lose the ability to contest the federal determination at the state level, and penalties apply on top of the additional tax.

What to Gather Before You Start

The corporation’s California corporation number, FEIN, and Secretary of State file number. If any of these changed since the original filing, note the discrepancy for Part V.³
A copy of the original return as filed, or the most recent 100X if you have amended this year before. You need those figures for column (a).
Federal Form 1120-X or the IRS Revenue Agent’s Report when the amendment stems from federal changes. Include the final federal determination and the schedules that explain the adjustments.
Supporting computations for anything complex: research credits, depreciation changes, NOL carrybacks.

Download the current version of Form 100X from ftb.ca.gov. Older versions may lack fields the FTB now requires and can be returned.

Filling Out the Form

Header

Enter the corporation’s name, address, California corporation number, FEIN, and Secretary of State file number. Check the box showing whether you are amending Form 100, 100W, or 100S, and enter the tax year. If the amendment is based on a final federal determination, check box C and enter the date the IRS finalized its changes.

The Three-Column Comparison on Side 1

Side 1 is where the math happens. Three columns:

Column (a): amounts from the original return as filed, or as last amended.
Column (b): the net change for each line, with increases and decreases clearly indicated.
Column (c): the corrected amounts. Column (a) plus or minus column (b) should equal column (c) on every line.

Work down the return methodically: gross income, deductions, net income, tax, credits, payments, and the final balance due or overpayment. Follow the line-by-line instructions so each entry matches the corresponding line on the original return. A mismatch between column (a) and the figures the FTB has on file is one of the most common reasons an amended return gets pulled for additional review.

Part V: Explanation of Changes

Explain, in detail, every change to the amounts on Side 1. The instructions require line-number references for both the original and the amended return, plus the underlying computations. If the name, address, corporation number, or FEIN differs from the original filing, explain that in Part V too.

Be specific. “Corrected depreciation” tells a reviewer nothing. “Revised MACRS depreciation on equipment placed in service 2023, increasing the deduction by $42,000 (see attached Schedule A)” is the level of detail that lets the FTB process the filing without writing back for documents. Put the corporation’s name and California corporation number on every attached schedule.

Signature

An officer authorized to act for the corporation must sign: president, vice president, treasurer, assistant treasurer, or chief accounting officer.⁶ A paid preparer who completed the form also signs and includes their PTIN.

Submitting the Return

E-File

The FTB accepts Form 100X electronically, and California law requires any business entity that prepares an original or amended return using tax preparation software to e-file.⁷ Confirm with your software provider that they support business amended returns. The FTB will consider waivers where technology constraints or undue financial burden make e-filing impractical.

Paper Filing

If you qualify for a waiver or are not using software, mail the completed return. The address depends on whether there is a payment:¹

Balance due with payment enclosed: Franchise Tax Board, PO Box 942857, Sacramento, CA 94257-0501.
Refund, no balance due, or balance paid electronically: Franchise Tax Board, PO Box 942857, Sacramento, CA 94257-0500.

If you e-filed and owe a balance, send Form FTB 3586 with your payment to PO Box 942857, Sacramento, CA 94257-0531. Electronic payment options include EFT, electronic funds withdrawal, Web Pay, and credit card.

Interest and Penalties on Additional Tax

Interest on additional tax runs from the original due date of the return, not from the date you file the 100X. For the period from July 1, 2025, through June 30, 2026, the FTB charges 7 percent on corporate underpayments.⁸ The rate adjusts twice a year, so check the current table if you are filing outside that window.

Late-filing penalties can apply if the FTB determines you should have amended sooner. The standard penalty is 5 percent of the unpaid tax for each month or fraction of a month the return is overdue, capped at 25 percent. Missing the six-month reporting window under RTC 18622 for federal changes can trigger additional penalties on top of that.

Processing Time and What Comes Next

Business amended returns move slowly. The FTB’s current estimate is roughly 12 months, and complex amendments or requests for more documents can push it further.⁹

When the review is done, you get a notice by mail. Approved amendments with a refund are paid out with any applicable interest. If the FTB needs more information, it sends a formal request; respond quickly, because delay at that stage adds months. If the FTB disagrees with your amendments, the notice will lay out the adjustments and how to protest.

1
Franchise Tax Board. 2025 Instructions for Form 100X Amended Corporation Franchise or Income Tax Return
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Franchise Tax Board. FTB Pub 1008 – Federal Tax Adjustments and Your Notification Responsibilities to California
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California Franchise Tax Board. California Form 100X – Amended Corporation Franchise or Income Tax Return
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California Legislative Information. California Revenue and Taxation Code 19306
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California Legislative Information. California Code RTC 18622 – Administration of Franchise and Income Tax Laws
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Office of the Law Revision Counsel. 26 USC 6062 – Signing of Corporation Returns
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Franchise Tax Board. e-file for Business
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Franchise Tax Board. Interest and Estimate Penalty Rates
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California Franchise Tax Board. Timeframes