Form 100X: California Amended Corporation Franchise or Income Tax Return Explained

Form 100X is the document California corporations use to correct a previously filed Form 100, Form 100S, or Form 100W. If your business discovers an error after submitting its original franchise or income tax return — whether it’s a missed deduction, an incorrect income figure, or a change resulting from an IRS audit — the Franchise Tax Board requires this amended return rather than a simple letter or phone call. It’s issued by the California Franchise Tax Board and applies to corporations operating or incorporated within the state.

The form is straightforward in purpose but detailed in execution. It walks through the original figures reported, the corrected figures, and the difference between the two, along with an explanation of why the change is being made. The FTB uses this information to recalculate tax owed, apply any additional payment due, or process a refund if the correction results in an overpayment.

Who Needs to File Form 100X

Any corporation that filed Form 100 (California Corporation Franchise or Income Tax Return), Form 100S (S Corporation), or Form 100W (Water’s-Edge) and later needs to change something on that return should use Form 100X. Common triggers include:

• A federal audit adjustment that changes taxable income reported to California
• Correcting math errors or omitted income discovered after filing
• Claiming a deduction, credit, or net operating loss carryback that wasn’t included originally
• Reporting changes required after an amended federal return (Form 1120X) was filed

S corporations and corporations electing water’s-edge treatment both use this same amended form, checking the appropriate box to indicate which original return is being corrected.

When to File

Generally, an amended return should be filed within four years from the original due date of the return, or one year from the date of overpayment, whichever period expires later — though corporations reporting a federal change should file within six months of the final federal determination. Filing late doesn’t necessarily block the correction, but it can affect whether a refund is still available or whether penalties and interest apply to additional tax owed. It’s worth checking the exact deadline against your original filing date since California’s statute of limitations doesn’t always mirror the IRS timeline.

How to Complete and Submit Form 100X

The 2025 version of Form 100X is a fillable PDF (AcroForm), meaning you can type directly into the fields on your computer before printing or saving. The form asks for the corporation’s name, California corporation number, FEIN, and the tax year being amended. From there, you’ll enter the originally reported amounts in one column and the corrected amounts in another, with a third column calculating the net change. A separate section requires a written explanation of each change — vague statements like “correction” aren’t sufficient; the FTB expects specifics, such as “disallowed deduction per IRS audit” or “corrected apportionment percentage.”

Corporations that also handle LLC-related filings should keep in mind that Form 100X doesn’t cover LLC fees or annual taxes — those follow separate procedures. For businesses managing multiple entity types, resources like FTB Form 3522 instructions for the LLC tax voucher or the guide to FTB Form 3536 estimated fees for LLCs can help clarify which form applies to which type of California filing obligation.

Once completed, Form 100X is mailed to the Franchise Tax Board at the address listed in the instructions — electronic filing options may vary depending on your tax preparation software. Attach any supporting schedules, federal audit documents, or amended federal forms that explain the change. If the amendment results in additional tax due, payment should accompany the return to limit interest accrual. If it results in a refund, processing times can run several months, so it’s reasonable to follow up with the FTB if you haven’t received a response within that window.