What Is Form 100: California Corporation Tax Return

California Form 100 is the annual Corporation Franchise or Income Tax Return that C-corporations file with the Franchise Tax Board to report income, claim deductions and credits, and calculate state tax. Net income is taxed at 8.84%, and most corporations owe a minimum franchise tax of $800 whether or not they turned a profit.¹ The return is due by the 15th day of the fourth month after the close of the taxable year, which is April 15 for calendar-year filers.²

Who Files Form 100

Form 100 is used by more than just standard C-corporations. The FTB instructions apply it to corporations, banks, financial corporations, real estate investment trusts, real estate mortgage investment conduits, regulated investment companies, Massachusetts or business trusts, publicly traded partnerships, exempt homeowners’ associations, political action committees, and LLCs or partnerships that have elected corporate tax treatment.³ If your LLC filed Federal Form 8832 electing to be taxed as a corporation, Form 100 is your return.⁴

S-corporations do not file Form 100. They file Form 100S, which reflects pass-through treatment.⁵

The filing requirement reaches beyond state borders. Revenue and Taxation Code Section 18601 requires every corporation subject to Part 11 of the code to file a return by the 15th day of the fourth month after year-end,⁶ and Section 23151.1 requires every corporation doing business in California to pay the annual franchise tax.⁷ An out-of-state corporation with California-source income, property, employees, or payroll in the state is on the hook.

The 8.84% Rate and the $800 Minimum Franchise Tax

California taxes corporate net income at a flat 8.84%. Banks and financial corporations pay a slightly higher rate, but 8.84% applies to the vast majority of C-corporations, and it applies to net income after deductions rather than gross receipts.¹

The $800 minimum franchise tax is separate from income tax and does not go away in unprofitable years. Under Revenue and Taxation Code Section 23153, the $800 is owed from the earlier of the date of incorporation, qualification, or commencement of business in California, and it continues each year until the corporation formally dissolves or withdraws.⁷

One exception matters for new businesses: the $800 minimum is waived for a corporation’s first taxable year. Any net income earned that year is still taxed at 8.84%.⁵

What You Need to Complete the Return

Three identification numbers go on the return: the Federal Employer Identification Number (FEIN), the California Secretary of State file number assigned when the entity was formed or registered, and the California corporation number. The SOS file number is the primary identifier for FTB purposes. A business that is not registered with the Secretary of State uses an FTB-issued ID number instead.⁸ The legal name on the return has to match what the Secretary of State has on file.³

On the financial side, you need gross receipts, cost of goods sold, business deductions, and any tax credits you plan to claim. The form asks you to specify your accounting period. Calendar-year filers use December 31; fiscal-year filers enter their beginning and ending dates. Your California taxable year must match the one you use for federal purposes unless the FTB has approved a different period.³

Corporations doing business in more than one state attach Schedule R to apportion income between California and other jurisdictions.⁹ Additional schedules handle net operating loss deductions, specific credits, and related-party transactions.

Deadlines and the Automatic Extension

Calendar-year corporations must file Form 100 and pay any tax owed by April 15. Fiscal-year filers use the 15th day of the fourth month after their year-end. If the due date falls on a weekend or holiday, it shifts to the next business day.²

California grants an automatic seven-month filing extension without a separate application, as long as the corporation is not suspended or forfeited. For a calendar-year filer, that pushes the return deadline to November 15. The extension does not extend the payment deadline. If you expect to owe tax and cannot file on time, pay by the original due date using Form FTB 3539 to avoid penalties and interest. If you owe nothing, no extension form is needed.¹⁰

Estimated Tax Payments

Corporations prepay California tax through quarterly estimated installments using Form 100-ES. The schedule is uneven and surprises many first-time filers. For calendar-year corporations:¹¹

April 15: 30% of estimated annual tax
June 15: 40% of estimated annual tax
September 15: no payment
December 15: 30% of estimated annual tax

Fiscal-year corporations pay on the 15th day of the 4th, 6th, 9th, and 12th months of their taxable year.

The prior-year safe harbor generally protects you from underpayment penalties if your installments equal or exceed the tax shown on last year’s return, prorated across the installments. Large corporations, meaning those with California net income of $1 million or more in any of the three preceding taxable years, can only use the prior-year safe harbor for the first installment and must base the rest on the current year’s expected tax. Newly formed corporations cannot use the prior-year exception in their first taxable year because there is no prior return to reference.¹¹

How to File and Pay

Any business that prepares its return using tax preparation software is required by California law to e-file.¹² In practice, most corporations file electronically. Paper filing remains available for returns prepared without software.

Paper returns go to one of two addresses depending on whether a payment is enclosed:

Without payment: Franchise Tax Board, PO Box 942857, Sacramento, CA 94257-0500
With payment: Franchise Tax Board, PO Box 942857, Sacramento, CA 94257-0501¹³

For payment, the FTB’s Web Pay service transfers funds from your business bank account at no cost.¹⁴ Credit cards are accepted but carry a 2.3% service fee.¹⁵ Check or money order with the return also works.

Penalties and Interest

The late-filing penalty is 5% of the unpaid tax for each month or partial month the return is overdue, capped at 25%. It applies to the balance remaining after any payments or credits made by the original due date. A corporation that files within the automatic seven-month extension period avoids this penalty as long as the balance was paid in full by the original deadline.¹⁶

Underpayment of estimated tax carries its own penalty, calculated at 4% for the period running through at least mid-2026. That is separate from the general interest rate on unpaid tax balances, which is 7% for corporate underpayments during the same period. Interest accrues from the original due date until the balance is paid in full and compounds on top of any penalties already assessed.¹⁷

Fixing a Mistake With Form 100X

To correct an error on a Form 100 that has already been filed, use Form 100X, the Amended Corporation Franchise or Income Tax Return. Wait until the original return has been processed before filing the amendment.¹⁸

The deadline for claiming a refund on an amended return is the latest of four years from the original due date, four years from the date the return was timely filed, or one year from the date the tax was paid. Returns filed before the due date count as filed on the due date for this calculation. If the amendment shows additional tax owed, file as soon as possible to limit interest charges.¹⁸

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Franchise Tax Board. Business Tax Rates
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Franchise Tax Board. Due Dates Businesses
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Franchise Tax Board. 2024 Instructions for Form 100
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Franchise Tax Board. LLC Treated as a Corporation Business Type
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Franchise Tax Board. S Corporations
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California Legislative Information. California Revenue and Taxation Code 18601
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California Legislative Information. California Revenue and Taxation Code 23151.1
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Franchise Tax Board. Business Entity ID Number Help
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Franchise Tax Board. 2024 Schedule R Apportionment and Allocation of Income California
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Franchise Tax Board. Extension to File
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FTB.ca.gov. 2026 Instructions for Form 100-ES Corporation Estimated Tax
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Franchise Tax Board. e-file for Business
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Franchise Tax Board. Mailing Addresses
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Franchise Tax Board. Payment Options
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Franchise Tax Board. Pay by Credit Card
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Franchise Tax Board. Common Penalties and Fees
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Franchise Tax Board. Interest and Estimate Penalty Rates
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Franchise Tax Board. Instructions for Form 100X Amended Corporation Franchise or Income Tax Return