The C-Corporation Foundation & When to File Form 1120

A C-Corporation (or 'C-corp') is a legal entity chartered by a U.S. state that is taxed separately from its owners under Subchapter C of the Internal Revenue Code. The 'C' comes from that subchapter, not from any classification. Every corporation formed under state law is a C-corp by default — it becomes an S-corp ONLY if it files Form 2553 and meets the S-corp eligibility rules (100 or fewer U.S. individual shareholders, one class of stock, etc.).

The single biggest thing to understand about C-corps: they pay tax at the entity level, and then again at the shareholder level when profits are distributed as dividends. This is 'double taxation' and it's the reason most small businesses prefer S-corps or LLCs. But C-corps exist for good reasons — attracting outside investment, offering tax-free fringe benefits to owner-employees, retaining earnings for expansion at the 21% flat rate, or holding real estate at arm's length.

As a preparer, your job on a C-corp return is fundamentally different from a 1040. On the 1040, everything flows to one person. On the 1120, you're preparing a return for a *separate legal person* — the corporation — that has its own books, its own tax year, its own bank account, and its own personality in the eyes of the IRS.

Every domestic C-corporation must file Form 1120 every year, even if it has no income and no activity. There is no minimum-income threshold, no 'small corporation' exception, and no way to skip a year without formally dissolving. This trips up first-time preparers who assume a dormant corp doesn't need to file — it does.

The corp must file even if:
- Total income is $0
- The corp had no bank account
- All shareholders are family members
- It's a 'shell' waiting for a future business

If a C-corp is dissolving mid-year, it files a final Form 1120 through the date of dissolution, and Box 'Final return' at the top of the form must be checked. If it converts to an S-corp mid-year via Form 2553, it files a short-year 1120 (C-corp period) and a short-year 1120-S (S-corp period).

For a calendar-year C-corp, Form 1120 is due the 15th day of the 4th month after year-end — April 15. This is different from calendar-year partnerships and S-corps (due March 15) and the same as individuals.

Fiscal-year corps (any year-end other than December 31) file by the 15th day of the 4th month after their year-end. A June 30 fiscal year is a special case — for tax years beginning before 1/1/2026, the 1120 for a 6/30 fiscal year is still due September 15 (not October 15), with an extended deadline of April 15 (a 7-month extension, not 6 months).

Extensions: File Form 7004 by the original due date for an automatic 6-month extension (5-month for a June 30 fiscal year — see above). The extension is of TIME TO FILE, not time to pay. Any tax owed must still be paid by the original due date, or interest and late-payment penalty (0.5%/month) start accruing.

The first tax year of a corporation is often the shortest. It begins on the earlier of the date the corp was formed (per state law) OR the date it acquired assets or began doing business. It ends on either 12/31 (calendar year) or the last day of a chosen fiscal year.

Startup costs (§195) and organizational costs (§248) are deductible up to $5,000 each in the first year — but this benefit phases out dollar-for-dollar if total startup or org costs exceed $50,000 (so if org costs are $54,000, only $1,000 is deductible immediately). Any remainder is amortized over 180 months (15 years). These deductions go on Form 4562, then Schedule K, and then flow to 'Other deductions' line 26 of Form 1120.

The choice of tax year is made on the FIRST filed return. Most C-corps pick a calendar year for simplicity, but a fiscal year can help align with a natural business cycle (e.g., a retail corp choosing January 31 to close the books after the holiday rush).