IRS Form 1120 “U.S. Corporation Income Tax Return” Explained

IRS Form 1120, U.S. Corporation Income Tax Return, is the annual federal income tax return for C corporations. Unlike partnerships and S corporations, a C corporation pays tax at the entity level, at a flat 21% federal rate. Shareholders then pay tax again on dividends they receive.

Who Files Form 1120

• Domestic corporations that haven’t elected S corporation status
• LLCs that have elected to be taxed as a corporation (Form 8832) and haven’t elected S status

S corporations file Form 1120-S instead and pass income through to shareholders on Schedule K-1. C corporations do not issue Schedule K-1s.

Due Date

Form 1120 is due by the 15th day of the fourth month after year-end, which is April 15 for calendar-year corporations. A six-month extension is available with Form 7004. An extension to file isn’t an extension to pay. Corporations expecting to owe $500 or more generally must make quarterly estimated tax payments.

Main Parts of the Return

• Page 1: gross receipts, cost of goods sold, other income, deductions, taxable income, tax and payments
• Schedule C: dividends received and the dividends-received deduction
• Schedule J: tax computation and credits
• Schedule K: other information, such as ownership, accounting method and NOLs
• Schedule L: balance sheet
• Schedules M-1 and M-2: reconciliation of book income to taxable income and analysis of retained earnings. Larger corporations with $10 million or more in assets file Schedule M-3.

Key Considerations

• Double taxation: profits are taxed at 21%, then dividends are taxed to shareholders, often at qualified dividend rates.
• Owner compensation: salaries paid to shareholder-employees are deductible but must be reasonable.
• Accumulated earnings and personal holding company taxes can apply to corporations that retain excess profits without a business need.
• Qualified small business stock (Section 1202) can allow shareholders to exclude gain on sale if requirements are met.

Choosing an Entity

Whether a C corporation, S corporation, partnership or sole proprietorship makes sense depends on your profits, plans to reinvest, exit strategy and state taxes. Diehl CPA can model the options. See our tax consulting services.

Get Help From Diehl CPA

Tax rules change often, and the right answer depends on your full financial picture. Diehl CPA, led by Mark Diehl, CPA, CFE, prepares returns and builds tax plans for investors, retirees and closely held businesses. Learn about our tax return preparation and investment tax planning services, or contact us to talk through your situation.

This article is general information, not tax or legal advice for your specific situation. Updated October 2026.