IRS Form 1120

Form 1120: U.S. Corporation Income Tax Return

A plain-English guide to IRS Form 1120 — what it's for, who files it, when it's due, and how to complete it.

Form number1120
Official nameU.S. Corporation Income Tax Return
CategoryBusiness
Typically dueApril 15 (calendar-year C corps)
Official IRS Form 1120 (PDF)

What is Form 1120?

Form 1120 is the income tax return C corporations file to report income, deductions and credits and to pay the flat 21% federal corporate tax.

The 1120 actually calculates and pays tax, unlike the pass-through returns — a flat 21% on the corporation's taxable profit. Because owner salaries and benefits are deductible before that 21% applies, a lot of tax planning for C corps is about the mix of salary versus retained profit versus dividends.

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Who files it

C corporations, including LLCs that elected corporate taxation.

When it's due

Form 1120 is generally due April 15 (calendar-year C corps). Deadlines can move for weekends and holidays, so confirm the exact date for the current year on IRS.gov.

Key lines and parts

Here's what you'll actually fill in on Form 1120:

  • Income and cost of goods sold
  • Deductions (salaries, rent, depreciation)
  • Taxable income and 21% tax
  • Schedule K — other information

How to file

File by the 15th day of the fourth month after year-end. Attach supporting schedules for cost of goods sold, dividends and more.

Common mistakes & penalties

The structural pitfall is double taxation: profit taxed at 21% inside the company, then taxed again as dividends to shareholders. C corps make the most sense when you're reinvesting profits or raising outside capital — not when you plan to pull all the money out each year.
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Frequently asked questions

What is IRS Form 1120?

Form 1120 is the income tax return C corporations file to report income, deductions and credits and to pay the flat 21% federal corporate tax.

Who has to file Form 1120?

C corporations, including LLCs that elected corporate taxation.

When is Form 1120 due?

Form 1120 is generally due April 15 (calendar-year C corps). Always confirm the current year's date on IRS.gov, since deadlines shift for weekends and holidays.

What's the most common mistake with Form 1120?

The structural pitfall is double taxation: profit taxed at 21% inside the company, then taxed again as dividends to shareholders. C corps make the most sense when you're reinvesting profits or raising outside capital — not when you plan to pull all the money out each year.