Form 1120 is the US Corporation Income Tax Return that domestic C corporations file each year to report income, deductions, credits and tax. It's due by the 15th day of the 4th month after the tax year ends, so 15 April for a calendar-year company, and every domestic corporation must file it whether or not it has taxable income.
If your company is a C corporation, or an LLC that elected to be taxed as one, this is your return. Unlike a partnership or S corporation, a C corporation pays tax itself, and that's the big difference. Here's what the IRS instructions say about who files, when and what it costs to be late.
Who files Form 1120
The IRS instructions say that, unless exempt under section 501, all domestic corporations, including corporations in bankruptcy, must file an income tax return whether or not they have taxable income. Domestic corporations file Form 1120 unless they're required, or elect, to file a special return.
| Your business | Usual federal return |
|---|---|
| C corporation | Form 1120 |
| Corporation or LLC that elected S corporation status | Form 1120-S |
| LLC with two or more members and no election | Form 1065 (partnership) |
| Single-member LLC with no election | Reported on the owner's return |
| Foreign-owned single-member US LLC | A pro forma Form 1120 with Form 5472 attached |
That last row surprises a lot of non-US founders, and it's easy to miss. The instructions say a domestic disregarded entity wholly owned by a foreign person is treated as a corporation for certain reporting rules, so it files a pro forma Form 1120 with Form 5472 by the due date, including extensions.
When Form 1120 is due
A corporation generally files by the 15th day of the 4th month after its tax year ends. There's one odd exception the instructions call out: a corporation with a fiscal year ending 30 June files by the 15th day of the 3rd month after its year ends. New corporations filing a short-period return, and corporations that dissolve, follow the same 4th-month rule from the end of the short period or the dissolution date. If a due date falls on a weekend or legal holiday, you're allowed to file on the next business day.
Need more time to file? Form 7004 gives an automatic extension, and it must generally be filed by the regular due date. But the extension is for filing, not paying: the instructions say tax due must be paid in full by the due date, not including extensions.
The tax rate and estimated payments
The Schedule J instructions tell corporations to multiply taxable income by 21%. A corporation must make quarterly estimated tax payments if it expects its total tax for the year, less credits, to be $500 or more, and those installments must be paid by electronic funds transfer. If it underpays, Form 2220 works out any penalty; if it overpays, Form 4466 can get a quick refund.
E-filing is the default for most companies
This is the rule that catches small corporations. For returns filed on or after 1 January 2024, a corporation must e-file Form 1120 if it files 10 or more returns of any type during the calendar year, and that count includes income tax, employment tax, excise tax and information returns. Four quarterly Forms 941, a Form 940 and a handful of 1099s can take a small company past 10 very quickly. The IRS may waive the rule for hardship.
Penalties for filing late
| Problem | What the instructions say (as of October 2026) |
|---|---|
| Late filing | 5% of the unpaid tax for each month or part of a month late, up to 25% of the unpaid tax |
| More than 60 days late | Minimum penalty of the smaller of the tax due or $525, for returns required to be filed in 2026 |
| Late or missed estimated tax | An underpayment penalty for the period of underpayment, worked out on Form 2220 |
| Interest | Charged on unpaid tax and on many penalties |
The good news for very small companies: some of the extra questions on the return don't apply if both total receipts and total assets at the end of the year are less than $250,000.
Paying the tax
The IRS asks corporations to pay electronically whenever possible, and to use direct deposit for refunds. EFTPS accepts same-day payments of $1 million or less if they're submitted before 3:00 p.m., and larger payments have a later cut-off. Any payment due on a weekend or legal holiday moves to the next business day.
Foreign owned companies have an extra filing, explained in our guide to the foreign owned company filing.
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A filing checklist
- Confirm the corporation's tax year and due date, including the 30 June exception.
- Close the books and reconcile income and expenses to your accounts.
- Work out tax at 21% on taxable income and subtract estimated payments already made.
- File Form 7004 by the due date if you need more time, and pay any tax due by the original date.
- Count your returns for the year; at 10 or more, e-file.
- Plan next year's estimated payments if tax will be $500 or more.
Clean books make this return far easier, so start with bookkeeping software for small businesses, and if you're issuing contractor forms, 1099 software helps keep the information-return count straight.
My verdict
Form 1120 comes with being a C corporation: the company files and pays tax at 21% every year, profit or not. The traps are paying late because you extended, missing quarterly estimates once tax reaches $500, and not realising you're over the 10-return e-file line. Diarise 15 April, pay what you owe on time, and let the extension cover the paperwork only.
Partnerships file a different return, covered in our guide to the partnership return.
Frequently asked questions
Who must file Form 1120?
All domestic corporations, unless exempt under section 501 or required to file a special return, whether or not they have taxable income.
When is Form 1120 due?
By the 15th day of the 4th month after the tax year ends, which is 15 April for calendar-year corporations. A 30 June year-end files by the 15th day of the 3rd month.
Does Form 7004 extend the time to pay?
No. It extends the time to file only. Tax due must be paid in full by the original due date.
What is the corporate tax rate on Form 1120?
The Schedule J instructions say to multiply taxable income by 21%.
What is the penalty for filing Form 1120 late?
Usually 5% of the unpaid tax per month or part month, up to 25%. For returns due in 2026 more than 60 days late, the minimum is the smaller of the tax due or $525, as of October 2026.
Do corporations have to e-file Form 1120?
Yes, if the corporation files 10 or more returns of any type in the calendar year, including employment and information returns.
