Which Business Tax Form Do I File? (1120 vs 1120-S vs 1065)

Fact-checked by the LegalGlass editorial team against primary sources · Published Aug 6, 2026 · Last updated Aug 6, 2026

Your entity type decides the form. A C corporation files Form 1120, an S corporation files Form 1120-S, and a partnership or multi-member LLC files Form 1065. A sole proprietor or single-member LLC files Schedule C with Form 1040. Form 1120-S and Form 1065 are pass-through returns; Form 1120 is taxed at the entity level.

Which Form Matches Your Entity

The business income tax form you file is set by your entity's federal tax classification, not by its name or state registration. The IRS Business Taxes overview states that "the form you use depends on how your business is organized." A limited liability company is the common source of confusion because an LLC is a state entity with no federal tax form of its own - it defaults to sole-proprietor or partnership treatment and can elect corporate treatment. Match your situation in the table, then read the section for your form.

Two follow-on returns apply to owners of pass-through entities. Wages from a sole proprietorship carry self-employment tax on Schedule SE, and owners of all four entity types may owe quarterly estimated taxes during the year. For the trade-offs between structures, see S-corp vs. LLC and LLC vs. S-corp tax.

Form 1120: C Corporations

Form 1120, U.S. Corporation Income Tax Return, is filed by a C corporation to report its income and figure its income tax. The IRS describes Form 1120 as the return corporations use to "report their income, gains, losses, deductions, credits" and to "figure their income tax liability." A C corporation is a separate taxpayer: it pays federal income tax at the entity level under 26 U.S. Code § 11, which imposes a tax on "the taxable income of every corporation." Since 2018 that rate has been a flat 21% under § 11(b).

Because the corporation pays tax on its profit and shareholders then pay tax again on dividends, C-corporation income can be taxed twice - the "double taxation" that pushes many small businesses toward pass-through status. A calendar-year C corporation files Form 1120 by the 15th day of the fourth month after year-end (April 15), the same fourth-month deadline set for corporations in 26 U.S. Code § 6072(b). An LLC reaches Form 1120 only by electing C-corporation treatment on Form 8832.

Form 1120-S: S Corporations

Form 1120-S, U.S. Income Tax Return for an S Corporation, is filed by a corporation (or an LLC) that has elected S-corporation status. The S corporation is a pass-through entity: per the Instructions for Form 1120-S, it reports each owner's share of income, deductions, and credits on Schedule K-1 (Form 1120-S), "Shareholder's Share of Income, Deductions, Credits, etc." Each shareholder then reports that pro rata share on their own Form 1040, so the profit is generally taxed once, at the shareholder level.

To be an S corporation, the entity must meet the eligibility rules of 26 U.S. Code § 1361: no more than 100 shareholders, only allowable shareholders (individuals, estates, and certain trusts, with no nonresident alien shareholders), and only one class of stock. The election is made on Form 2553. A calendar-year S corporation files Form 1120-S by March 15 - the 15th day of the third month under § 6072(b). Owner-employees of an S corporation must also take reasonable wages on a W-2, which is where the S election changes self-employment tax exposure.

Form 1065: Partnerships and Multi-Member LLCs

Form 1065, U.S. Return of Partnership Income, is filed by a partnership or a multi-member LLC that has not elected corporate treatment. Form 1065 is an information return - the entity itself pays no income tax. The Instructions for Form 1065 state that "a partnership doesn't pay tax on its income but passes through any profits or losses to its partners," who "must include partnership items on their tax or information returns." Each partner receives a Schedule K-1 (Form 1065) showing their distributive share.

The pass-through of tax to the partners rather than the entity is the rule of 26 U.S. Code § 701: "A partnership as such shall not be subject to the income tax… persons carrying on business as partners shall be liable for income tax only in their separate or individual capacities." A calendar-year partnership files Form 1065 by March 15, the 15th day of the third month under § 6072(b). By default, a single-member LLC does not use Form 1065 - with only one owner it is a disregarded entity that files Schedule C instead.

Schedule C: Sole Proprietors and Single-Member LLCs

Schedule C (Form 1040), Profit or Loss From Business, is filed by a sole proprietor or a single-member LLC to report business income and expenses on the owner's individual return. The IRS instructs taxpayers to "use Schedule C (Form 1040) to report income or (loss) from a business you operated or a profession you practiced as a sole proprietor." There is no separate business return: the net profit on line 31 flows to the owner's Form 1040 and to Schedule SE.

A single-member LLC that has not elected corporate treatment is a disregarded entity, so its owner files Schedule C exactly as a bare sole proprietor would. Because Schedule C is part of Form 1040, a calendar-year filer's deadline is April 15. For the full mechanics - the five parts, deductible expenses, and the line-31 flow - see our Schedule C guide.

Flow-Through vs. Entity-Level Tax

The single biggest difference among these forms is where the income is taxed. Three of the four returns are pass-through, and one is not. On Form 1065 and Form 1120-S, the entity computes income but reports each owner's share on a Schedule K-1; the owners pay the tax on their individual returns. On Schedule C, there is no entity at all - the owner reports directly. Only Form 1120 produces a tax bill for the business itself under § 11.

Pass-through income is generally subject to income tax once, at the owner level, while a C corporation's profit is taxed at the corporate level and again when distributed. This is the core reason most small businesses operate as pass-throughs and file Form 1120-S, Form 1065, or Schedule C rather than Form 1120.

Filing Deadlines by Form

Deadlines split cleanly by entity type under 26 U.S. Code § 6072. Pass-through entities that issue K-1s file first, so owners have their figures before their own returns are due; individuals and C corporations file a month later.

Corporations and partnerships request more time with Form 7004, an automatic six-month extension; sole proprietors use Form 4868 to extend Form 1040. An extension of time to file is never an extension of time to pay. For the full calendar including estimated dates, see when business taxes are due.

How to Change Which Form You File

You change the form by changing your entity's federal tax classification, not by filing a different return at random. An eligible entity elects corporate treatment on Form 8832, which moves it to Form 1120. To be taxed as an S corporation and file Form 1120-S, an eligible corporation or LLC files Form 2553 under § 1361. Adding a second owner to a single-member LLC generally moves it from Schedule C to Form 1065 by default.

Each change has a timing rule and downstream effects on payroll and self-employment tax, so review LLC vs. S-corp tax before electing, and Form 2553 for the election deadline. Once you have chosen, our guide on how to file business taxes walks the return end to end.

Common Mistakes

• Assuming "LLC" has its own tax form. It does not. An LLC files Schedule C, Form 1065, Form 1120-S, or Form 1120 depending on member count and any election.
• Missing the earlier March 15 deadline. Partnerships and S corporations file a month before the individual deadline under § 6072(b); owners waiting for April 15 file late.
• Treating an extension as more time to pay. Form 7004 and Form 4868 extend filing only; tax owed is still due at the original deadline.
• Filing Form 1120-S without a valid election. Without an accepted Form 2553, the IRS treats the entity as a C corporation and expects Form 1120.
• Forgetting the K-1. Partnerships and S corporations must issue Schedule K-1 to every owner; owners cannot complete their Form 1040 without it.

Related Guides

• Schedule C explained - the sole-proprietor and single-member-LLC return.
• Single-member LLC - why it files Schedule C, not Form 1065.
• Form 2553 explained - the election that moves you to Form 1120-S.
• S-corp vs. LLC - how the two structures compare.
• LLC vs. S-corp tax - the self-employment-tax math behind the choice.
• Self-employment tax - the tax on pass-through business income.
• Quarterly estimated taxes - how owners prepay during the year.
• When are business taxes due? - every deadline by entity.
• How to file business taxes - the step-by-step by entity.
• Business tax hub - all returns, schedules, and deadlines.

Frequently Asked Questions

What tax form does an LLC file?

It depends on member count and any election. A single-member LLC files Schedule C with Form 1040 by default. A multi-member LLC files Form 1065. An LLC electing S-corp status files Form 1120-S, and one electing C-corp status files Form 1120.

Does a partnership pay income tax on Form 1065?

No. Form 1065 is an information return. The partnership passes profit or loss through to its partners on Schedule K-1, and each partner pays income tax on their distributive share, per the IRS Instructions for Form 1065 and 26 U.S. Code § 701.

What is the difference between Form 1120 and Form 1120-S?

Form 1120 is the C corporation return, where the corporation pays income tax at the entity level. Form 1120-S is the S corporation return, a pass-through that reports income to shareholders on Schedule K-1 so tax is paid on their individual returns.

When are Form 1065 and Form 1120-S due?

Both are due by the 15th day of the third month after the end of the tax year - March 15 for a calendar-year filer - under 26 U.S. Code § 6072. Form 1120 for a calendar-year C corporation is due by the 15th day of the fourth month, April 15.

How do I change which business tax form I file?

Change your entity's tax classification. An eligible entity files Form 8832 to elect corporate treatment, or Form 2553 to elect S-corporation status. The election determines whether you file Form 1120, Form 1120-S, Form 1065, or Schedule C.

Do S corporations and C corporations file the same extension form?

Yes. Corporations and partnerships request more time to file with Form 7004, an automatic six-month extension. Sole proprietors extend Form 1040 with Form 4868 instead. An extension of time to file is not an extension of time to pay.

Sources

1. IRS - About Form 1120, U.S. Corporation Income Tax Return (C corporations report income and figure tax).
2. IRS - About Form 1120-S, U.S. Income Tax Return for an S Corporation.
3. IRS - About Form 1065, U.S. Return of Partnership Income.
4. IRS - About Schedule C (Form 1040), Profit or Loss From Business.
5. IRS - Instructions for Form 1120-S (March 15 due date; Schedule K-1 pass-through; Form 7004).
6. IRS - Instructions for Form 1065 (partnership pays no tax; passes profit/loss to partners).
7. IRS - Business Taxes ("the form you use depends on how your business is organized").
8. IRS - About Schedule K-1 (Form 1120-S).
9. IRS - About Schedule K-1 (Form 1065).
10. IRS - About Form 7004 (automatic 6-month extension for business returns).
11. IRS - About Form 8832, Entity Classification Election.
12. IRS - Single-Member Limited Liability Companies (disregarded entity files Schedule C).
13. Cornell LII - 26 U.S. Code § 6072, Time for filing income tax returns (3rd- vs 4th-month deadlines).
14. Cornell LII - 26 U.S. Code § 1361, S corporation defined (100-shareholder and one-class-of-stock limits).
15. Cornell LII - 26 U.S. Code § 701, Partners, not partnership, subject to tax.
16. Cornell LII - 26 U.S. Code § 11, Tax imposed on corporations (21% corporate rate).

LegalGlass provides general information for educational purposes and is not a law firm or a substitute for advice from a licensed attorney or tax professional. This page is information, not advice. Tax rates, forms, thresholds, and deadlines change; verify current requirements with the IRS before acting.