Small Business Tax Filing: A Step-by-Step Owner’s Guide

In the US, the federal income tax return a business files depends on how the IRS treats it: a sole proprietor reports business income on Schedule C with Form 1040, a partnership files Form 1065, a C corporation files Form 1120, and an S corporation files Form 1120-S. Each return has its own due date, and an extension gives more time to file, not to pay. This page quotes the IRS for each step and gives one worked example for 2025 returns filed in 2026.

Billed publishes this page. It gives general information about US federal tax filing, not tax advice. The rules are quoted from IRS pages on business structures, the IRS instructions for 2025 Forms 1065, 1120, 1120-S and Schedule C, Publication 509 (2026) and Publication 334 (2025), all opened on October 30, 2026; each source's date is given where it's quoted. State filing rules vary by state. Ask your accountant which returns apply to you.

1. Which return each business type files (US)

• Sole proprietor: the IRS's sole proprietorships page (last reviewed or updated June 28, 2026) lists "1040, U.S. Individual Income Tax Return" and "Schedule C (Form 1040 or 1040-SR), Profit or Loss from Business" for income tax. The 2025 Instructions for Schedule C say: "Use Schedule C (Form 1040) to report income or (loss) from a business you operated or a profession you practiced as a sole proprietor."
• Partnership: the IRS's partnerships page (June 8, 2026) says: "A partnership must file an annual information return to report the income, deductions, gains, losses, etc., from its operations, but it does not pay income tax." The return is "Form 1065, U.S. Return of Partnership Income".
• C corporation: the IRS's corporations page (June 17, 2026) lists "1120, U.S. Corporation Income Tax Return" for income tax.
• S corporation: the IRS's S corporations page (June 11, 2026) says: "In order to become an S corporation, the corporation must submit Form 2553, Election by a Small Business Corporation signed by all the shareholders." It lists Form 1120-S for income tax and says: "Shareholders of S corporations report the flow-through of income and losses on their personal tax returns and are assessed tax at their individual income tax rates."
• LLC: the IRS's LLC page (October 10, 2026) says: "Depending on elections made by the LLC and the number of members, the IRS will treat an LLC as either a corporation, partnership, or as part of the LLC's owner's tax return", which it calls a "disregarded entity".

2. When each return is due (US, 2025 returns)

• Form 1040 (with Schedule C): IRS Publication 509 (2026) says: "This form is due on the 15th day of the 4th month after the end of your tax year."
• Form 1065: the 2025 Instructions for Form 1065 say: "Generally, a domestic partnership must file Form 1065 by the 15th day of the 3rd month following the date its tax year ended as shown at the top of Form 1065." They add: "Calendar-year partnerships may therefore timely file their returns for the 2025 partnership year by March 16, 2026."
• Form 1120: the 2025 Instructions for Form 1120 say: "Generally, a corporation must file its income tax return by the 15th day of the 4th month after the end of its tax year." They give a different rule for a fiscal year ending June 30.
• Form 1120-S: the 2025 Instructions for Form 1120-S say: "For calendar year corporations, the due date is March 16, 2026 (March 15th falls on Sunday)."

3. Extensions: more time to file, not to pay (US)

• Individuals: IRS Publication 334 (2025) says Form 4868 is used "to request an automatic 6-month extension. For calendar-year taxpayers, this will extend the tax filing due date until October 15." It adds: "Filing an extension does not extend the time to pay your taxes, only the time to file the tax return."
• Businesses: the IRS's Form 7004 page (October 16, 2026) says: "Use Form 7004 to request an automatic 6-month extension of time to file certain business income tax, information, and other returns."
• Interest: the 2025 Instructions for Form 1120 say: "Interest is charged on taxes paid late even if an extension of time to file is granted."

4. Other filings, covered on other pages

• Self-employment tax: what is self-employment tax.
• Quarterly estimated tax: what is quarterly estimated tax.
• Forms 1099: what is a 1099 form.
• Deductions: guide to business tax deductions.
• Freelancers: freelance taxes.

5. Records to have ready

Your return draws on a full year of income and expense records; a profit and loss statement summarizes them. On how long to keep them, the IRS's How long should I keep records page (June 30, 2026) says: "The length of time you should keep a document depends on the action, expense, or event which the document records."

Worked example (sample figures, tax year 2025)

A sample business, invented for this example, is a domestic LLC with two members, a calendar tax year, and no Form 8832 election.

1. How it's treated: the IRS's LLC page says "a domestic LLC with at least two members is classified as a partnership for federal income tax purposes unless it files Form 8832 and affirmatively elects to be treated as a corporation." So the LLC files Form 1065.
2. Form 1065 due date: the 15th day of the 3rd month after December 31, 2025 is March 15, 2026, a Sunday. The next business day is Monday, March 16, 2026, the date the 2025 Instructions for Form 1065 give.
3. Each member's Form 1040: the 15th day of the 4th month after December 31, 2025 is Wednesday, April 15, 2026.
4. If a member files Form 4868: the filing date moves to Thursday, October 15, 2026, but any tax owed is still due by April 15, 2026.

The sample doesn't work out any tax. Ask your accountant which returns and dates apply to your business.

Where Billed fits

Billed's financial reporting page says: "All reports in Billed can be exported for use with your accountant, bookkeeper, or tax software." Its expense tracking page says: "Give every expense a category so your spending is already sorted when you file taxes or hand the year to your accountant."

When this guide isn't for you

This page covers federal income tax returns for small US businesses. If you have employees, sell in more than one state, have a fiscal tax year, or are changing how your business is classified, there are more forms and rules. State returns and due dates vary by state. Ask your accountant.

Related Articles

• Guide to Business Tax Deductions
• What Is Quarterly Estimated Tax?
• What Is Self-Employment Tax?
• Freelance Taxes: U.S. Essentials
• What Is a Profit and Loss Statement?

Frequently Asked Questions

Which tax return does a sole proprietor file?

The IRS lists Form 1040, U.S. Individual Income Tax Return, with Schedule C (Form 1040 or 1040-SR), Profit or Loss from Business, for a sole proprietor's income tax.

When is a 2025 partnership return due?

The 2025 Instructions for Form 1065 say: "Calendar-year partnerships may therefore timely file their returns for the 2025 partnership year by March 16, 2026."

When is a 2025 S corporation return due?

The 2025 Instructions for Form 1120-S say: "For calendar year corporations, the due date is March 16, 2026 (March 15th falls on Sunday)."

Does an extension give more time to pay?

No. IRS Publication 334 says "Filing an extension does not extend the time to pay your taxes, only the time to file the tax return."

How does the IRS treat an LLC?

The IRS says: "Depending on elections made by the LLC and the number of members, the IRS will treat an LLC as either a corporation, partnership, or as part of the LLC's owner's tax return (a 'disregarded entity')." Ask your accountant how your LLC is classified.