To file a company tax return, you match your entity type to the right IRS form, gather the financial records that back every line, and submit by the deadline that applies to your structure. A C-corporation files Form 1120. An S-corporation files Form 1120-S. A multi-member LLC that hasn’t elected corporate treatment files Form 1065 as a partnership. A single-member LLC that hasn’t elected corporate status doesn’t file a separate business return at all — its income goes on Schedule C of the owner’s Form 1040.1Internal Revenue Service. Single Member Limited Liability Companies
Pick the Right Form for Your Entity
Federal tax classification, not the name on your door, decides which return you file.
- Form 1120 is for C-corporations. Every domestic corporation that hasn’t elected S status has to file it, whether it had taxable income that year or not, unless it qualifies for exemption under Section 501. C-corporations pay a flat 21% federal income tax on taxable income.2Internal Revenue Service. Instructions for Form 1120 (2025)3Office of the Law Revision Counsel. 26 U.S. Code 11 – Tax Imposed
- Form 1120-S is for corporations that elected pass-through status under Subchapter S. The entity itself doesn’t pay income tax; profits and losses flow through to shareholders, who report them on their personal returns.4Internal Revenue Service. About Form 1120-S, U.S. Income Tax Return for an S Corporation
- Form 1065 is for partnerships and for multi-member LLCs that haven’t filed Form 8832 to elect corporate treatment. Like an S-corp, the partnership itself doesn’t pay federal income tax, but it still has to file the information return.5Internal Revenue Service. Instructions for Form 1065 (2025)
If you’re the sole owner of an LLC and never elected corporate status, the IRS treats the business as a disregarded entity. You file Schedule C with your personal Form 1040 instead of a separate business return.
Gather Your Records First
Have your Employer Identification Number ready — the nine-digit number the IRS assigned when you registered the business.6Internal Revenue Service. Get an Employer Identification Number Then pull your income statement and balance sheet for the tax year. These give you the gross receipts, cost of goods sold, and net income figures the return asks for. If you sell physical products, your beginning-of-year and end-of-year inventory totals need to be accurate, because cost of goods sold offsets gross receipts to arrive at gross profit.
Deductible expenses lower taxable income. Common categories are employee compensation, rent, business interest, insurance, advertising, professional fees, repairs, and state and local taxes the corporation paid. Keep the paperwork behind each category — leases, payroll records, insurance policies, vendor invoices — organized so you can produce it if the IRS examines your return.
Credits are different from deductions: they cut your tax bill dollar for dollar rather than just reducing taxable income. The research and development credit under Section 41 is one of the more widely claimed, but there are credits for energy investments, hiring from targeted groups, and employee benefits. Track them separately so they land on the right lines.
Always pull the current year’s form and instructions from the IRS website. The instructions walk through the return line by line and reflect recent code changes.7Internal Revenue Service. About Form 1120, U.S. Corporation Income Tax Return
Know Your Filing Deadline
Deadlines depend on entity type and tax year, and this is where filers get caught: S-corps and partnerships file a full month before C-corps.
- Form 1120 (C-corporations): the 15th day of the fourth month after the tax year ends. For a calendar-year corporation, that’s April 15.8Internal Revenue Service. Instructions for Form 1120
- Form 1120-S (S-corporations): the 15th day of the third month. For a calendar-year S-corp filing for 2025, the deadline is March 16, 2026, because March 15 falls on a Sunday.9Internal Revenue Service. Instructions for Form 1120-S (2025)
- Form 1065 (partnerships): also the 15th day of the third month. Calendar-year partnerships face the same March 16, 2026 deadline for tax year 2025.5Internal Revenue Service. Instructions for Form 1065 (2025)
When the due date falls on a Saturday, Sunday, or legal holiday, the deadline moves to the next business day. Fiscal-year filers count the same number of months from their year-end.
Extensions
If you need more time, file Form 7004 by the original due date for an automatic six-month extension.10Internal Revenue Service. About Form 7004, Application for Automatic Extension of Time to File Certain Business Income Tax, Information, and Other Returns The form asks for your company’s name, address, EIN, tax year ending date, and the type of return being extended.
An extension buys time to file, not time to pay. Any tax you expect to owe is still due on the original deadline, and you should estimate the liability and send payment with the extension to avoid interest.11Internal Revenue Service. Instructions for Form 7004 Keep the filed extension and payment confirmation in case the IRS later disputes your filing date.
Submit the Return and Pay the Tax
Most corporations file electronically through IRS-authorized e-file software. Certain large and mid-size corporations must e-file Forms 1120 and 1120-S; others can do so voluntarily.12Internal Revenue Service. E-File for Business and Self Employed Taxpayers E-filing gives you an immediate confirmation of receipt. Paper filing is still accepted; if you go that route, use certified mail to the processing center for your location so you have proof of the filing date.
Tax payments run through the Electronic Federal Tax Payment System (EFTPS), a free IRS tool that pulls directly from a business bank account.13Internal Revenue Service. Payments Enrollment takes time, so don’t leave it until the deadline. Save every payment confirmation number in case a payment isn’t credited correctly.
Electronic returns generally process within a few weeks. Paper returns can take several months. If the IRS finds a discrepancy, it will send a notice with the issue and any additional amount owed.
Pay Estimated Tax During the Year
Corporations that expect to owe $500 or more for the year have to make quarterly estimated tax payments rather than wait for the return.14Internal Revenue Service. Estimated Taxes For a calendar-year corporation, installments are due April 15, June 15, September 15, and December 15. Fiscal-year filers pay on the 15th day of their 4th, 6th, 9th, and 12th months.
Underpayment triggers an addition to tax calculated using the IRS underpayment interest rate, which for early 2026 sits at 7% for most corporations and 9% for large corporate underpayments.15Internal Revenue Service. Quarterly Interest Rates The charge runs from the date each installment was due until it’s paid or the return due date, whichever comes first.16Office of the Law Revision Counsel. 26 U.S. Code 6655 – Failure by Corporation to Pay Estimated Income Tax Paying in full with the return doesn’t erase it.
Deliver K-1s if You’re an S-Corp or Partnership
An S-corporation has to give each shareholder a Schedule K-1 reporting that shareholder’s share of income, losses, deductions, and credits. The K-1 is due on the same date as Form 1120-S — for calendar-year filers, March 16, 2026 for tax year 2025.9Internal Revenue Service. Instructions for Form 1120-S (2025) Shareholders need it to prepare their own returns, since the K-1 figures flow onto their Form 1040. It breaks out ordinary business income, rental income, interest, dividends, capital gains, and items the shareholder reports separately.17Internal Revenue Service. Shareholder’s Instructions for Schedule K-1 (Form 1120-S) (2025)
Partnerships filing Form 1065 have the same K-1 obligation to their partners. If you miss it, your owners can’t file on time.
What Late Filing and Late Payment Cost
The IRS imposes two separate penalties, and they can run at the same time.
C-corporation failure to file. For Form 1120, the penalty is 5% of the unpaid tax per month or partial month, capped at 25%.18Office of the Law Revision Counsel. 26 U.S. Code 6651 – Failure to File Tax Return or to Pay Tax If the return is more than 60 days late, the minimum penalty is the lesser of $525 or 100% of the unpaid tax for returns due after December 31, 2025.19Internal Revenue Service. Failure to File Penalty
S-corporation and partnership failure to file. The base penalty is $255 per shareholder or partner per month (or partial month) for returns due after December 31, 2025, up to 12 months.19Internal Revenue Service. Failure to File Penalty The bill scales with the number of owners, so a mid-sized S-corp that misses by a few months can face several thousand dollars in penalties.
Failure to pay. If you file on time but don’t pay what you owe, the IRS charges 0.5% of the unpaid tax per month, up to 25%.20Internal Revenue Service. Topic No. 653, IRS Notices and Bills, Penalties and Interest Interest also accrues on any unpaid balance from the due date. When both penalties apply in the same month, the IRS reduces the failure-to-file penalty by the failure-to-pay amount.
Fixing a Return After You’ve Filed
If you find an error later — a missed deduction, a misreported income figure, a credit you forgot — file Form 1120-X to correct a corporate return.21Internal Revenue Service. About Form 1120-X, Amended U.S. Corporation Income Tax Return You generally have three years from the date you filed the original return, or two years from the date you paid the tax, whichever is later. For claims based on bad debts or worthless securities, the window is seven years.22Internal Revenue Service. Instructions for Form 1120-X (Rev. December 2025)
Don’t sit on a known error. If you underpaid, interest keeps accruing. If you overpaid, the money stays with the IRS until you amend.
How Long to Keep Your Records
The IRS can generally audit returns filed within the last three years. Most audits target returns filed within the last two, but a substantial error can push the lookback to six years.23Internal Revenue Service. IRS Audits Match your retention to the risk:
- Three years for most income, deduction, and credit records.
- Six years if you failed to report income exceeding 25% of the gross income shown on the return.
- Seven years if you claimed a deduction for bad debts or worthless securities.
- Four years for employment tax records, measured from the date the tax is due or paid, whichever is later.
- Indefinitely if you never filed a return or filed a fraudulent one.
Records tied to property — purchase price, improvements, depreciation schedules — should be kept until the statute of limitations expires for the year you dispose of the asset.24Internal Revenue Service. How Long Should I Keep Records In practice, holding everything for at least seven years gives most companies a comfortable margin.