With a federal extension, business taxes are due six months after the original filing deadline: September 15 for calendar-year partnerships and S-corporations, and October 15 for calendar-year C-corporations and sole proprietors. The extension only buys time to file the return. Any tax you owe is still due on the original deadline, and interest starts running the next day.
Extended Filing Deadlines by Business Type
The extended deadline depends on your entity type, because each entity has its own original due date and the extension counts six months forward from there.
- Partnerships (Form 1065) and S-corporations (Form 1120-S): original deadline March 15, extended deadline September 15.
- C-corporations (Form 1120): original deadline April 15, extended deadline October 15.
- Sole proprietors (Schedule C on Form 1040): original deadline April 15, extended deadline October 15.
- Estates and trusts (Form 1041): original deadline April 15, but the automatic extension is 5½ months rather than six, making the extended deadline September 30.
Partnerships and S-corporations file earlier because they are pass-through entities. They don’t pay tax at the entity level; they issue Schedule K-1s to owners who then need that information for their own April 15 returns.
If any of these dates lands on a Saturday, Sunday, or federal holiday, it shifts automatically to the next business day.1Internal Revenue Service. When to File That applies to both the original and the extended deadline, and you don’t need to do anything to trigger the shift.
LLCs
An LLC has no tax classification of its own. It follows the deadline for whatever it has elected to be taxed as. A single-member LLC treated as a disregarded entity uses the sole proprietor calendar (April 15 original, October 15 extended). A multi-member LLC taxed as a partnership uses the partnership calendar (March 15, then September 15). An LLC that has elected S-corp or C-corp status follows the matching corporate dates.
Fiscal Year Businesses
The dates above assume a tax year ending December 31. If you use a fiscal year, your original filing deadline is generally the 15th day of the third month after year-end (partnerships and S-corporations) or the fourth month (C-corporations and sole proprietors), and the six-month extension counts forward from that date.2Internal Revenue Service. Instructions for Form 7004 (12/2025) A C-corporation with a fiscal year ending June 30, for example, has an original deadline of October 15 and an extended deadline of April 15 of the following year.
What You Still Owe on the Original Deadline
An extension is a filing extension, not a payment extension.3Internal Revenue Service. Get an Extension to File Your Tax Return Whatever tax you owe for the year is due on the original date: March 15 for partnerships and S-corporations, April 15 for C-corporations and sole proprietors. If you underpay, interest and penalties begin the day after.
Interest on Any Unpaid Balance
Interest runs from the day after the original deadline until you pay in full. The IRS compounds it daily and adjusts the rate each quarter. For the first quarter of 2026, the underpayment rate is 7 percent for most taxpayers. C-corporations with underpayments above $100,000 pay 9 percent.4Internal Revenue Service. Quarterly Interest Rates Interest cannot be waived, even in cases where penalties are.
The 90 Percent Rule for Corporations
A corporation can avoid the failure-to-pay penalty (but not the interest) by paying at least 90 percent of its total tax liability by the original due date and paying the rest by the extended deadline.5Internal Revenue Service. Instructions for Form 7004 (Rev. December 2025) That makes the tax estimate on Form 7004 more than a formality. If your estimate is too low and your payment falls short of 90 percent of the true liability, the penalty protection disappears.
Failure-to-Pay Penalty
If you don’t hit the 90 percent threshold, the failure-to-pay penalty is 0.5 percent of the unpaid amount for each month or partial month the balance stays outstanding, up to 25 percent.6Office of the Law Revision Counsel. 26 U.S. Code 6651 – Failure to File Tax Return or to Pay Tax It runs on top of interest.
Penalties for Missing the Extended Deadline
Blowing through both the original deadline and the extended one is far more expensive than paying late. The failure-to-file penalty is 5 percent of the unpaid tax for each month the return is late, capped at 25 percent.7Internal Revenue Service. Failure to File Penalty That is ten times the failure-to-pay rate, which is why filing the extension is worth doing even when you cannot pay the balance.
When both penalties apply in the same month, the failure-to-file penalty is reduced by the failure-to-pay penalty for that month. The combined charge for the first five months works out to 5 percent per month: 4.5 percent failure-to-file plus 0.5 percent failure-to-pay. After five months, the failure-to-file penalty maxes out at 25 percent, but the failure-to-pay penalty keeps accruing until the tax is paid or hits its own 25 percent cap.7Internal Revenue Service. Failure to File Penalty
If the return is more than 60 days late, a minimum penalty kicks in. For returns due after December 31, 2025, that minimum is $525 or 100 percent of the unpaid tax, whichever is less.7Internal Revenue Service. Failure to File Penalty
How to File for the Extension
The form depends on how your business reports income.
- Corporations, partnerships, and trusts file Form 7004, Application for Automatic Extension of Time to File Certain Business Income Tax, Information, and Other Returns.8Internal Revenue Service. About Form 7004
- Sole proprietors file Form 4868, which extends the entire Form 1040 (including Schedule C business income).9Internal Revenue Service. About Form 4868
Both forms require your EIN (or Social Security number for sole proprietors), the tax year ending date, and a good-faith estimate of your total tax liability.5Internal Revenue Service. Instructions for Form 7004 (Rev. December 2025) Work from your year-end profit and loss statements and prior returns. The number need not be perfect, but a large intentional underestimate can invalidate the extension.
The IRS Modernized e-File system produces an electronic acknowledgment when the request is accepted.10Internal Revenue Service. Modernized e-File (MeF) Program Overview Payments submitted with the request can go through the Electronic Federal Tax Payment System, IRS Direct Pay, or a debit or credit card.11Internal Revenue Service. Topic No. 304, Extensions of Time to File Your Tax Return If you mail the form instead, send it certified with a return receipt and keep the receipt in your tax file as proof of a timely filing.
Do not expect a confirmation letter. The IRS contacts you only if the extension is denied.2Internal Revenue Service. Instructions for Form 7004 (12/2025) A properly completed, timely filed form is automatically granted; your electronic acknowledgment or certified mail receipt is the proof you keep.
State Deadlines Are Separate
A federal extension does not automatically extend your state filing deadline everywhere. Most states accept a copy of the federal form in place of a state extension when no additional state tax is owed, but rules vary. Some require their own extension form. Others grant an automatic extension only if the full estimated state tax has been paid by the original date. Check with your state tax agency, because a state deadline can trigger its own penalties even when the federal extension is in place.