Suggested: desk calendar open to mid-October beside a tax return, calculator and laptop.
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Tax Deadline
October 15: An Extension to File Did Not Extend the Time to Pay
Final reminder for extended individual and C-corporation filers: balance due, interest accrual, and state deadline variation
October 15 is the last day to file extended 2025 returns, but payment was due in April. Unpaid tax has accrued interest at 7% a year plus monthly penalties. Pay and file now.
If you filed Form 4868 (individuals) or Form 7004 (C corporations) this spring, you bought more time to file your 2025 return. You did not buy more time to pay. The tax was due on the original April 15, 2026 date, and interest and penalties have been running on any unpaid balance ever since.
October 15, 2026 falls on a Thursday. With little time left, this is the last practical moment to limit what an underpayment costs. Below, we cover what the extension did and did not do, what an unpaid balance costs today, how state rules differ, and what to do before the deadline.
1. Who Is Affected?
This reminder applies to individuals and C corporations that requested an automatic extension:
| Filer | Extension form | Key dates |
|---|---|---|
| Individuals (Form 1040) | Form 4868 | Tax due April 15, 2026; extended return due October 15, 2026 |
| Calendar-year C corporations (Form 1120) | Form 7004 | Tax due April 15, 2026; extended return due October 15, 2026 |
| Fiscal-year C corporations | Form 7004 | Extension is generally six months from the original due date; June 30 year-ends may receive seven |
Taxpayers in federally declared disaster areas, in combat zones, or living outside the United States may have different dates. Confirm yours before assuming October 15 applies.
2. What the Extension Did, and Did Not, Do
Both forms give filing relief only. The IRS states on Form 4868 that it “doesn’t extend the time to pay taxes,” and the Form 7004 instructions say it “does not extend the time to pay tax.”
- What it extended: the date to file the return, generally six months, which is October 15 for calendar-year filers.
- What it did not extend: the date the tax was due, which remained April 15, 2026.
- What it required: a reasonable estimate of the tax owed, with payment of that estimate by April 15.
A timely extension protects you from the late-filing penalty if you file by October 15. It does nothing for the late-payment penalty or interest, which run from April 15 on any unpaid balance.
3. What an Unpaid Balance Costs
Three charges can apply to tax that was not paid by April 15:
- Interest. The IRS rate is 7% per year from October 1 to December 31, 2026, compounded daily for individuals (IRS Release IR-2026-98). Interest runs from the original due date until the tax is paid. Large corporate underpayments carry a higher 9% rate.
- Late-payment penalty. One-half of 1% of the unpaid tax for each month or part of a month, capped at 25%.
- C corporations. The Form 7004 instructions apply the same half-percent monthly penalty, capped at 25%, and charge interest from the regular due date until the tax is paid.
- Late-filing penalty. 5% of the unpaid tax for each month or part of a month a return is late, capped at 25%. A minimum penalty applies to returns more than 60 days late ($525 on Form 4868 for 2025 returns).
The filing penalty is ten times the monthly payment penalty. That is why filing on time still matters even if you cannot pay in full.
A simple example
Assume a $20,000 balance unpaid since April 15 and paid in full on October 15, 2026 (183 days), and assume for illustration that the 7% rate applied throughout:
- Interest: about $714 at 7% compounded daily.
- Late-payment penalty: six months at 0.5% is 3%, or $600.
- Total cost of waiting: about $1,314, or roughly 6.6% on top of the tax itself.
Under the Form 4868 rules, individuals avoid the late-payment penalty if at least 90% of the 2025 tax was paid by the April deadline through withholding, estimated tax, or extension payments, and the rest is paid when the return is filed. Interest still applies to the unpaid portion.
4. State Deadlines Do Not Always Follow the Federal Calendar
A federal extension does not always cover your state return, and state rules differ on forms, payment, and dates:
- Payment is usually still due in April. Most states treat an extension as filing-only and expect payment by mid-April. Iowa is reported to require payment by April 30.
- Some states need their own form. New York (Form IT-370) and Pennsylvania (Form REV-276) require a state extension request. North Dakota, by contrast, recognizes a federal extension automatically.
- A few extended due dates are not October 15. Reported dates include October 20 for Hawaii, October 31 for Iowa, and November 15 for Louisiana.
- Multistate filers need a state-by-state check. An individual with income in several states, or a corporation filing in several, may face a different rule and date in each.
State guidance changes from year to year and the dates above come from secondary guides. Confirm each state’s extended due date and payment rule with its revenue department before relying on them.
5. Action Plan Before October 15
This week
- Compute the actual balance. Use the near-final return rather than the April estimate, and subtract all payments already made.
- Pay electronically. Individuals can use IRS Direct Pay or EFTPS. Most C corporations must pay through EFTPS, per the Form 7004 instructions.
- File by October 15. Filing on time stops the 5% monthly late-filing penalty from starting.
- Handle the state return separately. Check the state’s form, due date, and payment rule.
- Keep confirmations. Save payment confirmation numbers and the filing acknowledgment.
If you cannot pay in full
File on time anyway and pay as much as you can now. The late-filing penalty is the larger charge, and paying down the balance reduces the base on which interest and the payment penalty accrue. The IRS also offers payment plan options for the remainder.
Bottom Line
An extension moved the filing deadline to October 15, but it never moved the payment deadline. Every day of unpaid tax since April 15 has added interest, and every month has added a penalty.
Pay what you owe and file by October 15. Then confirm state deadlines separately so a second balance does not follow the first.
How PNJ Can Help
PNJ’s tax team helps individuals, owners, and finance teams estimate the balance still owed, confirm what has already been paid, prepare and e-file extended returns, and review state extension rules. We also help set up quarterly estimates so next April does not bring the same surprise.
Talk with PNJ’s tax team before October 15 to confirm your balance due and filing status.
Sources and Professional Notes
- Internal Revenue Service, Form 4868 (2025), Application for Automatic Extension of Time To File U.S. Individual Income Tax Return, revised October 2025.
- Internal Revenue Service, Instructions for Form 7004 (12/2025), Application for Automatic Extension of Time To File Certain Business Income Tax, Information, and Other Returns, last reviewed April 30, 2026.
- Internal Revenue Service, Interest rates remain the same for the fourth quarter of 2026, IR-2026-98, August 21, 2026.
- Internal Revenue Service, Get an extension to file your tax return, IRS.gov, page last reviewed September 20, 2026.
- Intuit, State extension and due date information for individual returns, accountants.intuit.com (state forms and filing-only rules).
- FileTax, State Tax Extension Guide, filetax.com, September 24, 2026 (reported state extended due dates).
Disclaimer
This material is for general informational purposes only and does not constitute accounting, tax, or legal advice. Interest rates, penalties, and filing deadlines depend on an individual’s or entity’s facts, tax year, location, and filing history, and state rules vary. This article is a general summary and is not a substitute for reviewing the IRS and state guidance directly or obtaining advice based on specific circumstances. Consult qualified advisers before making payment, filing, or election decisions.