Check the year. The figures here are for tax year 2025. Limits and thresholds change, so confirm the current ones before you act.
If you filed Form 4868 by April 15, your 2025 federal return is due Thursday, October 15, 2026. That is the only thing the extension changed. Tax owed for 2025 was still due April 15, interest has been counting on any unpaid amount since then, and the failure-to-pay penalty may be running too.
So the question for the next two weeks is narrow: get the return filed by the 15th, and stop the meter on whatever you owe. Everything below is about doing those two things well.
What the extension did and did not do
The regulation behind Form 4868 is blunt. It grants an automatic six-month extension to file, and it says the extension "will not extend the time for payment of any tax due." The statute lets the IRS grant extensions of no more than six months, with an exception for taxpayers who are abroad. For a calendar-year 2025 return that lands on October 15, 2026.
The form also asked you to show the full amount you properly estimated as tax. The estimate matters for one thing, covered below: the 90% test.
Extended, or not
| Item | Date or status | Does the extension change it? |
|---|---|---|
| Filing the 2025 return | October 15, 2026 | Yes, six months later than April 15 |
| Paying the 2025 tax | April 15, 2026 | No |
| Interest on unpaid tax | Runs from April 15, 2026 | No |
| Failure-to-pay penalty | 0.5% per month from April 16, unless the 90% test is met | No, but the 90% test can excuse it |
| Failure-to-file penalty | 5% per month, only if you miss October 15 | Starts later, after the extended date |
| 2026 estimated tax, fourth payment | January 15, 2027 | No, it is a separate tax year |
The 90% test, and why it is already decided
The failure-to-pay penalty is 0.5% of the unpaid tax for each month or part of a month, up to 25% in total. The regulations carve out a presumption of reasonable cause for people on an automatic extension: no failure-to-pay penalty for the extension period if the tax still unpaid on April 15 is no more than 10% of the tax shown on the return, and you send the balance with the return. Payments that count toward the 90% are withholding, estimated tax payments and anything you paid with the extension.
Here is the part that matters now: that test was measured on April 15. You cannot go back and pass it by paying today. What you can still decide is how much more interest and penalty accrue. If you already met the test, pay the balance with the return on or before October 15 and the penalty never starts. If you did not, the penalty has been running on the unpaid amount since April 16, and each month you wait adds another half percent.
A worked example
Example, hypothetical: a couple's 2025 tax comes to $40,000. They extended, and by April 15 they had $37,000 in through withholding and estimates, which is 92.5%. The $3,000 balance is within the 10% cushion. If they file and pay it by October 15, there is no failure-to-pay penalty, only interest.
Change one input. Suppose only $30,000 was in by April 15, which is 75%. The unpaid $10,000 fails the test. Paying it on October 15 means six months of penalty, April 16 through October 15, at 0.5% each: $300. Now suppose they also miss October 15 and file on November 20. That adds a failure-to-file penalty of 5% for each of two months, $1,000, reduced by the $100 of failure-to-pay penalty for those same two months. The failure-to-pay penalty has by then run eight months, $400. Total penalty: $1,300 on the same $10,000.
Interest sits on top of all of this. The rate is the federal short-term rate plus 3 percentage points, reset each quarter, so I have not put a current rate in this article. The chart assumes 7% for illustration, compounded daily.
- Penalties
- Interest at an assumed 7%
| Penalties | Interest at an assumed 7% | |
|---|---|---|
| Paid 92.5% by April 15, balance with return Oct 15 | $0 | $107 |
| Paid 75% by April 15, balance with return Oct 15 | $300 | $357 |
| Paid 75% by April 15, filed and paid Nov 20 | $1,300 | $429 |
Filing late costs far more than paying late: the third bar is the second path plus a missed October 15.
Computed by script from 26 U.S.C. 6651 and 6601 mechanics; the 7% interest rate is an assumption, not the IRS rate.
How interest works
Interest is charged on tax not paid by the original due date, from that date to the day it is paid, and the law says to disregard extensions of time in finding that date. Interest compounds daily. It also applies to the failure-to-file penalty, running from the due date of the return including extensions.
Penalties carry a reasonable-cause exception in the statute. Interest simply follows the tax, so the practical lever is paying sooner. If a notice arrives, my page on IRS problem resolution covers the next steps. A payment stops interest on that amount from the day it is made, and a payment made before a month begins shrinks the base for that month's penalty.
- 10 daysApr 15: payment due, interest starts
- 2183 daysOct 15: extended filing deadline
- 3244 daysDec 15: more than 60 days late, minimum penalty applies
- 4275 daysJan 15, 2027: 2026 fourth estimate
The next two dates that matter for the 2025 return, and the point where the minimum late-filing penalty kicks in.
If you miss October 15 anyway
The failure-to-file penalty is 5% of the unpaid tax for each month or part of a month, up to 25%. In any month where both penalties apply, the filing penalty is reduced by the payment penalty, so the combined rate is 5% a month, not 5.5%. If the return is more than 60 days late, counting from the extended due date, there is a minimum: for 2025 returns Pub. 17 gives it as the smaller of $525 or 100% of the unpaid tax.
Notice what both penalties are based on: the tax left unpaid, not the whole liability. Money already paid in reduces the base, which is why paying early still helps even when the return itself is not ready.
Common mistakes
- Treating October 15 as a payment deadline and waiting to see the number, when interest on any balance has been running since April.
- Assuming a complete return is needed before paying. A payment can go in now on an estimate, and a good estimate stops interest on that amount.
- Missing the filing date because the money is not there. Failing to file costs ten times as much per month as failing to pay.
- Mixing up the 2025 balance with 2026 estimated tax. The fourth 2026 payment is due January 15, 2027, and an extension of the 2025 return does not touch it.
- Waiting on a late K-1 or brokerage statement without a backup plan. If a document is still missing, tell your preparer now, not on October 14.
- Assuming a second extension exists. An extension beyond six months is generally not granted, and the exceptions are mostly for people outside the country.
What to do before October 15
This week
Pull every document the return needs: W-2s, 1099s, K-1s, brokerage statements, mortgage interest and property tax statements. List anything still missing.
This week
Total what was paid in by April 15 and compare it to the tax. This tells you whether the 90% test was met and how large the unpaid balance is.
By about October 8
Make a payment toward the balance even if the return is not final. Use an estimate with some cushion, since an overpayment comes back as a refund.
By October 15
File the return. E-filed returns are timely if the transmitter's electronic postmark, in your time zone, is on or before the deadline. Paper returns need a postmark by the deadline.
If you cannot pay in full
File anyway and pay what you can. Then ask about a payment plan, which cuts the monthly penalty rate to 0.25% for returns filed by the extended date. If you want help getting the return finished, see return preparation.
