Missed the Tax Extension Deadline? The Penalty Just Went Up Tenfold.

A consultant filed an extension in April, paid $60,000 against a 2025 return she already knew would land near $78,000, and told herself she would finish it over the summer. It is September now, the return is still in a folder, and October 15 is four weeks out. Everyone who has missed the tax extension deadline believed what she believes right now: that a few more weeks cannot cost much. The few weeks are the expensive part. The penalty for filing late is ten times the penalty for paying late.

Form 4868 buys six months of filing time under Treas. Reg. §1.6081-4 and nothing else. The money was due April 15, 2026, so the failure-to-pay penalty of §6651(a)(2) and interest under §6601 have been running since April 16 on whatever the April payments did not cover. That part is already sunk. October 15 is where the second penalty starts.

The failure-to-file penalty of §6651(a)(1) is 5% of the unpaid tax for each month or part of a month the return is late, to a maximum of 25%. It is ten times the penalty already running. When both apply in the same month, §6651(c)(1) reduces the failure-to-file piece by the failure-to-pay piece, so the ceiling is 5% a month rather than 5.5%. Five months of lateness reaches the 25% cap, and it stops growing there even if the return never comes.

One thing that does not happen: the extension does not evaporate. The penalty period runs from October 15 rather than back to April 15, because §6651(a)(1) measures lateness from the due date determined with regard to any extension. That holds as long as the extension was valid, and validity has one condition. Treas. Reg. §1.6081-4(a)(3) requires Form 4868 to show the full amount properly estimated as tax for the year, and in Rev. Rul. 79-113 the IRS voided an extension reporting zero tax when the taxpayer had ample evidence of a liability. An estimate typed in as a placeholder is the one way to lose six months of protection you already had.

The 5% is not prorated. File on November 20 and the penalty period covers October 16 through November 15 as the first month and November 16 through 20 as the second, so a 36-day delay is charged as two months. Getting the return in on November 14 instead of November 16 is worth 5% of the balance. Not many deadlines move that much money in two days.

There is also a floor underneath the percentage. When a return arrives more than 60 days after its due date, the addition to tax under §6651(a) is at least the lesser of $525 or 100% of the tax required to be shown on the return. Rev. Proc. 2024-40 set that $525 for returns required to be filed in 2026. A return filed in January with a $400 balance produces a $400 penalty instead of a proportionate one.

In IR-2026-83, issued July 8, 2026, the IRS replaced First Time Abate with Automatic Exemption from Penalty, and the new mechanics are better. Rather than abating a penalty after it is assessed and after you call to ask, the IRS checks your compliance history while the return is processing, declines to assess the penalty at all, and sends a notice explaining why. It covers the failure-to-file, failure-to-pay, and failure-to-deposit penalties, applies to original returns beginning with tax year 2025, and fully replaces First Time Abate for returns with original due dates on or after January 1, 2027.

The test is a clean three years, meaning timely filing and timely payment for the three prior tax years. So a first-time late filer with a good record may well get the failure-to-file penalty waived without asking. That is still not a reason to let October 15 pass. Automatic Exemption from Penalty does not touch the tax or the interest, and the eligibility test is what makes it expensive to spend: file late in 2026 and the three prior years are no longer clean for 2027, 2028, or 2029, which is the stretch where a real problem is likely to turn up. Anyone who fails the test argues reasonable cause under §6651(a) instead, a facts case made on Form 843, the same claim form the COVID-era penalty refunds run through.

Both penalties are percentages of the unpaid balance, so what you have in the bank on October 15 does not change what filing costs. Filing is free. Someone who overpaid in April and files in December owes no penalty at all. The person who guessed low and waits is paying 5% a month on a return that would have cost nothing to send.

One rule punishes doing this in the wrong order. §6651(h) cuts the failure-to-pay penalty from 0.5% to 0.25% a month for any month an installment agreement under §6159 is in effect, but only for an individual who filed the return by the due date including extensions. Blow past October 15 and that discount is gone for the year, no matter what plan you set up in March. File first, then arrange the money. A short-term payment plan runs up to 180 days with no setup fee, and a long-term agreement costs $22 online with direct debit. The CP14 notice post has the full fee table.

Interest is the charge nobody gets out of. The rate for individual underpayments is 7% for the quarter beginning October 1, 2026 under Rev. Rul. 2026-15, compounded daily, and §6404(e) permits abatement only when the delay was the IRS's own.

If your April payments covered the year, none of the above applies and both penalties compute to zero. The deadline that matters then is the refund window in §6511, three years from the original due date, which is April 15, 2029 for a 2025 return. Miss that one and the money stays with the Treasury. Taxpayers in a federally declared disaster area get postponed dates under §7508A, and the IRS publishes the current list of covered areas.