A filing extension and the penalties it does not stop
The calculator is set to a return filed on extension by October 15. Enter the tax that was still unpaid on April 15 and when you pay it: the result shows the late payment penalty and interest that the extension leaves in place.
- Extended filing date, 2025 returns
- October 15, 2026
- Payment due date
- April 15, 2026
- Late payment penalty during the extension
- 0.5 % a month
- Interest rate now
- 7 %
Balance on October 20, 2026
$9,203
$603 added to $8,600 of tax since April 15, 2026; interest now runs at about $2 a day.
| Unpaid tax | $8,600 |
| Failure to file penalty | $0 |
| Failure to pay penalty, 7 months | $301 |
| Interest, compounded daily | $302 |
Penalties of IRC 6651 and interest at each quarter's underpayment rate (7 % in the fourth quarter of 2026). Estimate, not tax advice.
| Month ending | Penalties | Interest | Balance |
|---|---|---|---|
| May 15, 2026 | $43 | $43 | $8,686 |
| June 15, 2026 | $86 | $87 | $8,773 |
| July 15, 2026 | $129 | $134 | $8,863 |
| August 15, 2026 | $172 | $187 | $8,959 |
| September 15, 2026 | $215 | $241 | $9,056 |
| October 15, 2026 | $258 | $294 | $9,152 |
| October 20, 2026 | $301 | $302 | $9,203 |
Compiled by Radif Partners · How we calculate · rules and rates read on irs.gov · not tax advice, the IRS decides
An extension gives six more months to file the return, not to pay the tax. For 2025 returns the filing date moves from April 15, 2026 to October 15, 2026, which removes the failure to file penalty for anyone who files by then. The tax itself was still due on April 15, 2026: anything unpaid on that date carries the late payment penalty of 0.5 % a month and interest at the quarterly rate, 7 % in the fourth quarter of 2026, exactly as if no extension existed. Take $8,600 unpaid in April and a return filed on October 15, 2026. With the extension, the balance on October 20, 2026 is $9,203. Without it, the same filing date is six months late and adds $1,935 of late filing penalty, for $11,205. Paying $7,200 of the tax in April would have brought the extended case down to $1,498.
Three ways the IRS accepts an extension
The IRS extension page lists three routes to the October date. You can pay all or part of the tax online and mark the payment as an extension payment, which counts as the request without a separate form. You can file the request electronically through Free File, which has no income limit for extensions. Or you can mail Form 4868, on which you estimate the year's tax and subtract what was already paid. The request has to be made by the April due date. U.S. citizens and resident aliens living outside the country on that date get an automatic two-month extension, and people in a declared disaster area may get more time.
What the extension changes in the numbers
Only the filing penalty depends on the filing date. With the extension, the failure to file penalty counts from October 15, 2026 instead of April 15, 2026. Without one, a return filed in October is six months late and the filing penalty has already reached its 25 % ceiling, reduced by the late payment penalty of the same months. The table runs $8,600 of 2025 tax through five situations.
| Situation | Late filing | Late payment | Interest | Owed at the end |
|---|---|---|---|---|
| Extension, filed October 15, 2026 | $0 | $301 | $302 | $9,203 |
| Extension, $7,200 paid in April, filed October 15, 2026 | $0 | $49 | $49 | $1,498 |
| No extension, filed October 15, 2026 | $1,935 | $301 | $369 | $11,205 |
| Extension, filed November 30, 2026 | $774 | $344 | $382 | $10,100 |
| No extension, not filed yet | $1,935 | $301 | $369 | $11,205 |
Unpaid in April, filed on extension
Balance on January 20, 2027
$6,405
| Unpaid tax | $5,800 |
| Failure to file penalty | $0 |
| Failure to pay penalty, 10 months | $290 |
| Interest, compounded daily | $315 |
Interest and the late payment penalty from April
The IRS interest page says it plainly: underpayment interest applies even if you file an extension, and extensions to file do not extend the date for payment. The late payment penalty runs at 0.5 % a month on the unpaid tax from April 15, 2026. Between April and October that is six months, or 3 % of whatever was not paid, plus about half a year of interest.
This is why the amount paid in April matters more than the extension itself. In the table, the person who paid $7,200 with the extension request ends with $1,498, against $9,203 for the person who paid nothing until October. A payment made with the extension also counts as the request, so it does two jobs.
Missing the extended date
An extension buys one fixed date. A return filed after October 15, 2026 is late from that date: in the table, a return filed on November 30, 2026 picks up $774 of late filing penalty for 2 months counted from the extended date. More than 60 days after it, the minimum penalty of $525 for returns due in 2026 can also apply. Interest on a filing penalty runs from the extended due date when an extension was filed.
The 2025 return on a calendar
For a 2025 Form 1040, the dates line up as follows. April 15, 2026 is the day both the return and the tax are due, and the last day to ask for an extension. From the next day, any tax not paid starts accruing interest daily and the late payment penalty monthly. October 15, 2026 is the last day to file without a failure to file penalty for anyone who asked in time. A few weeks after the return is processed, the IRS mails a bill for the balance, with the penalty and interest counted to the bill date and a date to pay. Paying by that date stops further interest on the amount shown; paying later adds it again until the day of payment.
When the return shows a refund or no balance
If withholding and estimated payments covered the tax, an extension changes nothing in money terms: there is no unpaid tax for either penalty to apply to. The extension is still useful for the paperwork. For a person who owes and cannot pay, the order of steps is the one the IRS itself gives: file by the date that applies, pay what you can, and ask for a short-term plan or an installment agreement for the rest. The page on owing more than you can pay sets the two penalties side by side.
These are estimates from the published rates and the dates in the IRS pages, not tax advice. Disaster postponements, which change both dates for a county, are not modelled, and the IRS figure on your notice prevails.