Key takeaways
- Form 1120 for tax year 2026 is due April 15, 2027, and a timely Form 7004 moves the filing date to October 15, 2027.
- Form 7004 extends the time to file only. Tax not paid by April 15, 2027 accrues interest and the late payment penalty from that date.
- The late filing penalty is 5% a month, up to 25%. The late payment penalty is 0.5% a month, up to 25%.
- Estimate the balance before April 15 and pay it with the extension request, not in October.
The dates for tax year 2026
A calendar-year C corporation files Form 1120 for the tax year that ends December 31. The IRS instructions for Form 1120 state that a corporation generally must file its return by the 15th day of the 4th month after the end of its tax year. For a calendar-year corporation that is April 15. The 2026 return is therefore due Thursday, April 15, 2027.
Calendar three dates for the 2026 return: April 15, 2027, the original filing date and the last day to file Form 7004; the same day for paying any tax still owed; and October 15, 2027, the extended filing date, which is a Friday.
The extension request is due on the original date, not the extended one. The IRS instructions say Form 7004 generally must be filed on or before the due date of the return. A Form 7004 sent on April 16, 2027 is late, and the corporation does not get the automatic extension.
What Form 7004 does and does not do
Form 7004 is the request for an automatic extension of time to file. The IRS instructions state that the automatic extension period for time to file is generally 6 months, and for tax years beginning in 2026 it is 6 months. Six months from April 15 is October 15, so a calendar-year Form 1120 moves from April 15 to October 15.
The IRS instructions also state that Form 7004 does not extend the time to pay any tax due. Interest is charged on any tax not paid by the regular due date of the return, even when an extension is granted. The payment question is settled before April 15, not after.
Decision rule: if the corporation’s estimate shows a balance due, pay the estimated balance by April 15 and file Form 7004 the same day. If the estimate shows no balance, the extension only buys filing time. State returns follow their own deadlines and extension rules, so check each state separately.
Late filing: 5% a month, up to 25%
If the return is filed after its due date, including the extended date, the failure to file penalty is 5% of the unpaid tax for each month or part of a month the return is late. The cap is 25% of the unpaid tax. The base is the unpaid tax, meaning the tax shown on the return less payments already made, not the gross tax.
The minimum penalty applies to a return more than 60 days late. The IRS table lists $525 for returns due after December 31, 2025, which covers tax year 2025 returns due in 2026; the minimum for a tax year 2026 return due in 2027 is adjusted annually, so confirm the figure in the instructions for that year before quoting it. The penalty is the smaller of the tax due or that amount, so a corporation that owes less than $525 pays its tax, not $525. The IRS adjusts this figure, so check the instructions for the year the return is due before quoting it.
The two penalties interact. In each month when both the failure to file and failure to pay penalties run, the 0.5% failure to pay charge reduces the 5% failure to file charge, so the combined charge is 5% a month for the first five months. After that the failure to file penalty stops at its 25% maximum while the failure to pay penalty keeps running.
Late payment: a separate 0.5% a month, up to 25%
The late payment penalty applies to tax not paid by its due date, whether or not a return was filed. It is 0.5% of the unpaid tax for each month or part of a month the tax remains unpaid, and it stops at 25% of the unpaid tax. The IRS also states that interest is charged on penalties, and the start date for that interest depends on the penalty type.
A corporation that files on extension and still owes tax on April 15 pays this charge from April 15 onward. The extension does not stop it, and it is the most common surprise when a corporate return is filed in October with a balance that was never paid.
Worked example: a $40,000 balance on April 15, 2027
The numbers below are illustrative, not IRS figures. Assume a calendar-year corporation’s 2026 Form 1120 shows $100,000 of tax, and it paid $60,000 through estimated payments. The unpaid balance is $40,000 on April 15, 2027.
Case A: the Form 7004 is filed by April 15, 2027, and the return and the $40,000 are both paid on October 15, 2027. There is no failure to file penalty, because the return was filed by the extended date. The failure to pay penalty runs for six months, April 15 to October 15, so it is 0.5% times 6, or 3% of $40,000, which is $1,200. Interest also runs on the $40,000 for that period.
Case B: the return and the $40,000 are both paid on December 1, 2027. That is 47 days after the extended date, which is two months counting a part month as a full month. The failure to file penalty is 5% a month on $40,000 for those two months, reduced by the 0.5% failure to pay charge for the same months, so it is 4.5% times 2, or $3,600. The failure to pay penalty runs for eight months, April 15 to December 1, so it is 0.5% times 8, or $1,600. The total is $5,200 before interest. The return is not more than 60 days late, so the $525 minimum does not apply.
Case C: the $40,000 is paid on April 15, 2027 with the Form 7004, and the return is filed on October 15, 2027. Neither penalty applies. Compared with Case A that saves $1,200, and compared with Case B it saves $5,200 plus interest.
Fiscal year corporations and June 30 year-ends
A C corporation with a fiscal year follows its own year-end, using the same 15th day of the 4th month rule. A corporation with a June 30 year-end whose tax year begins July 1, 2026 ends June 30, 2027, so its return is due October 15, 2027. Six months later is April 15, 2028, which is a Saturday, so the weekend rule decides the actual day. Calendar the date only after checking that rule.
The IRS instructions for Form 7004 say the 7-month extension for C corporations with June 30 year-ends applies only to tax years beginning before January 1, 2026. A June 30 corporation whose tax year begins in 2026 gets the 6-month extension, the same as a calendar-year corporation. A fiscal-year client that still uses the 7-month assumption from an earlier year needs its date recalculated.
Failure modes and fixes
Failure: the client thinks the extension covers the payment. Fix: put the payment on the April 15 line of the calendar and ask for the balance estimate in March, so the payment is not discovered in October. The client should be told in writing that a filing extension does not extend the time to pay.
Failure: the Form 7004 is filed after April 15. Fix: treat the return as due April 15 with no extension. Penalties run from that date, so the firm should tell the client the same week that the request was late and what the bill now looks like.
Failure: the firm files in 2027 without a schedule of the balance. Fix: keep one schedule per client with the tax shown on the return, the estimated payments made, the unpaid balance, the April 15 payment date and the October 15 filing date. The penalty math in the worked example can be run from that schedule in minutes.