C Corporation tax returns (Form 1120) are due on the 15th day of the 4th month after the fiscal year ends. For calendar-year corporations, this is April 15, 2026 for tax year 2025. With a 6-month extension (Form 7004), the deadline becomes October 15, 2026.

S Corporation tax returns (Form 1120-S) are due on the 15th day of the 3rd month after the fiscal year ends. For calendar-year S-Corps, this is March 15, 2026 (March 16 since the 15th is Sunday) for tax year 2025. Extended deadline is September 15, 2026.

If a tax deadline falls on a Saturday, Sunday, or legal holiday, the deadline automatically moves to the next business day. For example, March 15, 2026 falls on Sunday, so S-Corp and Partnership returns are due March 16, 2026.

File Form 7004 (Application for Automatic Extension) by the original due date. This grants an automatic 6-month extension to file (not to pay). You can file electronically through tax software or IRS e-file. Payment is still due on the original date.

Corporations expecting $500+ in tax must pay quarterly estimates. For calendar-year corporations: Q1 (April 15), Q2 (June 15), Q3 (September 15), Q4 (December 15). Use Form 1120-W to calculate and pay via EFTPS.

Failure to file: 5% of unpaid tax per month (max 25%). Failure to pay: 0.5% per month (max 25%). S-Corp/Partnership late filing: $220 per shareholder/partner per month (max 12 months). Interest also accrues on unpaid balances.

Yes! An extension to file is NOT an extension to pay. You must estimate and pay your tax liability by the original due date to avoid penalties and interest. The extension only gives you more time to complete and submit the return.

Due dates are calculated from your fiscal year end. C-Corps: 15th of 4th month after year end. S-Corps/Partnerships: 15th of 3rd month. For example, a June 30 fiscal year end C-Corp would file by October 15 (4 months later).

Use EFTPS (Electronic Federal Tax Payment System) at eftps.gov. You must enroll in advance. You can also use IRS Direct Pay for same-day payments. Credit/debit card payments are available through approved processors with fees.

Yes, S-Corporations must provide Schedule K-1 to shareholders by the filing deadline (March 15 or extended date). This allows shareholders to file their personal returns. Late K-1s can delay shareholder filings and may require them to file extensions.