S Corp vs C Corp Deadlines 2027: 1120-S vs 1120
17 October 2026 · Educational information · No affiliate links · 3 min read
General information, not tax, legal or financial advice. We are not the IRS. Check IRS.gov or a qualified tax professional. Sources read on October 17, 2026. Dates after 2026 are expected and amounts for 2026 returns may change. This page contains no affiliate links.
Short answer: Form 1120-S (S corporation) is due the 15th day of the 3rd month after year-end: expected March 15, 2027 for a calendar year. Form 1120 (C corporation) is due the 15th day of the 4th month: expected April 15, 2027. An automatic extension is available with Form 7004. The instructions (12/2025) say the period is generally 6 months, so a calendar-year Form 1120 is expected to be extended to October 15, 2027 (a Friday). The extension is for filing, not for paying.
| Form 1120-S | Form 1120 | |
|---|---|---|
| Rule | 15th day of 3rd month | 15th day of 4th month |
| Calendar year 2026, expected | March 15, 2027 (Monday) | April 15, 2027 (Thursday) |
| Automatic extension | 6 months with Form 7004 | 6 months with Form 7004 (the 7-month rule applies only to June 30 year-ends beginning before 2026) |
| Extended date, expected | September 15, 2027 | October 15, 2027 (a Friday; calculated from the 6-month rule in the Form 7004 instructions) |
| Payment | not extended | not extended |
Key differences in practice
- An S corporation generally passes income to shareholders through Schedule K-1, due by the return due date; see K-1 due dates.
- A C corporation files and pays its own tax with Form 1120.
- Both: Form 7004 does not extend the time to pay tax due.
Penalties
S-corporation late-filing penalties are per shareholder per month; see the penalty guide. Form 1120 penalties were not researched here.
FAQ
Which should I be? That is a tax and legal choice outside this page; ask a professional.
What we could not verify
- Form 1120 penalty rules.
Sources (official)
- IRS, Instructions for Form 1120-S (2025) (read October 17, 2026)
- IRS, Instructions for Form 7004 (12/2025) (read October 17, 2026)
- IRS, Instructions for Form 1120 (read October 17, 2026)