What is Form 1120 / 1120-S?

Form 1120 — the C-corp return

C-corporations pay a flat 21% federal tax on profits, reported on Form 1120. Foreign-owned single-member LLCs also file a pro-forma 1120 — a mostly blank cover page — purely as the vehicle for Form 5472.

Form 1120-S — the S-corp return

S-corporations generally pay no federal income tax themselves. The 1120-S reports the company's results and issues each shareholder a Schedule K-1 carrying their share to their personal return. Owner salary must run through payroll first.

Deadlines

• 1120-S: March 15 (calendar year), extendable to September 15
• 1120: April 15, extendable to October 15
• Late 1120-S penalties accrue per shareholder, per month — the expensive one to forget