Who files a real 1120
Any US entity treated as a C-corporation for US tax purposes files a complete Form 1120. That includes:
- A US corporation (Inc, Corp)
- A single-member LLC that filed Form 8832 to be taxed as a corporation
- A single-member LLC that filed Form 2553 to be taxed as an S-corporation (which then files 1120-S, not 1120; but 8832 + 2553 is the usual path)
For a complete 1120, income, deductions, and tax are calculated. Schedules C, J, K, L, M-1, M-2 attach depending on size.
Who files a pro-forma 1120
A single-member LLC that is a US disregarded entity owned by a foreign person files a pro-forma 1120 as a cover for the Form 5472 information return. Only the identifying header and name/EIN sections are filled in; the schedules remain blank.
Due date
The 15th day of the 4th month after the tax year end — 15 April for a calendar year — extendable to 15 October with Form 7004.
See also: Form 5472, pro-forma 1120.