Why S corporations file March 15 not April 15

Quick answer: S corporations use a March deadline (rather than April) because they are "pass-through" entities that file an information-style return — the corporation itself generally doesn't pay income tax, but each shareholder needs their share of income/deductions (Schedule K-1) early enough to complete their own individual return, which is due in April.

Explanation:

For a calendar-year S corporation, the documents show the return (Form 1120-S) is due about a month before the individual deadline. Per the tax calendar, S corporations must file a 2025 calendar year income tax return (Form 1120-S) and pay any tax due, and provide each shareholder with a copy of their Schedule K-1 (Form 1120-S), Shareholder's Share of Income, Deductions, Credits, etc.¹ by March 16 (the 15th falls on a weekend, so the deadline shifts).

This pattern is confirmed in the general fiscal-year rules too: Form 1120-S (or Form 7004) is due on the 15th day of the 3rd month after the end of the corporation's tax year, and each shareholder must receive a copy of their Schedule K-1 (Form 1120-S) by the 15th day of the 3rd month after the end of the corporation's tax year.²

Compare that to regular C corporations, which follow the April timeline instead: Generally, a corporation must file its income tax return by the 15th day of the 4th month after the end of its tax year.³

Why the earlier date makes sense: S corporation income isn't taxed at the entity level — it flows through to the shareholders' individual returns. Shareholders need their K-1 figures in hand well before their own Form 1040 is due (April 15), so the IRS built in a one-month head start for S corps (and partnerships, which follow the same March 15/16 schedule) versus the April deadline used by regular C corporations and individuals.

What it depends on:

• Whether the S corp uses a calendar year or a fiscal year (fiscal-year S corps use the 15th day of the 3rd month after their own year-end, not necessarily March).
• Whether an extension (Form 7004) was filed, which pushes the return deadline to September 15 for calendar-year S corps, though shareholder K-1s must still go out by that same extended date.

Since deadlines and extension mechanics can get complicated depending on your corporation's specific tax year and filing history, it's worth confirming your exact due dates with a CPA.