2026 Federal Tax Filing Deadlines for US Startups

Tax season isn't a single date on the calendar. It's a rolling series of deadlines that catch founders off guard every year, and missing even one can mean penalties, interest, or worse. If you're running a US startup, here's exactly what you need to know about federal filing deadlines for 2026.

Your Entity Type Determines Your Deadlines

The most important thing to understand upfront: your filing calendar depends entirely on how your company is structured. S-corps, C-corps, partnerships, and single-member LLCs all operate on different schedules.

S-Corps and Partnerships (Multi-Member LLCs)

March 16, 2026 is your primary deadline. (March 15 falls on a Sunday, so it rolls to the next business day.) S-corps file Form 1120-S, while partnerships file Form 1065. Both must also furnish Schedule K-1 to all partners or shareholders by this date.

If you need more time, file for a 6-month extension. Your final extended deadline is September 15, 2026. Note that an extension gives you more time to file, not more time to pay. Any tax owed is still due by the original deadline.

C-Corps

April 15, 2026 is when Form 1120 is due. This is also when your first quarterly estimated tax payment for 2026 is due.

With a 6-month extension, the final deadline moves to October 15, 2026.

Single-Member LLCs and Individuals

April 15, 2026 is when Form 1040 is due, and it's also the date to file for an automatic extension (Forms 4868 or 7004) if you need one. The extended deadline is likewise October 15, 2026.

Fiscal Year Startups

Not everyone runs on a January–December calendar. If your fiscal year ends on a different date, your filing deadline is the 15th day of the fourth month after your fiscal year closes. For example, if your fiscal year ends June 30, your deadline is September 15.

Quarterly Estimated Tax Payments

Both corporations and individuals are generally required to pay taxes quarterly throughout the year rather than in one lump sum at filing time. Missing these payments can result in underpayment penalties.

Individual founders typically have a Q4 estimated payment too, but that falls on January 15, 2027, which is outside this calendar year.

Payroll and Information Returns

If you have employees or pay contractors, you have a separate set of obligations that begin in January.

January 31 / February 2, 2026 January 31 is a Saturday, so the deadline moves to Monday, February 2. By this date, W-2s must be furnished to employees, 1099s to contractors, and Form 940 (Annual Federal Unemployment Tax) must be filed.

February 28 / March 2, 2026 Paper filings for 1099-MISC, 1099-NEC, and W-2 forms are nominally due February 28, but since that falls on a Saturday, the deadline moves to March 2.

March 31, 2026 Electronic filing deadline for Form 3921, which applies if your startup issued incentive stock options (ISOs) that were exercised during the prior year.

Delaware-Specific Deadlines

Most US startups, particularly VC-backed ones, are incorporated in Delaware. If that's you, two additional state-level deadlines apply:

• March 16, 2026: C-Corp Annual Report and Franchise Tax
• June 1, 2026: LLC Franchise Tax ($300 flat fee)

These are state obligations, not federal, but they're easy to miss if you're only tracking IRS deadlines.

FBAR: Foreign Bank Account Reporting

If your startup holds funds in foreign bank accounts and the combined balance exceeded $10,000 at any point during 2025, you're required to file an FBAR (FinCEN Form 114). The initial deadline is April 15, 2026, with an automatic extension to October 15, 2026.

A Few Things Worth Flagging

Extensions don't extend your payment deadline. This is the most common misconception. Filing for an extension only gives you more time to submit paperwork. You still owe any taxes due by the original deadline. Pay what you estimate you owe to avoid penalties and interest.

Weekend and holiday rule. When any federal deadline falls on a weekend or federal holiday, it automatically moves to the next business day. All adjusted dates above already account for this.

State deadlines vary. This guide covers federal obligations. State and local tax deadlines don't always align with federal dates, so check your specific state requirements separately.

Tax laws change. The dates above are accurate as of publication, but tax regulations can shift. Always verify with the IRS or a qualified tax professional before filing.

Quick Reference Calendar

Set calendar reminders at least two to three weeks before each date, not the night before. The IRS does not accept "I forgot" as an explanation.

Don't Track This Alone

Inkle helps US startups stay on top of tax deadlines, filings, and compliance so you can focus on building.