Startup C Corp filing deadline 2025 and other business tax deadlines

What's the C corp filing deadline in 2025?

The C corp filing deadline in 2025, for your 2024 tax year IRS Form 1120, is April 15th, 2025. If you file an extension before that date using Form 7004, you may receive an additional six months to file, pushing the deadline to October 15th, 2025.

This assumes your C corporation’s tax year is the standard calendar year. If your business has elected a fiscal tax year, your corporate income tax filing deadline is the 15th day of the fourth month after the end of the year.

Other C corp tax deadlines

Here are some of the other most significant 2025 tax deadlines for C corporations.

Form 3921: Exercise of an Incentive Stock Option

Corporations that had an employee exercise an Incentive Stock Option (ISO) during the tax year must file Form 3921 for each one. You must send Copy A to the Internal Revenue Service and Copy B to the employee in question.

If you prefer to file paper copies, you must do so by February 28, 2025. If you file electronically, you have until March 31, 2025.

Estimated tax payments

If you expect your corporation to owe $500 or more in federal business taxes for the year, you generally must make quarterly estimated tax payments. Here are the due dates, which differ slightly from the deadlines for individuals:

• April 15, 2025
• June 15, 2025
• September 15, 2025
• December 15, 2025

Once again, this assumes a December 31 year-end. If you use an alternate fiscal year, the deadlines are the 15th day of the fourth, sixth, ninth, and twelfth months of the year.

LLC tax deadlines

A limited liability company (LLC) generally doesn't have unique tax deadlines because the IRS will treat them as a corporation, partnership, or disregarded entity, depending on your elections as the owner and the number of members.

If your LLC is taxable as a corporation or partnership, it’s subject to the same tax deadlines as the corresponding business structure. 

If it’s a disregarded entity, you should report its activities on your personal tax return, subject to individual tax deadlines.

S corporation tax deadlines

Here are the most notable tax deadlines for S corporations in 2025.

Form 1120-S: Income Tax Return for an S Corporation

The S corp filing deadline in 2025—for your 2024 tax year Form 1120-S—is a month before the C corp deadline: March 17, 2025. An extension grants you an additional six months to file, pushing the deadline to September 15, 2025.

If your S Corporation elects a non-calendar tax year, your filing deadline is the 15th day of the third month after the end of the year.

Form K-1

S corporations must issue a Form K-1 to each shareholder, reporting their portion of its activities for the year. The deadline is March 17, 2025.

If you file an extension for your S corp, the deadline becomes September 15, 2025. However, that may force shareholders to extend their individual returns as well since they’re originally due by April 15, 2025.

Form 3921: Exercise of an Incentive Stock Option

S corporations may also have to file Form 3921 if one or more employees exercise an ISO during the tax year. The deadlines are the same as they are for C corps: February 28, 2025, for paper copies and March 31, 2025, for electronic copies.

Federal quarterly estimated tax payments

The estimated tax deadlines for S corporations are the same as the deadlines for C corporations. Here are the due dates in 2025:

• April 15, 2025
• June 16, 2025
• September 15, 2025
• December 15, 2025

If you use a fiscal tax year, the deadline for each estimated tax payment is the 15th day of the fourth, sixth, ninth, and twelfth months.

Sole proprietor and partnership tax deadlines

Here are the main tax deadlines for sole proprietorships and partnerships in 2025.

Form 1040: Individual Income Tax Return (Schedule C)

Sole proprietors report their business owner activities on their personal tax returns—Form 1040—using Schedule C. The deadline is April 15, 2025, but you can file a six-month extension to push the deadline to October 15, 2025.

Form 1065: Return of Partnership Income

Partnerships must report their business activities on Form 1065, the tax return for partnerships. The deadline is March 17, 2025, but you can file an extension to push it out six months to September 15, 2025.

If your partnership elects a tax year different than the calendar year, its Form 1065 is due on the 15th day of the third month after the tax year.

Form K-1

Partnerships must issue tax Form K-1 to each partner, reporting their share of the partnership’s activities for the year. The deadline is March 15, 2025. As with S corps, you can file an extension to push the due date to September 15, 2025.

Federal quarterly estimated tax payments

Estimated tax payments for sole proprietorships and partnerships are due by the following dates:

• April 15, 2025
• June 16, 2025
• September 15
• January 15, 2026

If you use a fiscal tax year, which is rare for sole proprietorships but more common for partnerships, each tax deadline is the 15th day of the fourth, sixth, ninth, and thirteenth months after the start of the year.

How filing tax extensions works

Tax extensions are relatively easy to file, and as long as you complete them correctly, the IRS usually accepts them automatically. However, they only extend the time to file your return. They don’t give you any extra time to pay.

If you owe the IRS on tax day, filing an extension will help you avoid late filing penalties, but it won't save you from being penalized for underpayment.

When you’re not sure how much you owe, you can avoid penalties by paying enough throughout the year to reach the lower of the following:

• 90% of your current year’s tax liability
• 100% of your liability on the previous year’s tax filing

You may also be able to avoid penalties if you owe less than $1,000 in federal income tax after subtracting withholdings and business tax credits.

Navigate tax season with Zeni’s expert help

Keeping up with never-ending tax obligations can be overwhelming—especially on top of growing your business—but Zeni makes it easy.

Our startup tax accountant experts have years of experience keeping businesses like yours compliant while hunting down as many tax savings opportunities as possible. 

We’ve helped our clients capture more than $6.5 million through the R&D tax credit alone.

Schedule a demo today to see how Zeni can help you streamline your financial reporting, optimize your tax preparation, and navigate your most complex financial decisions.