September 15, 2025 IRS Deadlines: Q3 Estimated Taxes, S-Corp 1120-S, and Partnership 1065 (Extended)

September 3, 2025

Two big obligations land on this date.

  • Your third-quarter estimated tax payment (individuals).

  • Final filing for S-corporations and partnerships on extension.

You can finish both in one focused session today.

What’s due on September 15, 2025

  • Q3 estimated tax for individuals.
    The third payment for the 2025 tax year is due September 15, 2025. Quarterly dates for 2025 are April 15, June 16, September 15, and January 15, 2026. If a due date falls on a weekend or legal holiday, the next business day applies. IRS+1

  • S-corporations on extension (calendar year).
    File Form 1120-S and furnish Schedule K-1 to each shareholder by this final extended date. Extension is via Form 7004 (6 months). IRS

  • Partnerships on extension (calendar year).
    File Form 1065 and furnish Schedule K-1 to each partner by this final extended date. Extension is via Form 7004 (6 months). IRS

  • Note for certain C-corps.
    A corporation with a fiscal year ending June 30 files Form 1120 by the 15th day of the third month after year-end, which falls in September. IRS+1

Q3 estimated tax: do you need to pay?

You do if both are true:

  • You expect to owe at least $1,000 after withholding and credits.

  • Your withholding and estimates will be less than 90% of this year’s tax or 100% of last year’s tax (110% if last year’s AGI was above the high-income threshold). IRS+1

This is the safe-harbor standard. Hitting it avoids an underpayment penalty even if your final bill ends up higher. IRS

Estimate your Q3 in minutes

  • Open last year’s return and find “total tax.”

  • If prior-year AGI was below the high-income line, use 100% of that number; if above, use 110%.

  • Divide by four.

  • Subtract what you paid for Q1 and Q2.

  • Pay the difference for Q3.

Prefer a current-year view? Use the worksheet in Form 1040-ES and recalc next quarter if income shifts. IRS

How to pay today

  • IRS Direct Pay from a bank account, or EFTPS for business payments.

  • Keep the confirmation number with your records. IRS

If you filed an extension: S-corporation (Form 1120-S)

  • File the return by September 15, 2025.

  • Furnish K-1s to shareholders with the filing.

  • Handle K-2/K-3 if a shareholder requested international detail by the “1-month date.”

Details you can use:

  • The “1-month date” is one month before you file the 1120-S. For tax year 2024 calendar-year S-corps on extension, the latest one-month date is August 15, 2025. IRS

  • If a shareholder requested K-3 by that date, you must file K-2/K-3 and furnish K-3 to that shareholder. IRS

The extension moved the filing date, not any payments. Keep payroll and employment tax deposits on their own schedules. IRS

If you filed an extension: Partnership (Form 1065)

  • File the return by September 15, 2025.

  • Furnish K-1s to partners with the filing.

  • Provide K-3 if a partner requested it by the “1-month date” or if you otherwise meet K-2/K-3 filing triggers.

Key timing:

  • For tax year 2024 calendar-year partnerships on extension, the latest one-month date is August 15, 2025. IRS+1

Quick checklist you can run now

  • Confirm your safe-harbor target (90% current-year or 100%/110% prior-year). IRS

  • Pay Q3 via Direct Pay or EFTPS. Save the receipt. IRS

  • If you’re an S-corp or partnership on extension:

    • Finish the return.

    • Issue K-1s with the filing.

    • Include K-2/K-3 if a timely request came in by August 15, 2025. IRS+1

  • Edge case: C-corp with June 30 year-end? Confirm your September due date. IRS

If cash is tight

  • Boost workplace withholding now to cover Q4.

  • Pay what you can for Q3 and true-up by January 15.

  • If retired, you can solve shortfalls with withholding from IRA or pension distributions; the IRS treats withholding as paid evenly through the year. This can help you meet safe harbor without four separate estimates. IRS

Where a tax advisor helps today

  • Confirms your safe-harbor number and Q3 amount. IRS

  • Checks whether K-2/K-3 applies this year and handles one-month-date requests. IRS+1

  • Reviews S-corp payroll, officer comp, and retirement funding before year-end.

  • Plans next steps if you expect a sale or partner changes later this year.

Related reads you can open now

FAQ

What exactly is due on September 15, 2025?
Q3 estimated tax for individuals, plus extended 1120-S and 1065 returns with K-1s for calendar-year S-corps and partnerships. IRS+1

How do I pay my Q3 estimate?
Use IRS Direct Pay or EFTPS and keep the confirmation. IRS

What is the safe-harbor rule?
Avoid penalties by paying 90% of this year’s tax or 100% of last year’s tax (110% for higher incomes), or owing less than $1,000 after credits. IRS

Do I need to send K-3s with my 1120-S or 1065?
If a shareholder or partner requested K-3 by the one-month date (for calendar-year filers on extension, August 15, 2025), you must include K-2/K-3 and furnish K-3 to the requester. IRS+1

What if I miss September 15?
Pay as soon as you can. Penalties and interest can apply. Hitting safe harbor for the year can reduce further penalties. IRS

Does the date ever move?
If the deadline falls on a weekend or legal holiday, the IRS moves it to the next business day. IRS

I’m short on cash today. Any options?
Increase workplace withholding for the rest of the year. Retirees can use withholding from IRA or pension distributions; the IRS treats withholding as paid evenly through the year. IRS

Block one hour.

  • Send your Q3 payment.

  • File your extended 1120-S or 1065 and deliver K-1s.

  • Add a calendar hold for your January 15 payment and any year-end moves.

Ready to talk strategy? Start here.

Visit contact physiciantaxsolutions.com to schedule a consultation and learn how we can help you take control of your tax strategy today.

This post serves solely for informational purposes and should not be construed as legal, business, or tax advice. Individuals should seek guidance from their attorney, business advisor, or tax advisor regarding the matters discussed herein. physiciantaxsolutions.com assumes no responsibility for actions taken based on the information provided in this post.

 

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