September 15 Deadlines: Q3 Estimates and Extended Business Returns
· RKube Services
Tuesday, September 15, 2026 is the due date for Q3 estimated taxes and extended S-corp and partnership returns. Here's what to check in your books in the next two weeks.
If you extended your S-corp or partnership return back in March, it probably felt like September was a long way off. It's now twelve days away.
Tuesday, September 15, 2026 is a double deadline for a lot of business owners. Here's what's due and what's worth checking in your books before then.
What's Due on September 15
Extended 2025 S-corp and partnership returns
Calendar-year S corporations (Form 1120-S) and partnerships (Form 1065) that filed an extension by March 16, 2026 have until September 15, 2026 to file their 2025 returns. There's no further extension.
Late filing penalties for these returns are charged per shareholder or partner, per month, so a late return with several owners gets expensive quickly. And the K-1s these returns produce are what the owners need to finish their own extended 1040s, due October 15, 2026. A late business return pushes everyone downstream.
Q3 2026 estimated tax payment
The third-quarter estimated payment for individuals is also due September 15. For calendar-year C corporations, the third installment of 2026 estimated tax falls on the same date.
If you're self-employed, an S-corp owner with pass-through income, or have significant investment income without enough withholding, this likely applies to you.
Source: IRS: Estimated Taxes
How Much to Pay: The Safe Harbor
For individuals, you generally avoid the federal underpayment penalty if your total payments for the year (withholding plus estimates) are at least the smaller of:
- 90% of this year's tax, or
- 100% of last year's tax (110% if last year's AGI was over $150,000, or over $75,000 if married filing separately)
The prior-year option is useful when income is hard to predict, because you already know the number.
Worked Example
An S-corp owner had total federal tax of $32,000 on her 2025 return, with AGI over $150,000. Her salary withholding for 2026 is running at about $1,000 a month.
- Safe harbor target: $32,000 × 110% = $35,200
- Expected 2026 withholding: $12,000
- Remaining through estimates: $23,200, or $5,800 per quarter
She paid $5,800 in April and June. If she pays $5,800 on September 15 and again in January, she's covered on the federal side even if the business has a much bigger year. She'd still owe the difference when she files, so we'd also project actual 2026 tax and set money aside.
The trap: if her 2025 return is one of the extended ones, the "last year's tax" number isn't final until the return is done. Finishing the return early helps the estimate math too.
What to Check in Your Books in the Next Two Weeks
If your 2025 business return is still open, the preparer needs clean 2025 books. For the Q3 estimate, you want a reliable 2026 year-to-date picture. Either way, the list is similar:
- Reconcile every bank and credit card account through August 31. Unreconciled accounts are the main reason year-to-date profit is wrong.
- Clear out uncategorized transactions. A big "Ask My Accountant" balance means your profit number is a guess.
- Review owner draws, distributions, and payroll. For S-corp owners, make sure distributions are booked as distributions, not expenses, and that you're paying yourself a reasonable salary through payroll.
- Record fixed asset purchases properly. Equipment and vehicles should be on the balance sheet, not expensed on a whim. Depreciation decisions come later, at tax time.
- Tie out loan balances to lender statements, splitting principal from interest.
- Collect W-9s from contractors now. For payments made in 2026, the 1099-NEC reporting threshold rises to $2,000, but you still want W-9s on file before January.
You can pay federal estimates through IRS payment options such as Direct Pay or EFTPS. State estimates are separate, so check your state's schedule.
How RKube Helps
- We finish extended 1120-S and 1065 returns and get K-1s to owners in time for their October 15 deadline.
- We calculate Q3 estimates using both the safe harbor and a year-to-date projection, so you know the minimum and the realistic number.
- We reconcile and clean up your 2026 books so the projection rests on real numbers.
- We flag S-corp payroll and distribution issues before they become year-end problems.
If your books aren't ready for either deadline, our free 30-day bookkeeping trial is a low-pressure way to start, or call (984) 234-7030: https://rkubeservices.com/campaign
Final Thoughts
September 15 isn't a deadline you can extend your way out of. Two weeks is enough time to reconcile, review, and pay, as long as you start this week.
This is general information. Your estimate and filing situation may differ, so check with a tax professional.
RKube Services
Bookkeeping, tax preparation, payroll, and proactive tax planning.