Tuesday, September 15, 2026 is the most crowded date on the fall tax calendar for business owners. Several separate obligations land on the same day and they have nothing to do with each other. Missing one because you were focused on another is the most expensive mistake we see in September. Here is what is due, what late costs, and how to get the money to the IRS correctly.
Three deadlines, one day
| What is due September 15, 2026 | Who it applies to |
|---|---|
| Third estimated tax payment for tax year 2026 | Individuals and businesses paying quarterly |
| Form 1120-S and Form 1065 for calendar year 2025, on extension | S corporations and partnerships that filed an extension in March |
| Forms 1042, 8804, 8805, 8813 and 3520-A, on extension | Filers with foreign partner or foreign trust reporting |
| Monthly payroll tax deposit | Employers on the monthly deposit schedule |
(Source: IRS third quarter tax calendar.)
The trap is the second row. Many owners assume the March extension carried them to October. October is for Form 1040 and Form 1120. Partnerships and S corporations run out of road in September, and there is no second extension behind it.
The penalty that surprises people: $255 per partner, per month
A late partnership or S corporation return does not draw one flat penalty for the business. It draws a penalty for every owner:
- Form 1065 filed late — IRC §6698: $255 per month, per partner.
- Form 1120-S filed late — IRC §6699: $255 per month, per shareholder.
Run the math on a small partnership. Four partners, filed three months late:
$255 × 4 partners × 3 months = $3,060.
That bill exists even if the partnership owed no tax at all. The penalty is for not filing, not for not paying. A break-even year and a forgotten deadline still produce a four-figure charge.
An extension to file is not an extension to pay
An extension buys time to send the paperwork. It never buys time to send the money. Tax owed was due on the original deadline, and interest has been running since then. When you file in September or October, you settle a balance that has already been growing.
Filing on extension without paying is still better than not filing — failure to file runs at 5% per month against 0.5% for failure to pay — but it is not free.
How much do you actually have to send? The safe harbor
For the Q3 estimated payment, you are not required to guess your final 2026 tax perfectly. You need to clear a threshold.
First, the obligation itself. Per Form 1040-ES for 2026, you must pay estimated tax if both are true: you expect to owe at least $1,000 after withholding and refundable credits, and your withholding plus credits will be less than the smaller of 90% of your 2026 tax or 100% of your 2025 tax (that 2025 return must cover 12 months).
| Threshold | Based on | Condition |
|---|---|---|
| 90% | 2026 tax (current year) | Always available; requires projecting the year in progress |
| 100% | 2025 tax (prior year) | 2025 AGI of $150,000 or less; 2025 return must cover 12 months |
| 110% | 2025 tax (prior year) | 2025 AGI above $150,000, or above $75,000 if filing married filing separately for 2026 |
(Source: IRS Form 1040-ES 2026, page 1.)
None of this applies the same way if two-thirds or more of your gross income comes from farming or fishing. Those filers are calculated under separate rules.
One more thing about the penalty for underpaying estimates: it is not a flat annual percentage. It is computed period by period on Form 2210. A strong fourth quarter does not undo a short third quarter.
Three ways to pay, and the one that will not work today
IRS Direct Pay. Pays straight from your bank account with no prior enrollment. This is the right tool for a payment you are making this week.
EFTPS. Built for businesses and useful for scheduling payments ahead. It carries a hard limitation: it requires enrollment in advance. If you are not already enrolled, EFTPS will not get this payment in on September 15. Use Direct Pay now and enroll for future quarters.
Your IRS.gov online account. Shows balance, payment history and scheduled payments. This is where you verify a payment landed where it should.
Bank debit costs nothing. Card payments carry processor fees, which is not trivial on a large estimate.
The wrong-year error
When you pay, you have to select the year 2026 and the payment type “estimated tax.” Money applied to the wrong year or the wrong period leaves your 2026 account looking unpaid. The notice arrives months later, interest has been accruing the whole time, and untangling it takes weeks. Save the confirmation number. It is the difference between a correction that takes days and one that takes months.
If you cannot pay the full amount
Send what you have. Interest is charged on the unpaid balance, so a partial payment on September 15 shrinks the base the charge is calculated on. Paying nothing because you cannot pay everything is the worst option.
Penalties and interest in effect
| Charge | Rate | Cap |
|---|---|---|
| Underpayment interest (non-corporate) | 7% per year, compounded daily | None |
| Overpayment interest (non-corporate) | 7% per year | |
| Failure to pay | 0.5% per month | 25% |
| Failure to pay after notice of levy | 1% per month | 25% |
| Failure to pay under an approved installment agreement | 0.25% per month | 25% |
| Failure to file | 5% per month | 25% |
| Minimum failure-to-file penalty, more than 60 days late | Lesser of $525 or 100% of the tax | |
| Form 1065 late (§6698) | $255 per month, per partner | |
| Form 1120-S late (§6699) | $255 per month, per shareholder |
(Sources: the 7% rate is set by Rev. Rul. 2026-10 for July–September 2026 and remains unchanged for October–December 2026 under Rev. Rul. 2026-15, announced in IR-2026-98 on August 21, 2026. Minimum failure-to-file penalty for returns filed in 2026: Rev. Proc. 2024-40 §.53.)
Tennessee disaster relief does not cover these dates
This needs to be said clearly, because it is circulating locally. The IRS granted relief for Winter Storm Fern and expanded it on April 15, 2026 to all 95 Tennessee counties, Montgomery County included, with a postponed deadline of June 8, 2026.
That relief does not extend the September or October 2026 deadlines. They stand as normal. If someone told you the disaster declaration bought you more time this quarter, it did not.
What comes after September 15
| Date | What is due |
|---|---|
| September 30, 2026 | Forms 730 and 2290 |
| October 13, 2026 | Employee tip reporting for September ($20 or more in tips) |
| October 15, 2026 (Thursday) | Form 1040, Form 1120 and Form 1040-NR on extension; FBAR (FinCEN Form 114) under its automatic extension |
| January 15, 2027 | Fourth estimated payment for 2026 |
(Source: IRS third and fourth quarter tax calendars.)
Two notes on that list. The FBAR applies if the combined value of your foreign financial accounts exceeded $10,000 at any point during the year — a peak, not a year-end balance. And the January 15 payment is not required if you file your 2026 return by February 1, 2027 and pay the full balance then.
Frequently asked questions
I filed an extension for my two-member LLC in March. Do I have until October?
No. A multi-member LLC generally files Form 1065, and the extended Form 1065 is due September 15, 2026. October 15 is for Form 1040 and Form 1120. Once September passes, the §6698 penalty starts accruing at $255 per month for each partner.
My S corp had a loss. Do I still have to file on time?
Yes, and this is where the penalty hurts most. The §6699 penalty is charged for the late filing itself, not for unpaid tax. A loss year with three shareholders and a late return still generates $255 per shareholder per month.
Can I set up EFTPS today and use it on the 15th?
Plan on no. EFTPS requires advance enrollment and that process is not instant. Use IRS Direct Pay for this payment — no registration, no fee from your bank account — and complete your EFTPS enrollment for future quarters.
How do I confirm my payment was applied to the right year?
Check your online account at IRS.gov a few days after paying. It shows the balance, the history and the period each payment was applied to. Keep the confirmation number from the payment itself; without it, fixing a misapplied payment takes far longer.
Let’s get September 15 handled
Jimenez Consulting Tax Services works with partnerships, S corporations and self-employed owners across Clarksville and Montgomery County. We calculate what you owe this quarter, get your 1065 or 1120-S filed before the penalty clock starts, and confirm your payment posts to the right year and period. If the cash is not there this month, we build a realistic plan instead of letting interest run.
Call (931) 802-8620 or visit us at 2225 Lowes Dr W Ste B, Clarksville, TN 37040. Bilingual service available.