September 15 is an important tax checkpoint for many taxpayers and business owners, but not every September deadline works the same way. For 2026, the date can involve a federal estimated-tax payment, extended federal business returns, a California S corporation filing deadline, and certain California estimated-tax considerations.
One particularly important distinction for California taxpayers is that the federal system generally requires a third individual estimated-tax installment on September 15, while California's standard installment schedule assigns 0% of the required annual payment to the third installment. Understanding that difference can help business owners avoid treating federal and California payment calendars as if they were identical.
What is due on September 15, 2026? For calendar-year taxpayers, September 15, 2026 is the third federal estimated-tax payment deadline for individuals using Form 1040-ES. It is also the extended federal filing deadline for calendar-year S corporations and partnerships that timely requested extensions for their 2025 returns. California also identifies September 15 as an estimated-tax payment date, but under its standard individual installment schedule, no percentage of the required annual payment is allocated to the third installment. California S corporations using the automatic extension generally have an extended filing deadline of September 15, while California partnerships generally receive a seven-month extension.
1. The Third Federal Estimated-Tax Payment Is Due September 15
| Payment | Due Date |
|---|---|
| First | April 15, 2026 |
| Second | June 15, 2026 |
| Third | September 15, 2026 |
| Fourth | January 15, 2027 |
The IRS specifically identifies September 15, 2026 as the due date for the third Form 1040-ES payment. Estimated tax is generally used to pay tax on income not fully covered by withholding — self-employment income, business income, interest, dividends, rental income, capital gains, and other taxable income without sufficient withholding.
Why review rather than simply repeat the September payment? Because the appropriate amount may depend on what actually happened during 2026 — current business profit, owner compensation, investment gains or losses, rental income, withholding, and prior estimated payments can all shift the projection used earlier in the year. See Quarterly Estimated Taxes 2026.
2. California's September 15 Rule Is Different
| Installment | Standard Percentage |
|---|---|
| April 15 | 30% |
| June 15 | 40% |
| September 15 | 0% |
| January 15, 2027 | 30% |
The Franchise Tax Board states that no installment is due for the third required installment under the standard calculation. This is one of the clearest examples of why federal and California tax payment schedules should not be treated as interchangeable. See California Estimated Tax Payments 2026.
3. Does a 0% California Installment Mean Nothing Needs Review?
No. The standard 0% third installment means California generally does not allocate another portion of the required annual payment to September 15 under its regular schedule — it does not necessarily mean the taxpayer's California tax position should be ignored until January. A September review can still help confirm whether projected California income has materially changed, whether earlier estimated-tax assumptions remain reasonable, and whether withholding or business income shifted enough to warrant the annualized-income method.
Taxpayers whose 2026 California AGI is $1 million or more ($500,000 married/RDP filing separately) generally must calculate estimated tax using their 2026 current-year tax rather than the prior-year comparison — making current-year projections particularly important near those thresholds.
4. September 15 Is Also a Federal Business Extension Deadline
The IRS's 2026 tax calendar identifies September 15 as the extended federal filing deadline for calendar-year S corporations filing Form 1120-S and partnerships filing Form 1065 when a timely six-month extension was requested. September can therefore involve two distinct workstreams — current-year 2026 planning, and completion of the extended 2025 business return — which should not be confused.
5. California S Corporations Also Have a September 15 Extended Deadline
For a calendar-year California S corporation, the Franchise Tax Board lists September 15, 2026 as the extended filing date for the 2025 Form 100S. California automatically allows qualifying S corporations a six-month extension to file — but an extension to file is not an extension to pay. The S corporation's tax liability remains due by the original filing deadline even when the return itself is filed under extension.
6. California Partnerships Do Not Follow the Same September 15 Extension Date
Federally, a calendar-year partnership that timely requested an extension generally files its Form 1065 by September 15. California, however, generally provides partnerships and many non-corporate business entities with an automatic seven-month extension — the FTB states the extended due date is the 15th day of the 10th month after the close of the tax year, which for a calendar-year partnership generally means October 15 rather than September 15. That difference should be tracked deliberately rather than assumed identical to the federal date.
7. Extended Filing Does Not Mean Extended Payment
This principle applies across every September deadline discussed above. An extension generally changes when the return may be filed, but does not necessarily change when tax was required to be paid. For taxpayers working through an extended 2025 return while also making 2026 estimated payments, it can be useful to keep separate records for prior-year balance due, extension payments, current-year estimated payments, withholding, and entity-level tax payments.
8. September 15, 2026 at a Glance
| Area | September 15 Treatment |
|---|---|
| Federal individual estimated tax | Third 2026 payment generally due |
| California individual estimated tax | Standard third installment percentage is 0% |
| Federal S corporation return | Extended 2025 Form 1120-S generally due |
| California S corporation return | Extended 2025 Form 100S generally due |
| Federal partnership return | Extended 2025 Form 1065 generally due |
| California partnership return | Generally extended to October 15 |
| Current-year projection | Review income, withholding and prior payments |
| Extension payments | Filing extension generally does not extend payment deadline |
This table is a general educational summary. Different rules may apply based on entity type, fiscal year, income pattern, or other facts.
9. What Should Business Owners Review Before September 15?
A practical review can begin with four categories: current-year income (business income, owner compensation, rental income, capital gains); taxes already paid (federal and California estimated payments, withholding, extension payments); extended business returns (Form 1120-S, Form 1065, California Form 100S status); and year-end planning (expected changes between September and December, fixed-asset activity, and California-specific adjustments).
10. How This Fits Into Year-End Tax Planning
September 15 arrives late enough in the year that many taxpayers have several months of actual results available, making it a natural checkpoint before more detailed year-end planning begins. Rather than asking only "how much do I need to pay," the broader question is whether the tax projection still reflects what actually happened during 2026 — spanning business profitability, estimated taxes, owner compensation, withholding, California conformity differences, and bookkeeping completeness. See 2026 Year-End Tax Planning Timeline for California Business Owners and Tax Planning in Los Angeles.
Compliance Resources and Tools
- Internal Revenue Service — 2026 Form 1040-ES, Estimated Tax for Individuals (accessed September 9, 2026).
- Internal Revenue Service — Third Quarter Tax Calendar (accessed September 9, 2026).
- California FTB — 2026 Instructions for Form 540-ES (accessed September 9, 2026).
- California Franchise Tax Board — Business Due Dates (accessed September 9, 2026).
- California Franchise Tax Board — Extension to File (accessed September 9, 2026).
- September 15, 2026 is the third federal individual estimated-tax payment deadline.
- California follows a different standard installment structure: 30% / 40% / 0% / 30%.
- California's standard third installment is 0%, but September can still be an important planning checkpoint.
- Taxpayers with 2026 California AGI of $1 million or more (or $500,000 married/RDP separately) generally must base estimated tax on current-year California tax.
- Calendar-year federal S corporations and partnerships with timely extensions generally have a September 15 extended filing deadline.
- California S corporations generally also have a September 15 extended filing deadline.
- California partnerships generally receive a longer seven-month extension, producing an October deadline.
- An extension to file generally does not extend the time to pay.
References
- Internal Revenue Service. 2026 Form 1040-ES, Estimated Tax for Individuals. U.S. Department of the Treasury. Accessed September 9, 2026.
- Internal Revenue Service. Third Quarter Tax Calendar. U.S. Department of the Treasury. Accessed September 9, 2026.
- California Franchise Tax Board. 2026 Instructions for Form 540-ES. ftb.ca.gov (accessed September 9, 2026).
- California Franchise Tax Board. Due Dates: Businesses. State of California. Accessed September 9, 2026.
- California Franchise Tax Board. Extension to File. State of California. Updated March 18, 2026. Accessed September 9, 2026.
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