Sept 15 Is Almost Here: Your Final Q3 Tax Countdown Checklist
With eight days left until September 15, now is the time to move from planning to completion.
This deadline affects several types of small business owners. You may need to:
Make your third 2026 estimated tax payment.
File an extended 2025 partnership or S corporation return.
Provide or review partner and shareholder Schedule K-1 information.
Deposit August payroll taxes if you are a monthly depositor.
That may sound like a lot. It does not have to feel overwhelming.
I recommend working through each item in order. Review your books. Confirm what applies to you. Schedule payments and filings early. Keep your records together.
The goal is simple: accurate numbers, organized records, and less stress.
What is due on September 15?
September 15 is an important federal tax date for many calendar-year businesses and business owners.
Here are the main items to review.
1. Third 2026 estimated tax payment
The third 2026 estimated tax payment is due September 15, 2026.
For estimated tax purposes, this payment period covers income earned from June 1 through August 31, 2026. The payment may apply to individuals who receive income from:
Sole proprietorships.
Single-member LLCs.
Partnerships.
S corporations.
Freelance or contract work.
Investments.
Rental property.
Other income without enough tax withholding.
For many pass-through business owners, the business income flows through to the owner’s personal tax return. The owner may then need to make the estimated tax payment using Form 1040-ES.
Calendar-year corporations should also review their third estimated tax installment for 2026. Use your current business results, including the June 1–August 31 period, as part of your updated tax projection.
The exact payment depends on your business structure, income, deductions, credits, withholding, and payments already made.
2. Extended 2025 partnership and S corporation returns
If your calendar-year partnership or S corporation requested a timely extension, September 15 is generally the extended federal filing deadline for the 2025 return.
This includes:
Form 1065 for partnerships.
Form 1120-S for S corporations.
Schedule K-1 information for partners and shareholders.
An extension gives additional time to file. It does not automatically extend the time to pay tax.
Partnerships and S corporations generally pass income through to their owners. The owners remain responsible for their personal tax obligations, including estimated tax payments.
If you are still waiting for final numbers, do not delay your records review. Your tax preparer needs complete bookkeeping information to finish the return and prepare accurate K-1s.
3. August monthly payroll tax deposit
If your business is a monthly payroll tax depositor, federal employment taxes withheld from August payroll are generally due by September 15.
This may include:
Federal income tax withholding.
Employee Social Security tax.
Employee Medicare tax.
The employer share of Social Security and Medicare tax.
Payroll tax deposits are separate from estimated income tax payments and business return filings. They follow your assigned payroll deposit schedule.
If you are a semiweekly depositor, your August payroll deposits follow the semiweekly schedule instead. If you are unsure which schedule applies, review IRS employment tax due dates or check your payroll records.
The IRS generally requires federal tax deposits to be made electronically through options such as EFTPS.
Your final eight-day checklist
You do not need to solve every tax question at once. Use this simple process.
Step 1: Confirm which deadlines apply
Start by making a short list for your business.
Ask:
Do I need to make a third estimated tax payment?
Does my business have an extended Form 1065 or Form 1120-S?
Are partner or shareholder K-1s ready for review?
Is my business a monthly payroll tax depositor?
Have all August payroll deposits been scheduled?
Does my state have a separate deadline?
Not every business owner has every obligation. Confirm what applies before calculating amounts.
Step 2: Bring your bookkeeping up to date
Your tax estimate is only as reliable as your records.
Review your books through August, if possible. Gather:
Year-to-date profit and loss statements.
Bank statements.
Credit card statements.
Accounts receivable.
Accounts payable.
Payroll reports.
Owner draws or distributions.
Estimated tax payment confirmations.
Major purchase and asset records.
Receipts for significant expenses.
Prior-year tax returns.
Partnership or S corporation documents.
Form 7004 extension confirmation.
Reconcile your business bank and credit card accounts. Confirm that income is recorded once. Check that expenses are categorized correctly.
If personal transactions are mixed into business accounts, identify them now. Clean records make the tax review easier.
You do not have to fix everything in one sitting. Start with cash, income, payroll, and the largest expenses. A clear foundation is enough to move forward.
For more practical preparation ideas, read our guide to getting your books ready before Q4.
Review the estimated tax safe harbors
Estimated tax payments help you pay tax during the year instead of waiting until you file your return.
In general, you may avoid an underpayment penalty if your withholding and estimated payments reach one of these safe-harbor amounts:
90% of your expected 2026 tax liability.
100% of your 2025 tax liability.
110% of your 2025 tax liability if your 2025 adjusted gross income was more than $150,000.
For married taxpayers filing separately, the higher-income threshold is generally $75,000.
To review the prior-year safe harbor:
Find your 2025 total tax.
Determine whether the 100% or 110% amount applies.
Add estimated payments already made.
Add federal withholding.
Compare the total with your safe-harbor target.
Calculate the amount needed for the September payment.
These are general rules. Special rules may apply if your income is uneven, your prior-year return covered less than 12 months, or you earn income from farming or fishing.
If your income changes significantly during the year, the annualized income method may provide a more accurate result. IRS Publication 505 explains estimated tax calculations and annualized income.
Understand the Q3 underpayment interest
The estimated tax underpayment rate for the third quarter of 2026 is approximately 7% annualized.
The rate applies when required tax payments are too low or paid late. The calculation is based on the amount underpaid and the length of time it remains unpaid. The amount may compound daily.
This is not a reason to panic. It is a reason to review your numbers now.
If your estimate shows a shortfall, you may be able to:
Increase your September estimated payment.
Adjust your fourth estimated payment.
Increase federal withholding from wages.
Use the annualized income method.
Review deductions and credits.
Confirm the calculation with your tax professional.
The best payment is not always the largest payment. The right amount depends on your complete tax picture.
Finish extended returns and K-1s
If your partnership or S corporation filed an extension, use this final review list:
Confirm Form 7004 was filed on time.
Reconcile all business accounts.
Review payroll records.
Confirm income and expenses.
Review fixed assets and major purchases.
Confirm partner or shareholder ownership percentages.
Check capital accounts and distributions.
Review K-1 information.
Confirm any tax due has been addressed.
Verify that the return can be filed by September 15.
The late-filing penalty for many partnership and S corporation returns is generally $235 per partner or shareholder per month, including each month or part of a month that the return is late. The penalty can apply for up to 12 months.
Reasonable-cause exceptions may apply. Do not assume an extension protects you after September 15. Confirm your filing status and contact your tax professional if records are incomplete.
Complete the payroll review
If you are a monthly depositor, confirm that your August payroll tax deposit includes the correct amounts.
Check:
August payroll dates.
Federal withholding.
Social Security and Medicare taxes.
Employer payroll tax amounts.
Deposit schedule.
Payment account.
Electronic payment confirmation.
Save the confirmation with your payroll records. Depositing payroll taxes does not replace the requirement to file the related payroll tax return.
If your business had $100,000 or more in accumulated employment taxes on a day during the deposit period, the next-day deposit rule may apply. Review the IRS employment tax guidance if this situation may affect you.
Schedule everything before the deadline
Do not wait until September 15 if you can complete the work earlier.
Before submitting, confirm:
The correct tax year.
The correct payment period.
The payment amount.
The payment method.
The bank account.
The filing status.
The recipient information for K-1s.
The payment and filing confirmations.
Then save every confirmation.
You do not have to manage this alone
September 15 is easier when your books are current and your next steps are clear.
I help small business owners organize their records, understand their numbers, and move from financial uncertainty to perfect order. We can review your bookkeeping, prepare your tax information, and help you approach the deadline with a simple plan.
You can learn more about Jenni’s Business Services, explore our business resources, or book a consultation.
Start with one step today. Gather your records. Confirm what applies. Then complete each item in order.
With a calm review and accurate books, September 15 can be manageable and stress-free.
This article provides general educational information. Tax rules, filing requirements, penalty calculations, payroll deposit schedules, and state deadlines vary by taxpayer and business structure. Please confirm your specific obligations with a qualified tax professional or the IRS.
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