What Is the September 15th Tax Deadline?

The September 15th tax deadline affects specific groups of taxpayers, marking a final filing cutoff or a required payment point in the tax year. For millions of individuals and businesses, this date represents a convergence of different tax obligations. Understanding whether this deadline applies to your specific financial situation is a part of maintaining compliance with federal tax laws and avoiding potential penalties for non-payment or late filing.

Key Tax Filings and Payments Due

The September 15th deadline is the extended filing deadline for many businesses. Partnerships, including multi-member LLCs taxed as partnerships, must file Form 1065 by September 15th if they secured an extension from the original March 15th due date. S corporations that filed for an extension must also submit their Form 1120-S by this date. These extensions grant an additional six months to file the returns that report the business’s income and deductions.

Beyond annual return filings, September 15th is a payment deadline for those required to make quarterly estimated tax payments. This applies to individuals who are self-employed, freelancers, or have substantial income from sources other than wages. These taxpayers use Form 1040-ES to pay their tax liability for the third quarter, which covers income earned from June 1 to August 31.

For individuals who requested a six-month extension to file their annual income tax return, the final day to submit their Form 1040 is October 15th.

The Extension to File Is Not an Extension to Pay

A common misunderstanding is that an extension to file also postpones the deadline to pay any taxes owed. The six-month extension, often requested using Form 4868 for individuals, only provides more time to submit the tax return. It does not grant more time to pay the tax liability; payment was still due on the original April 15th deadline.

Any tax not paid by that original due date began to accrue interest. The IRS charges interest on the unpaid balance, and this amount compounds daily from the original due date until the tax is paid in full.

In addition to interest, a failure-to-pay penalty may also be applied to the unpaid tax balance. This penalty starts accruing the day after the original tax-filing deadline. Taxpayers who filed an extension but did not pay their estimated tax bill are subject to both interest and penalty charges.

Penalties for Non-Compliance

Failing to meet tax deadlines can result in specific financial penalties from the IRS. If you have an extension and do not file your return by the due date, the failure-to-file penalty is 5% of the unpaid taxes for each month or part of a month that the return is late. This penalty is capped at 25% of your unpaid tax liability. If a return is over 60 days late, the minimum penalty is the lesser of $510 or 100% of the tax owed for returns due in 2025.

A separate penalty exists for not paying the taxes owed. The failure-to-pay penalty is 0.5% of the unpaid taxes for each month or part of a month the taxes remain unpaid, and this is also capped at 25% of the unpaid tax amount. If both penalties apply in the same month, the failure-to-file penalty is reduced by the amount of the failure-to-pay penalty.

On top of these penalties, the IRS charges interest on underpayments. Interest can be applied to the unpaid tax as well as on the penalties themselves, compounding the amount owed over time. The interest rate can fluctuate and is determined quarterly, based on the federal short-term rate plus 3 percentage points.

Submitting Your Tax Payment

You can make a payment to the IRS using several methods:

• IRS Direct Pay allows you to pay directly from a checking or savings account at no cost and does not require pre-registration.
• The Electronic Federal Tax Payment System (EFTPS) is a free online service operated by the Treasury Department that allows for scheduled payments for all types of federal taxes.
• Payments can be made through third-party processors using a debit card, credit card, or a digital wallet, though these services charge a fee.
• You can mail a check or money order made payable to the “U.S. Treasury” with a payment voucher, such as Form 1040-V, to ensure the payment is applied correctly.

Do not send cash through the mail.