The September 15 Estimated Tax Payment, and What to Do If You Are Behind

For a calendar-year taxpayer, the third estimated tax payment of 2026 is due Tuesday, September 15, 2026. It covers the period from June 1 through August 31 — the quarters are not equal lengths, which is the first thing that catches people out.

September matters more than the other three dates for a practical reason rather than a legal one. It is the last payment with enough of the year left to change the outcome. Miss it and you are managing a penalty; catch it and you are still managing a tax bill, which is a much better problem.

Who this actually applies to

You generally have to make estimated payments for 2026 if you expect to owe at least $1,000 in tax after subtracting your withholding and refundable credits. That threshold catches more people than they expect, because it is about what is left unpaid, not about how much you earn.

• Sole proprietors and single-member LLCs, where nothing is withheld from anything.
• S corporation owners, whose W-2 covers the wages and not the distributions.
• Partners receiving a K-1, which reports your share of income whether or not cash arrived.
• Landlords with rental income that no one withholds against.
• Anyone with a large one-off event this year — a sale, a Roth conversion, an exercised option, a capital gain.

Note the last one. A W-2 employee with steady withholding can still land in estimated tax territory after a single transaction, and there is no notice in advance.

The two safe harbors, and why the second is usually the right one

What the penalty actually costs

The underpayment charge is interest, not a flat fee. For the calendar quarter that began July 1, 2026 the rate on individual underpayments is 7%, compounded daily. The rate is reset quarterly, so a shortfall running across several quarters is not charged at one figure.

It is also charged quarter by quarter. Paying the whole year's tax in January does not undo a shortfall from April — the money was late for three quarters and the charge runs for those quarters. That is the single most common misunderstanding we see about estimated tax, and it is why the September payment is worth making on time even if the April one was not.

If you are already behind

South Carolina in 2026 is not South Carolina in 2025

South Carolina has its own estimated payment requirement, filed on SC1040ES, on the same quarterly calendar as the federal one. Ordinarily the state estimate is the easy half: take last year's state number, adjust for anything obvious, pay a quarter of it.

That will not work for 2026. Governor McMaster signed H. 4216 into law on March 30, 2026, and it changes the state individual income tax substantially beginning with the 2026 tax year:

• The rate structure is now two tiers — 1.99% on income under $30,000, and 5.21% on income of $30,000 and above, minus $966.
• South Carolina is decoupled from the federal standard and itemized deductions, and federal adjusted gross income is now the starting point for the state return.
• A new South Carolina Income Adjusted Deduction replaces the federal standard deduction: $15,000 single or married filing separately, $22,500 head of household, $30,000 married filing jointly or surviving spouse. These may be reduced based on income.
• The state Earned Income Tax Credit is limited to $200.

Every one of those moves the state number in a different direction depending on who you are, and the SCDOR has said further guidance is coming later in the year. The honest position in August 2026 is that a state estimate built by copying last year's figure is a guess, and worth revisiting before September 15 rather than after April.

How to pay

• Federal: IRS Direct Pay from a bank account, EFTPS, a debit or credit card, or the Form 1040-ES vouchers by mail. Direct Pay is free and gives you a confirmation number worth keeping.
• South Carolina: MyDORWAY, or the SC1040ES voucher by mail.
• Record the date and the amount somewhere you will find them in March. Reconstructing four payments from bank statements a year later is a slow way to lose an afternoon.

The one after this is the one people forget

The fourth payment for 2026 is due January 15, 2027 — after the year has ended, when the income is known and the instinct is to wait for the return. Waiting costs you the last quarter's interest. Put it in the calendar now, while you are already thinking about it.

If your 2026 income has moved materially from 2025 — a new entity, a first employee, a property sale, a business bought or sold — the September payment is the right moment to have the year recalculated rather than repeated. That is a short conversation, and it is a great deal shorter in September than in April.