If you underpaid your April and June estimates, writing a bigger check on September 15 will not fix it.
The underpayment penalty in IRC § 6654 is calculated per installment period, on that period’s shortfall. Paying late settles the balance. It does not undo the penalty already running on the earlier quarters.
Two provisions in the statute can. Both are underused, and both are still available this month.
1. Wage withholding is treated as paid evenly across the whole year
Under IRC § 6654(g), federal income tax withheld from wages is deemed paid in four equal parts on the four installment dates, no matter when it was actually withheld. Withholding in December counts as though a quarter of it arrived on April 15.
So if there is a W-2 in your household — yours or a spouse’s — raising that withholding now reaches backward and can cure an April and June shortfall. An estimated payment cannot do that. It is credited on the day you send it and not one day earlier.
For a 1099 contractor with a W-2 spouse, this is the most useful move still available in September.
2. If your income arrived unevenly, annualize it
IRC § 6654(d)(2), computed on Form 2210, Schedule AI. The default rule assumes you earned in a straight line and owed 25 percent each quarter. That is not how a commission or a seasonal contract works.
Annualizing recalculates each installment from what you had actually earned by that date, at 22.5, 45, 67.5 and 90 percent for the periods ending March 31, May 31, August 31 and December 31. A closing that funded in August is not income you were required to pay tax on in April.
It is more work at filing. On genuinely lumpy income it often reduces or eliminates the earlier quarters’ penalty.
The safe harbor, stated precisely
This one is easy to carry the wrong version of. You are inside the safe harbor if you pay at least 90 percent of this year’s tax, or 100 percent of last year’s tax — but 110 percent of last year’s if your prior-year AGI was over $150,000, or $75,000 if married filing separately. If you are above that line and you pay 100 percent, you are not in the harbor.
And the penalty is not small money. The underpayment rate is 7 percent a year, compounded daily, for the quarter beginning October 1, 2026, and it is not deductible.
The four installment dates
- January 1 – March 31: due April 15
- April 1 – May 31: due June 15
- June 1 – August 31: due September 15
- September 1 – December 31: due January 15
Calendar-year filers. Every date in this cycle falls on a weekday, so nothing shifts — but when one lands on a weekend or a legal holiday it moves to the next business day under IRC § 7503. Fiscal-year filers substitute the corresponding months under IRC § 6654(k).
Get the full guide
Download Your September Tax Checkpoint (PDF) — six pages covering the deductions that cut both your income tax and your 15.3 percent self-employment tax, the two that only cut one of them, the two provisions above, and a short checklist for the rest of the month.
If you are behind on Q1 or Q2, my first question is not how much you can send by the deadline. It is whether there is any wage withholding in your household. If you would like a second set of eyes on your own numbers, call (713) 730-9792 or get in touch.
General information current as of September 2026, not advice for your situation. Whether any provision described here applies to you, and what it produces if it does, depends on facts this article cannot know. Deanna R. Ngueket, CPA, LLC — certified public accountant licensed in the State of Texas.
