The third estimated tax payment for 2026 is due September 15. It is the one most worth paying attention to, because it arrives while there is still time to change the outcome of the year.
The 2026 schedule
- April 15, 2026
- June 15, 2026
- September 15, 2026
- January 15, 2027
Note that these are not evenly spaced quarters, which trips people up. The third payment covers June through August, a three-month stretch, while the second covers only two.
The safe harbor, and the part that catches higher earners
You avoid an underpayment penalty by paying the smaller of:
- 90% of the tax shown on your 2026 return, or
- 100% of the tax shown on your 2025 return
There is also a de minimis rule: if you owe less than $1,000 after withholding and credits, no penalty applies.
Now the part that catches people. If your 2025 adjusted gross income exceeded $150,000 — or $75,000 if married filing separately — the prior-year safe harbor rises from 100% to 110% of last year’s tax.
That extra 10% is not optional and it is not widely known. A Fresno business owner who had a strong 2025, dutifully paid in exactly 100% of last year’s tax, and assumed they were protected can still owe a penalty.
Why prior-year safe harbor is usually the smart play
The 90% current-year test requires you to predict this year accurately. For most business owners that is genuinely hard: revenue moves, a large equipment purchase lands, a client pays late.
The prior-year test requires only arithmetic on a return that already exists. You know last year’s tax. Multiply by 100% or 110%, divide by four, pay it.
It is not always the cheapest option in cash flow terms. If this year is materially worse than last, paying 110% of a big prior year ties up money you need. But it is the certain one, and certainty has value.
California runs on a different schedule
California does not follow the even federal pattern. The state front-loads estimated payments across the year rather than spreading them evenly, which means matching your California payments to your federal ones will leave you short early and over-paid late.
This is one of the more common errors we see on returns prepared without local knowledge.
Use September as a checkpoint, not just a payment
This is the real argument for taking the September payment seriously. By mid-September you have eight months of actual results. That is enough to project the year with reasonable confidence, and there are still three and a half months in which to act.
Things that are still available in September and gone in January:
- Retirement plan decisions. Some plans must be established before year end, not merely funded by the filing deadline.
- Equipment timing. Assets must be placed in service by December 31, which means ordering with installation time to spare.
- Income and expense timing. Accelerating a deduction or deferring a receipt only works with runway.
- Entity elections. Most have deadlines well before year end.
- Charitable strategy. Donor advised funds and appreciated securities take time to execute.
A payment made without looking at any of that is just a payment. A payment made alongside a projection is tax planning.
If you have underpaid so far
Do not skip the payment to avoid confronting it. The penalty is calculated by period, so a shortfall in an earlier quarter keeps accruing until it is made up. Paying more in September stops the meter on the amount you catch up.
If your income was genuinely uneven across the year — a large one-off sale, a seasonal business — the annualised income method on Form 2210 may reduce or eliminate the penalty by matching payments to when the income actually arrived. It requires records that support the timing, which is another argument for books that are closed monthly.
Talk it through with a Fresno CPA
We run projections ahead of each estimated payment so the number is deliberate rather than a guess, and so the planning conversation happens while it can still change something.
Schedule a meeting or call 559-372-9213.
This post is general information, not advice for your specific situation. Tax figures are current for the year stated and many are indexed annually. Talk to a CPA before acting on any of it.
