If your income does not come with withholding attached, your third quarter estimated tax payment is due Tuesday, September 15, 2026. That deadline applies to most self-employed people, partners, S corporation shareholders, landlords, and anyone with meaningful income from investments or a side business.
It is an easy one to let slide. There is no form to file and nobody sends you a bill, so the deadline passes quietly and the consequences show up months later. Here is what is actually at stake and how to handle it well.
Who needs to make a payment
Generally, you owe estimated payments if you expect to owe at least $1,000 in tax after subtracting your withholding and refundable credits. For business owners, that threshold gets crossed easily.
One thing owners frequently miss: if you take a salary from your own S corporation, your W-2 withholding counts toward your total, so you may need smaller estimates than a sole proprietor with the same income. If you have not looked at how your salary and estimates fit together, that is worth checking now rather than in April.
The safe harbor is the number that actually protects you
You do not have to predict your final tax bill perfectly. The safe harbor rules give you a defined target, and hitting any one of them protects you from an underpayment penalty even if you end up owing more in April:
- 90 percent of your 2026 tax liability, or
- 100 percent of your 2025 tax liability, or
- 110 percent of your 2025 tax liability if your 2025 adjusted gross income was more than $150,000, or $75,000 if married filing separately
The prior-year options are the practical ones for most people, because you already know that number. It is printed on last year’s return. If your income is climbing, paying 100 or 110 percent of last year’s tax keeps you penalty-free even though you will owe more at filing, and that is often the right cash flow decision. Just be clear with yourself that you are deferring the balance, not avoiding it.
Two things worth repeating. First, the safe harbor protects you from the penalty, not from the tax. The rest comes due with your return. Second, and this trips people up, hitting the safe harbor total is not enough on its own. The amount has to be paid on time across the installment periods, through withholding or timely estimates. Paying the full safe harbor number in a lump in January does not undo the quarters you missed along the way.
Why “I will catch up in January” does not work
This is the most common and most expensive misunderstanding about estimated taxes.
The penalty is not a single charge assessed on your April balance. It is calculated period by period. Miss the September payment and interest starts running on that shortfall from September 16, even if you send a large payment in January. Making up the dollars later stops the meter, but it does not undo the months that already accrued.
The rate matters here too. The IRS resets it quarterly, and it rose to 7 percent effective July 1, 2026, compounded daily. That is a meaningful cost on a five-figure shortfall carried for several months, and it is not deductible.
There is one useful exception. Withholding, unlike estimated payments, is treated as paid evenly across the year no matter when it actually happened. If you or your spouse have W-2 income, increasing withholding late in the year can retroactively cure an earlier shortfall. It is a genuinely useful tool if you find yourself behind in the fall.
If your income has been uneven, say so
Many businesses do not earn evenly across the year. Construction, tourism, and seasonal retail can have a quiet spring and a very good summer, which describes plenty of businesses in Colorado and Hawaii alike.
Paying four equal installments when your income was lopsided can create a penalty on paper even though you paid the right total. The annualized income installment method, computed on Schedule AI of Form 2210, lets you match payments to when you actually earned the money. It takes more record keeping, so it pays to know before year-end whether you plan to use it.
A short checklist for September 15
- Run your numbers through August. Six or seven months of real results is enough to see whether your original estimates still make sense.
- Look for the things that change the answer. A strong quarter, an equipment purchase, a new hire, or the sale of an asset can all move your liability substantially.
- Confirm your safe harbor target. Pull last year’s return, note the total tax, and check whether the 110 percent rule applies to you.
- Do not forget the state. If you are in Colorado, that is Form DR 0104EP. In Hawaii, it is Form N-200V. Each has its own requirements, and paying the IRS is only part of the job.
- Pay electronically and keep the confirmation. IRS Direct Pay, EFTPS, or your IRS online account all work, and each gives you a record. Your IRS online account is also the fastest way to confirm payments already credited for the year.
The bottom line
The September 15 estimated tax payment is a checkpoint, not just a due date. It is the last good opportunity to look at how the year is really going and adjust before the numbers are locked in. Owners who treat it that way rarely get surprised in April.
If you would like a second set of eyes on your estimates before the deadline, reach out to Key2 Accounting. We help small business owners across Colorado and Hawaii keep estimated payments accurate and cash flow predictable. Our post on why year-round tax strategy beats last-minute filing explains why this checkpoint matters more than most owners expect.