September 15: The Deadline That Hits Business Owners Four Ways

There's a version of this conversation that happens every October. An owner comes in, we look at the year, and it turns out they've been sending in the same quarterly payment they set up two years ago — back when the business was half the size. Nobody told them to change it. Nothing in the system flags it. The number just quietly stopped being right.

The other version is the one where they haven't sent anything at all, because nobody ever explained that a business doesn't withhold.

What's actually due on September 15

September 15, 2026 is the Q3 federal estimated tax deadline — covering income you earned June through August. For business owners it's a busier date than that, because it's also the extended filing deadline for calendar-year S-corporations (Form 1120-S) and partnerships (Form 1065). And if you're in New York, it's a PTET estimated payment date too.

So one date, several obligations, and they have nothing to do with each other except the square on the calendar.

Who actually has to make estimated payments

The short version: if income reaches you without anyone withholding tax from it, that's you.

• Sole proprietors and single-member LLCs — Schedule C profit. Nothing is withheld, ever.
• Partners — your K-1 share is taxable whether or not a dollar was distributed to you.
• S-corp owners — your W-2 salary is withheld. Your distributions and your share of remaining profit are not.
• Landlords — rental income arrives whole.
• Anyone with meaningful 1099, interest, dividend or capital-gain income.

The federal threshold is low: you generally owe estimated tax if you expect to be short by $1,000 or more after withholding and credits. That catches almost every profitable business.

The part that surprises people: self-employment tax

Most owners budget for income tax and forget that they're also paying both halves of Social Security and Medicare. An employee splits that with their employer. You are both.

Here's an ordinary set of numbers — hypothetical, but the shape is one we see constantly:

Sole proprietor, single, $150,000 of net Schedule C profit. Self-employment tax: $21,194. Federal income tax: $22,191. Total federal: $43,385 — about $10,846 a quarter. Amount anyone withheld on your behalf: $0.

The self-employment tax is nearly as large as the income tax. That's the line item that turns a comfortable year into an April problem, and it's the one people leave out when they estimate in their head. (Social Security stops at the $184,500 wage base for 2026; Medicare's 2.9% doesn't stop at all, and an extra 0.9% kicks in over $200,000 single / $250,000 joint.)

Safe harbor: switch off the penalty without predicting the year

§6654 charges an interest-style penalty for paying late during the year, not for owing at the end of it. And it gives you three ways out — hit any one and the penalty disappears, even if you still write a large check in April:

• 90% of this year's tax, or
• 100% of last year's tax, or
• 110% of last year's tax if your prior-year AGI was over $150,000 ($75,000 if married filing separately).

For a business owner the prior-year number is the one worth using, and not because it's easier — because you cannot forecast your own year. A December contract, a slow autumn, one large receivable landing on the wrong side of the 31st, and your 90%-of-current-year estimate was wrong through no fault of yours. Last year's number is already on your return. It can't move.

The trade-off is honest: if this year is much better than last, safe harbor protects you from the penalty and leaves you a real balance due at filing. Know which of those two problems you're solving.

The move most owners miss: use a paycheck if you have one

Here's the rule worth the price of admission. Tax withheld from wages is treated as paid evenly across the entire year, no matter when it was actually withheld — that's §6654(g). Estimated payments get no such treatment; they count on the day you make them.

Which means if you're behind in August, an estimated payment fixes Q3 forward and leaves the earlier quarters underpaid. But withholding retroactively covers the whole year.

Two ways that applies to owners, and both get missed:

If you run an S-corp, you have a paycheck — but know what it can and can't carry. Your payroll covers reasonable compensation only. The tax on your share of the company's remaining profit is a separate problem, it flows through on your K-1, and for most profitable S-corps it's the larger of the two. Payroll was never withholding against it.

What the paycheck gives you is a lever: raise the federal withholding on that salary for the rest of the year — Step 4(c) of your Form W-4 — and those dollars get §6654(g)'s ratable treatment, curing earlier quarters. Most S-corp owners never think of this because they file themselves mentally under "estimated payments" and stop looking.

The ceiling is real, though. You can only withhold out of a paycheck that exists. If your reasonable comp is $80,000 and your profit share is $300,000, there is not enough salary left in the year to withhold the tax on $380,000 — you'll run out of paycheck before you run out of gap. In that case you use the salary to carry what it can, and an estimated payment for the rest. Raising reasonable compensation to create withholding room is not the answer: reasonable comp is determined by the value of the services you perform, not by what's convenient for cash flow, and moving it for tax-timing reasons invites exactly the scrutiny you don't want.

If you're a sole proprietor married to a W-2 earner, their W-4 can do the work — but only on a joint return. File jointly and withholding is withholding: it doesn't matter which spouse's paycheck it came out of, it applies against the one combined liability, and it's still ratable. Your spouse adding an amount to Step 4(c) can cure your underpaid quarters.

File separately and this strategy does not exist. On married-filing-separately returns each spouse's withholding is credited to that spouse's own return. Your spouse could withhold an extra $20,000 and it would do nothing for your underpayment — it would sit on their return, likely as a refund, while your penalty kept running.

Which is why this is a conversation before it's a form. Filing status is not a lever you pull for one purpose. Couples file separately for reasons that have nothing to do with estimated tax — income-driven student-loan repayment calculated on one spouse's income, keeping liability separate where there's a business with exposure, immigration or state-residency situations, a separation in progress. And MFS carries its own costs: the prior-year safe harbor threshold drops to $75,000 of AGI instead of $150,000, and a long list of credits and deductions narrow or disappear.

So the honest version is: if you already file jointly, this is one of the best moves available to you. If you file separately, or you're not sure why you file the way you do, the withholding question is downstream of a bigger one — and switching filing status to chase a withholding fix can cost more than the penalty you were trying to avoid. That's worth twenty minutes with us before anyone changes a W-4.

Same dollars either way. The mechanism decides whether they fix the whole year or only the rest of it.

What to do before September 15

1. Pull last year's total tax from your 1040 — the total tax line, not the refund or the balance due. Multiply by 1.1 if your prior-year AGI was over $150,000. That's your safe-harbor target.
2. Add up what's actually been paid in this year — estimates made, plus any withholding from a W-2 (yours or a spouse's).
3. The difference is the whole problem, expressed as one number.
4. If you have wages, fix it with withholding first. Divide the gap by remaining pay periods, put it on Step 4(c). This gets weaker every pay period you wait.
5. Pay the rest as an estimate — September 15 for Q3, January 15, 2027 for Q4.
6. If your entity return is on extension, don't let the two blur. The 1120-S or 1065 filing due that same day is a separate obligation, and a late one carries a per-partner, per-month penalty that has nothing to do with §6654.

If you're a New York business

September 15 is also a PTET estimated payment date — for both the New York State and the separate New York City regimes. PTET lets the entity pay state tax at the entity level and deduct it federally, sidestepping the individual SALT cap entirely, and it survived the 2025 legislation that many expected to curtail it.

The catch is that the election is annual and the deadline is March 15 of the tax year itself — there is no retroactive election. If you're already in for 2026, September 15 is a payment date. If you're not, the thing to calendar is March 15, 2027. We file PTET for clients and it's one of the few genuinely large levers left for a profitable pass-through in New York.

Common questions

I missed the June payment. Is it too late to fix?

No, and waiting makes it worse rather than neutral. The §6654 penalty works like interest — it accrues on the amount you're behind, for the length of time you're behind. Closing the gap in August costs less than closing the same gap in January. There's no cliff you've already gone over.

My income is seasonal. Do I really owe the same amount every quarter?

Not necessarily. The default assumes even income, but the annualized income installment method (Form 2210, Schedule AI) lets you match payments to when you actually earned the money. It's genuinely useful for a business with a lopsided year — a landscaper, a tax practice, a retailer. It's also fiddly enough that it's worth having someone run it rather than guessing.

I take distributions, not a salary. What do I pay on?

If you're an S-corp taking distributions without reasonable compensation, that's a separate and more urgent conversation than estimated taxes — it's one of the most commonly examined positions in small-business tax. On the estimates themselves: you owe on your share of the entity's profit, whether or not it was distributed to you.

What if I owe less than $1,000?

Then the estimated tax penalty generally doesn't apply, and you can settle up at filing. The threshold is what you'll be short after withholding and credits — not what you'll owe in total.

I have RSUs as well as business income.

Then you have both problems, and they interact — RSU income raises your total, which can push your business income into a higher bracket than you planned for. Vesting is withheld at a flat 22%, which is usually short. See RSU estimated tax payments and the W-4 fix, and estimate the gap with the RSU tax calculator.

When it's worth a conversation

If your business looks roughly like last year and you're inside the prior-year safe harbor, this is a thirty-minute job with your last return and a calculator.

Where it stops being that: a year that looks nothing like the previous one, seasonal income where annualizing would genuinely help, an S-corp where the salary-versus-distribution split hasn't been revisited recently, multi-state work, or a PTET election in play. Those are situations where the rule of thumb produces a confidently wrong answer.

We'd rather have that conversation in August than in April.