There is a version of this conversation that happens every October. An owner comes in, we look at the year, and it turns out they have 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 have not sent anything at all, because nobody ever explained that a business does not 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 is a busier date than that, because it is also the extended filing deadline for calendar-year S-corporations (Form 1120-S) and partnerships (Form 1065). And if you are in New York, it is 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 is 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.
It also assumes you know your profit, and that is where this goes wrong most often. Contractors whose job costing is off and agencies billing pass-through media as revenue are both working from a profit figure that is not real. An estimate built on a wrong number is wrong in the same direction every quarter, and you find out in April.
The part that surprises people: self-employment tax
Most owners budget for income tax and forget that they are also paying both halves of Social Security and Medicare. An employee splits that with their employer. You are both.
Take an ordinary set of numbers. Hypothetical, but the shape is one we see constantly: a sole proprietor with $150,000 of net Schedule C profit.
Self-employment tax on that is $21,194. Not a rounded illustration, an actual figure: 15.3% of 92.35% of the profit, which is $138,525 of SE base. Amount anyone withheld on your behalf: $0. That bill exists before a single dollar of income tax is calculated.
That is the line item that turns a comfortable year into an April problem, and it is 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% does not stop at all, and an extra 0.9% starts over $200,000 single or $250,000 joint.
Income tax then sits on top, and it is the harder half to state in a sentence, because for most pass-through owners the §199A qualified business income deduction takes 20% of the profit out of the calculation before the brackets touch it. That deduction is worth several thousand dollars on the numbers above, and any estimate that leaves it out will overstate what you owe. Which is a nicer error than the other direction, but it is still an error, and it is why the arithmetic in your head is usually not the arithmetic on the return.
The reliable way round it is the one in the next section. Do not forecast this year. Look up last year.
If you've been sending the same quarterly payment for two years without revisiting it, that number is almost certainly wrong now, in one direction or the other. Overpaying is an interest-free loan to the IRS. Underpaying is a penalty. We'll tell you which one you're doing.
Have us check the numberSafe 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 is 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 cannot 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 are solving.
The move most owners miss: use a paycheck if you have one
Here is 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 is §6654(g), which deems the withholding credit to be estimated tax and deems "an equal part of such amount" paid on each due date. Estimated payments get no such treatment. They count on the day you make them.
Which means if you are behind in August, an estimated payment fixes Q3 forward and leaves the earlier quarters underpaid. 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. 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 is 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, on 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 will 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 is convenient for cash flow, and moving it for tax-timing reasons invites exactly the scrutiny you do not want. This is the point where the clever answer is worse than the boring one, and the boring one is: pay the estimate.
The withholding fix has a filing-status trap
This is the part every other article on this subject leaves out, and it is the one that decides whether the advice above works at all.
If you are 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 does not matter which spouse's paycheck it came out of. It applies against the one combined liability, and it is 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, most likely as a refund, while your penalty kept running.
Which is why this is a conversation before it is 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 is 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 are 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.
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
- Pull last year's total tax from your 1040, the total tax line, not the refund and not the balance due. Multiply by 1.1 if your prior-year AGI was over $150,000. That is your safe-harbor target.
- Add up what's actually been paid in this year: estimates made, plus any withholding from a W-2, yours or a spouse's.
- The difference is the whole problem, expressed as one number.
- 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.
- Pay the rest as an estimate. September 15 for Q3, January 15, 2027 for Q4.
- 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 are already in for 2026, September 15 is a payment date. If you are not, the thing to calendar is March 15, 2027. We file PTET for clients and it is 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 are behind, for the length of time you are behind. Closing the gap in August costs less than closing the same gap in January. There is no cliff you have 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 is genuinely useful for a business with a lopsided year: a landscaper, a tax practice, a retailer. It is also fiddly enough that it is worth having someone run it rather than guessing.
I take distributions, not a salary. What do I pay on?
If you are an S-corp taking distributions without reasonable compensation, that is a separate and more urgent conversation than estimated taxes. It is 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 does not apply, and you can settle up at filing. The threshold is what you will be short after withholding and credits, not what you will owe in total.
Does the QBI deduction change what I should be paying in?
It changes the amount, not the method. §199A can take 20% of qualified business income off your taxable income, which is a large enough swing that an estimate built without it will be too high. But it phases out for specified service businesses above certain income, and it interacts with W-2 wages paid and property held, so it is not a flat 20% for everyone. This is another argument for the prior-year safe harbor: last year's return already has your actual QBI deduction baked into the number.
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 are 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 has not 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.
That owner in October had been sending the same payment for two years. Nobody set it wrong. It was correct the day it was set, and then the business grew around it, quietly, while the number sat still.
So pull last year's total tax. Pull what you have paid in so far. Subtract.
The number was right once. That is the whole trouble with it.
Send us last year's 1040 and a note of what you've paid in so far this year. We'll tell you your safe-harbor target, the size of the gap, and whether your filing status makes the withholding fix available to you at all.
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