If you run a sole proprietorship, an S-corp where you take owner distributions, or any business that doesn’t withhold taxes from a regular paycheck, you have a deadline in nine days that’s easy to lose track of: the third-quarter estimated tax payment, due September 15, 2026.
Unlike April’s filing deadline, there’s no annual reminder blitz for this one. No accountant newsletter blast, no software pop-up on your phone. It just quietly shows up on the calendar, and a lot of local business owners miss it — or pay the wrong amount — because nothing about their day-to-day operations flags that it’s coming.
Who actually owes a Q3 payment
The rule from the IRS is straightforward on paper: if you expect to owe at least $1,000 in tax for the year after subtracting withholding and credits, you’re generally required to make estimated payments. In practice, that covers most owners of restaurants, salons, contracting businesses, retail shops, and professional service firms who pay themselves through distributions or draws rather than a W-2.
It also catches people who don’t think of themselves as “self-employed” in the traditional sense — an S-corp owner who takes a modest salary but a larger year-end distribution, a spouse who does the books and picks up 1099 income on the side, or an LLC member who assumed the business’s tax prep would sort it out at filing time. According to the IRS’s guidance on estimated tax, the obligation to pay quarterly exists independently of how your business is structured — what matters is whether tax is being withheld as income is earned.
The math that keeps you out of penalty territory
The IRS gives you two “safe harbor” paths, and you only need to hit one of them to avoid an underpayment penalty:
- Pay at least 90% of what you’ll actually owe for the current tax year, or
- Pay at least 100% of what you owed last year (bumped to 110% if your prior-year adjusted gross income was above $150,000, or $75,000 if you’re married filing separately).
The second option is the one most owners should lean on if this year’s income is hard to predict — which, for a seasonal restaurant, a contractor coming off a slow spring, or a retailer riding a volatile sales year, is often the case. Pull last year’s total tax liability off your return, divide by four, and as long as your cumulative payments match that number, you’re protected even if this year turns out to be a record one.
Where owners get tripped up is assuming a rough guess is good enough. The IRS doesn’t require precision, but it does compound underpayments across the year: pay too little in April and June, and you’re playing catch-up with real dollars by September, plus the penalty clock has already been running since the quarter you fell short — not just from the annual filing deadline. The Q3 payment is your best remaining checkpoint to true up before that gap grows further.
Why this year’s estimate is trickier than usual
If you’ve made changes to how you pay yourself or your staff this year, your Q3 estimate is probably not just “last quarter’s number again.” A few things worth checking before you calculate the payment:
- New payroll rules affecting take-home pay. With the “no tax on tips” provision now in effect for 2026, tipped employees are seeing different withholding this year, and owners who also earn tip income personally (bartenders-turned-bar-owners, for instance) may find their own liability shifted in ways last year’s return doesn’t reflect.
- A stronger or weaker season than planned. If revenue ran hotter than expected this summer, your April estimate based on last year’s numbers may now be too low — even under the safe harbor, if you’re using the “90% of current year” method rather than the prior-year method.
- A change in business structure. Anyone who elected S-corp status, brought on a partner, or restructured how distributions are paid this year should recalculate rather than repeat a prior payment amount, since the underlying liability itself has changed, not just the size of the business.
The Small Business Administration’s guidance on managing business finances points to the same core habit that solves most of this: reviewing profit and loss monthly rather than reconstructing it at tax time, so numbers like this are already sitting in your books instead of requiring a scramble every quarter.
What to actually do this week
- Pull year-to-date profit through August, not a projection from January.
- Check it against your last two estimated payments — if income is running ahead of or behind last year’s pace by more than 10-15%, adjust this payment rather than repeating the same figure.
- Confirm the safe harbor path you’re using (90% current-year or 100/110% prior-year) and make sure this payment keeps you on that path cumulatively, not just for the quarter.
- Pay through IRS Direct Pay or EFTPS by September 15 — a paper check mailed on the 14th that arrives late doesn’t count as timely.
None of this requires new software or a bookkeeper on retainer. It requires ten minutes with your P&L and a calculator. The owners who get burned aren’t the ones with complicated finances — they’re the ones who let a quiet deadline slide because nothing on their desk was flashing red about it. This one is. Mark it, calculate it, and pay it before the 15th.
