By the time September arrives, taxes are probably the last thing on your mind. Summer is winding down, spring filing is behind you, and the third-quarter estimated payment due on September 15 feels like a formality. For most business owners, it is whatever they paid last quarter, sent off without much thought.That habit is where the money leaks.By September, you can see most of the year: Two-thirds of your income is already on the booksYou know whether the year is running ahead of plan or behind itThe spring projection your estimates were built on is probably out of dateThe Q3 payment is a great opportunity to true up before the year closes. Skipping that recalculation is one of the most common and most avoidable mistakes I see.I’m a CPA and head of Tax at Gelt, and here is what the conversation with your own CPA should cover before the deadline. Recalculate the number — don’t repeat itMost business owners pay their Q3 estimate by copying the Q2 figure forward. That works only if nothing changed, and for a growing business, something almost always has. A strong sales quarter, a large client payment, a capital gain or a major asset purchase can all push your income far from what you projected in April. If your estimates are still built on that spring number, you are likely to be underpaying, or worse, overpaying, and not find out for months to come.About Adviser IntelThe author of this article is a participant in Kiplinger’s Adviser Intel program, a curated network of trusted financial professionals who share expert insights on wealth building and preservation. Contributors, including fiduciary financial planners, wealth managers, CEOs and attorneys, provide actionable advice about retirement planning, estate planning, tax strategies and more. Experts are invited to contribute and do not pay to be included, so you can trust their advice is honest and valuable.The fix is to rerun the projection with actual numbers through August:Pull your year-to-date income and compare it to the figure your estimates were based onAdd any one-time events you’re still expecting that may not have been in the original planRecalculate what you owe for the full year, then check it against what you have paid so farAs a CPA, I’d recommend doing this in early September, not on September 14. If the review turns up a shortfall, you want time to act on it.Know the number that protects youYou do not have to predict your tax bill perfectly to avoid a penalty. The IRS gives you a safe harbor, and hitting it is the goal.You generally avoid an underpayment penalty if you pay the smaller of two amounts:90% of what you owe this yearOr 100% of what you owed last yearIf your adjusted gross income last year was over $150,000, that second figure rises to 110%.A few numbers worth keeping in mind:You face a penalty only if you are short by $1,000 or more after withholding and creditsThe penalty is really interest, currently 7% a year compounded daily, charged on what you underpaidIt runs from each missed deadline until you pay, so a Q3 shortfall keeps costing you until you close itFor most business owners, the prior-year safe harbor is the one to lean on, because it is a known, fixed number. You already know last year’s tax. Paying 100%, or 110% if you are over the income threshold, across four even installments is the cleanest way to stay protected when this year’s income is hard to pin down.Use withholding as a late-year fixIf your September review turns up a gap, there is a tool most business owners overlook.Estimated payments count only for the quarter you actually make them. Withholding works differently. The IRS treats withholding as if it were paid evenly across all four quarters, even if it all came out of a December paycheck. If you or a spouse has W-2 income, increasing that withholding late in the year can patch an earlier shortfall in a way a catch-up estimated payment cannot.There is also relief if your income is genuinely uneven. The annualized income installment method lets you match your payments to when you actually earned the money, so a large third or fourth quarter is not treated as income you should have paid tax on back in April. If most of your income lands later in the year, this can lower or even erase a penalty. It takes more documentation, so it is a conversation to have with your CPA rather than a box to check on your own.At Gelt, we treat the September estimate as a planning moment, not just a payment. It is the point where the year is finally clear enough to act on, and there is still time left to act.Make September 15 a checkpoint, not just a paymentWhat makes this deadline matter, beyond the payment itself, is what it sets up. A wrong Q3 estimate does not stay contained in Q3. It follows you into the final January 15 installment and into the bill you settle in April.Looking for expert tips to grow and preserve your wealth? Sign up for Adviser Intel, our free, twice-weekly newsletter.When you recalculate now, you get more than a correct payment. You get an early read on where the year will land, and that gives you room to make real moves before December, such as adjusting your compensation, timing a large purchase, funding a retirement plan or accelerating a deduction.So before September 15, ask your CPA three questions: What do I actually owe for the year based on income through August?Am I on track to hit my safe harbor?If I am short, do I fix it with an estimated payment, with withholding or by annualizing my income?Those three questions turn a routine deadline into the most useful tax checkpoint of your year.Related ContentWhen Are Estimated Tax Payments Due in 2026?I’m a CPA: This Is the High Earner’s Guide to Winning Your 2026 Tax Season6 Ways to Use AI to Improve Your Financial LifeAsk the Tax Editor, June 19: Estimated Tax Payments and WithholdingThe Top 10 Side Gigs For Retirees In 2026This article was written by and presents the views of our contributing adviser, not the Kiplinger editorial staff. 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The September 15 Tax Conversation You Should Be Having Right Now
