The September 15 Estimated Tax Deadline Is Trickier in 2026 – Here’s Why

A new tax law, a tighter AMT phase-out, and a safe harbor that’s easy to get wrong this year. Here’s what business owners, physicians, dentists, and executives need to check before the third 2026 estimated tax payment comes due.

The 2026 estimated tax deadline lands on September 15 this year, and it’s the one most people misjudge. This is the third of four quarterly payments, covering income earned between June 1 and August 31. For most W-2 employees, withholding handles this automatically. Business owners, physicians and dentists with practice income, and anyone with a large capital gain, K-1 distribution, or bonus this year have to send the IRS a check themselves. And 2026 has more moving parts than usual.

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You avoid an underpayment penalty by paying at least 90% of this year’s tax, or 100% of last year’s tax, by each due date. That threshold rises to 110% if your prior-year AGI was over $150,000 ($75,000 if married filing separately). The catch in 2026: OBBBA’s permanent 100% bonus depreciation, higher Section 179 limits, the increased SALT cap, and a tighter AMT phase-out all changed the underlying numbers, so relying on last year’s return without a quick recheck can mean overpaying or underpaying by more than usual. The current IRS underpayment interest rate is 7% annually, compounded daily.

What the September 15 Payment Actually Covers

The IRS splits the tax year into four uneven payment periods, not four equal quarters. The September 15 installment covers income from June 1 through August 31. It’s the shortest of the four periods, but the calculation is the same. Add up what you owe so far, subtract what you’ve already paid through withholding and prior installments, and send the difference. Business owners and practice owners often owe more at this checkpoint than earlier in the year. A strong summer, a large K-1 allocation, or proceeds from a partial practice sale can all drive second-half revenue swings.

The Safe Harbor Rules That Actually Protect You

Missing the exact dollar amount isn’t usually what triggers a penalty. Missing the safe harbor is. You avoid an underpayment penalty if your withholding and estimated payments, combined, equal at least 90% of this year’s total tax. The alternative threshold is 100% of last year’s total tax. If your prior-year adjusted gross income was over $150,000 (or $75,000 if married filing separately), that prior-year threshold rises to 110%. Most years, the 100%-or-110%-of-last-year rule is the easy, low-stress option. This year, it’s worth double-checking rather than assuming it still applies cleanly.

Why 2026 Is Different: OBBBA Reshuffled the Numbers

The One Big Beautiful Bill Act’s provisions are now fully in effect for 2026. Several of them change how much tax business owners and practice owners actually owe. That means last year’s return is a less reliable stand-in for this year’s liability than usual. Bonus depreciation is back to a permanent 100% for qualifying property placed in service after January 19, 2025. The Section 179 expensing limit rose to $2.5 million. The SALT deduction cap increased to $40,400 for 2026 before it phases down for higher earners. Each of these can meaningfully lower a business owner’s or practice owner’s 2026 tax bill compared to 2025.

That sounds like good news, but it cuts both ways. Anyone paying 100% or 110% of last year’s tax under the safe harbor could be significantly overpaying. And a business that skipped a big equipment purchase last year could be underpaying if it’s leaning on old numbers. The AMT phase-out also got tighter this year. The higher exemption amounts stayed, but the income level where the exemption starts phasing out dropped to $500,000 single and $1,000,000 joint. And the phase-out rate doubled from 25% to 50%. That change hits harder for anyone with a large capital gain, a practice sale, or concentrated equity income landing in 2026.

What Happens If You Underpay or Miss It

The penalty is really just IRS interest on the shortfall, charged for each day it’s outstanding. The IRS sets the rate quarterly. It currently sits at 7% annually, compounded daily, through the end of 2026 (see the IRS’s Q4 2026 rate announcement). It isn’t deductible, and paying late costs more than just the shortfall. You also owe a running interest charge, calculated separately for each installment period from its original due date. A payment that’s a few weeks late for the smallest of the four installments rarely amounts to much. A six-figure underpayment tied to a practice sale or a large distribution is a different story.

Owner and Practice-Specific Wrinkles Worth Checking

Business owners taking S-corp distributions on top of a reasonable W-2 salary need to true up both pieces. Don’t just true up the salary withholding. Physicians and dentists often see the biggest swings in the June–August window. That’s especially true if they’re receiving K-1 income from a practice or mid-transaction on a DSO sale, as summer patient volume and installment-sale payments come in. Owners in the middle of a business sale structured as an installment sale need to estimate tax on each payment as it’s received. Don’t wait to handle it all at year-end. None of these situations are unusual. They’re just easy to get wrong when you run the safe harbor math on autopilot instead of checking it against this year’s actual numbers.

Don’t Run the September Payment on Last Year’s Numbers

The safe harbor rules haven’t changed. What changed is how much you can rely on them without checking. Bonus depreciation, Section 179, the SALT cap, and the AMT phase-out all moved at once this year. A mid-year projection — even a rough one — is worth ten minutes before September 15. That’s especially true for business owners, physicians and dentists with practice income, and anyone who had a large one-time event this year.

Finally, one more installment remains after this one: the fourth-quarter payment due January 15, 2027. Our guide to year-end moves explains how to time it alongside Roth conversions and other December decisions.