Tax Check-In: A Practical Checklist Before the September 15th Tax Deadline
Whether you make quarterly estimated payments or plan to adjust your withholding instead, here's exactly what to check before September 15.
We wrote a few weeks ago about why the third quarter is worth a genuine tax projection check-in, whether you make estimated payments or rely on withholding. With the September 15 deadline five days out, here's the practical version — what to actually check and do before it arrives.
This isn't a repeat of that earlier conversation. It's the action list.
If You Make Estimated Payments
Confirm your Q3 payment amount reflects your actual year-to-date income, not the number you projected back in April. If your income has moved meaningfully in either direction, recalculate against the safe harbor targets: 90% of this year's total liability, or 100% of last year's (110% if your prior-year AGI exceeded $150,000). Make the payment through IRS Direct Pay, EFTPS, or a mailed check with Form 1040-ES. Direct Pay is generally the fastest for a one-time payment, and it confirms receipt immediately rather than relying on mail.
If You Rely on Withholding Instead
You don't have a September 15 deadline in the same sense, but this is still the right week to check your year-to-date withholding against the same safe harbor targets. If you're behind, a W-4 adjustment or a one-time withholding election on an upcoming bonus, RSU vest, or IRA distribution can still close the gap for the full year — a real advantage withholding has over estimated payments, since it's treated as paid evenly across the year regardless of when it actually happens.
Either Way, Check These Three Things
First, any realized capital gains or harvested losses from earlier in the year that haven't been factored into your running total. Second, whether a bonus, RSU vest, or business distribution changed your income picture since your last projection. Third, whether you're still on track for the safe harbor threshold that actually applies to you — the 100/110% of last year's liability is often the simpler target if this year's income is higher than expected, since it doesn't require knowing your final 2026 number precisely.
What Happens If You're Behind
An underpayment penalty is calculated based on how much you owed and for how long, not as a flat fee — so catching a shortfall now, even five days before the deadline, still reduces what you'd otherwise owe. It's not a reason to skip the check because you assume it's too late.
The Windward Approach
We treat this week's check-in as a natural extension of the tax projections we build with clients throughout the year, not a separate fire drill. If your income has shifted since your last conversation with us, this is the week to flag it.
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