The September 15 Tax Conversation You Should Be Having Right Now
Rather than simply repeating your previous estimated tax payment for the September 15 deadline, treat it as a strategic "true-up" moment to recalculate your income, ensure you're avoiding penalties and plan smart tax moves before the year ends.
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.
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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 books
- You know whether the year is running ahead of plan or behind it
- The spring projection your estimates were built on is probably out of date
The 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 it
Most 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.
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 on
- Add any one-time events you're still expecting that may not have been in the original plan
- Recalculate what you owe for the full year, then check it against what you have paid so far
As 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 you
You 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 year
- Or 100% of what you owed last year
If 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 credits
- The penalty is really interest, currently 7% a year compounded daily, charged on what you underpaid
- It runs from each missed deadline until you pay, so a Q3 shortfall keeps costing you until you close it
For 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 fix
If 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 payment
What 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.
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.
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