Do I need to pay Q4 estimated taxes? Run one number before January 15.
The fourth installment for 2026 is due January 15, 2027, and whether you owe it comes down to one number. If you skipped the first three, December withholding fixes more of the penalty than a January check does.
A consultant whose practice doubled in 2026 opens the books in November and realizes nobody sent the IRS anything but his wife's paycheck withholding all year. The question he types is the one I get every December: do I need to pay Q4 estimated taxes, and if I already missed three, is the fourth one even worth sending? It usually is, but a January check is not the cheapest fix, and the cheapest fix has a December 31 deadline.
How to tell if you need to pay Q4 estimated taxes.
Section 6654 does not care what you owe in April. It cares whether enough arrived by each of four dates, and the target is called the required annual payment. Take the lesser of 90% of the tax on your 2026 return or 100% of the tax on your 2025 return. If your 2025 adjusted gross income was over $150,000 ($75,000 married filing separately), the prior-year figure becomes 110%. That one rule is why most people with a growing business should anchor on last year: it is a known number, and this year's is a guess.
Then subtract what you have paid. Count every estimate sent for 2026 and every dollar of withholding you expect through December 31, from your W-2, a spouse's W-2, or your own S corporation payroll. If what is left is above zero, that is your Q4 payment. The safe harbor calculator does this arithmetic in about a minute and tells you which of the two targets is cheaper.
Two exits from the penalty are worth knowing and rarely worth planning around. If the balance due on the return after withholding is under $1,000, there is no penalty at all (§6654(e)(1)). And §6654(h) waives the fourth-installment penalty if you file the 2026 return and pay in full by January 31. January 31, 2027 is a Sunday, so the deadline rolls to Monday, February 1. That works for a W-2 household with nothing pending. Anyone waiting on a K-1 or a brokerage 1099-B in mid-February cannot use it, and it never touches the first three quarters.
Three ways to cover the same shortfall.
Here is the consultant's year. His 2025 joint return showed $48,000 of tax on $260,000 of AGI, so his target is 110% of that, or $52,800, which is $13,200 a quarter. His 2026 tax will be about $71,000, and 90% of that is $63,900, so last year's number is the cheaper target by $11,100. His wife's job will withhold $22,000 for the year. He paid no estimates. The shortfall is $30,800, and he has three ways to send it.
- Required annual payment (110% of $48,000)
- $52,800
- W-2 withholding for the year
- $22,000
- Shortfall, $7,700 in each quarter
- $30,800
- Pay nothing until April 15, 2027
- about $1,434
- File and pay in full by February 1, 2027
- about $978
- Send $30,800 as a Q4 estimate on January 15
- about $902
- Add $30,800 of December W-2 withholding
- $0
Tax year 2026, married filing jointly. Penalty computed as simple interest from each 2026 installment date (April 15, June 15, September 15, January 15) at 7%, the underpayment rate in effect for the fourth quarter of 2026; the IRS compounds daily and resets the rate each quarter, so actual figures run slightly different. Assumes the 2025 return showed AGI over $150,000.
Why December withholding beats a January check.
A January 15 payment is applied to the earliest unpaid installment first (§6654(b)(3)), so it does cover April, June, and September. It just covers them late, and the penalty for each one runs until the day the money shows up. That is the $902.
Withholding gets different treatment. Under §6654(g)(1), income tax withheld from wages is treated as paid in four equal parts on the four due dates, no matter when during the year it actually came out of a paycheck. If his wife files a new Form W-4 with $30,800 on line 4(c) spread across her December paychecks, the IRS treats $7,700 of it as having arrived on April 15, another $7,700 on June 15, and so on. Every quarter is on time, after the fact. The bill is still the same $30,800. The penalty is gone.
The catch is mechanical: the paychecks have to be big enough to absorb it, and payroll systems need lead time. A year-end bonus is the easiest place to put extra withholding. An S corporation owner can do the same thing with his own payroll, which is the move in running S corp payroll in December. A household with no W-2 income at all has no withholding to adjust, so January 15 is its best option.
What I would actually do.
If you are on track, send the Q4 payment and move on. If you are behind and anyone in the household has a paycheck, change the W-4 before the last December payroll closes and skip the January estimate. If nobody has a paycheck, pay by January 15 and accept the smaller penalty. Do not plan on the February 1 filing exit unless your return is done by the time your W-2 arrives.
I would also keep the penalty in proportion. It is interest, not a fine, and $902 on a $71,000 tax bill is not worth borrowing against a credit card to avoid. What it is worth is setting up 2027 so this does not repeat: 110% of 2026's tax, split four ways, starting April 15.
