Consider a hypothetical taxpayer named Alex. He earns $100,000 in wages and expects about $12,000 of federal income tax to come out of his paychecks this year. His 2025 return showed $14,000 of total tax. Then, in November, he sells appreciated stock and realizes a $30,000 long-term capital gain. Assume he files as single, his 2025 adjusted gross income was below $150,000 and that return covered a full 12 months.
Nothing about Alex's job changed. His tax picture did. The stock sale normally does not come with payroll withholding, so the gain may leave him short under the federal pay-as-you-go system. The amount he ultimately owes, the amount needed to reach a safe harbor and the date a payment is due are three different calculations.
First, find the safe-harbor number
Publication 505's general rule says that, in most cases, an individual must pay estimated tax for 2026 if both conditions apply: the person expects to owe at least $1,000 after subtracting withholding and tax credits, and withholding and credits are expected to be less than the smaller of 90% of current-year tax or 100% of prior-year tax. For higher-income taxpayers, the prior-year percentage generally rises to 110% when prior-year AGI exceeds $150,000, or $75,000 for a married taxpayer filing separately.
Under the assumptions above, Alex's prior-year safe-harbor target is $14,000. With $12,000 of projected wage withholding, he is $2,000 short of that target. But $2,000 is not an estimate of the tax on his gain. It is the additional payment or withholding needed to reach this particular penalty safe harbor. He could still have a balance due with his 2026 return even after reaching $14,000.
The November date changes the timing analysis
Publication 505 says that if a taxpayer first has income on which estimated tax is required in a later payment period, the first payment generally moves to that period's due date. For income first arising after Aug. 31, the table points to Jan. 15 of the following year. That may help Alex, but the sale date alone does not settle the issue: his wage withholding may already have been insufficient before November.
The penalty calculation asks a related but different question. Under the regular Form 2210 method, most taxpayers start with one-fourth of the required annual payment for each installment. Schedule AI, the annualized-income installment method, can lower or eliminate earlier required installments when income was uneven — including when a large capital gain arrived late in the year. Publication 505 says a taxpayer who uses this method must file Form 2210 with the return, and the Form 2210 instructions require Schedule AI to be used for every payment period once it is used for any one period.
Alex therefore needs to test whether the November gain was the first event that created an estimated-tax need and whether Schedule AI produces a better result than the regular installment calculation. Only then can he tell what a Jan. 15 estimated payment would accomplish.
Late wage withholding has a different timing rule
For estimated-tax penalty purposes, federal income tax withheld from wages is generally treated as though it were paid in equal portions on the installment due dates. If a taxpayer instead establishes the actual withholding dates, the amounts are treated as paid on those dates. Both rules appear in IRC §6654(g)(1).
The default equal-allocation rule is why extra withholding from late-year paychecks can sometimes repair an earlier-period shortfall more effectively than a late estimated-tax payment. A W-2 employee who discovers a problem in November should not assume Form 1040-ES is the only way to catch up.
Extra withholding is not an automatic penalty eraser. Alex's result still depends on his required annual payment, the installment method, total withholding, prior estimated payments and the Form 2210 calculation. If the late capital gain is the main reason his income jumped, Schedule AI may already reduce the earlier required installments.
Payroll may be the binding deadline
On Form W-4, Step 4(c) asks for an additional amount to withhold from each pay period — not a yearly total. If Alex needs $2,000 more and has two paychecks that can still reflect the change, that would mean $1,000 of additional withholding on each check. With three checks, it would be about $667 per check.
The employer controls when the revised form reaches payroll. IRS Publication 15 says a replacement Form W-4 must take effect no later than the start of the first payroll period ending on or after the 30th day after the employer receives it, although an employer may act sooner. A late-November request can therefore miss one or more December payroll cutoffs. Alex should ask payroll for the effective paycheck before relying on the strategy, and he should plan for the corresponding drop in take-home pay.
Start with the return, pay stub and payroll calendar
Before choosing between withholding and an estimated payment, I would put four items side by side:
- the 2025 return's total tax, AGI and filing status;
- year-to-date federal withholding and projected withholding from the remaining checks;
- a current estimate of 2026 income, deductions, credits and any estimated payments already made; and
- the employer's deadline for changing withholding on the remaining payroll dates.
Those figures show the safe-harbor target, whether the $1,000 balance threshold is relevant, and whether the late gain should be tested under Schedule AI. They also prevent a common mix-up: reaching a penalty safe harbor does not mean the tax on the stock sale has been fully paid.
For calendar-year taxpayers, the regular 2026 estimated-tax due dates are April 15, June 15 and Sept. 15, 2026, and Jan. 15, 2027, subject to weekend, holiday and special rules. As of Sept. 22, 2026, the next regular installment date is Jan. 15, 2027.
Sources
Primary and controlling sources last checked Sep. 22, 2026.