The fourth-quarter estimated tax payment for 2026 covers income earned September 1 to December 31 and is due January 15, 2027. A tax firm should flag clients with self-employment, investment or irregular income in early December compare what they have paid with the safe harbor (90% of this year's tax or 100% of last year's, 110% if prior AGI topped $150,000) and send each a specific amount and link.
Key takeaways
- The final 2026 estimated payment is due January 15, 2027 and covers income from September 1 to December 31, 2026.
- Clients generally avoid the underpayment penalty by paying the smaller of 90% of 2026 tax or 100% of 2025 tax, rising to 110% if 2025 AGI was above $150,000.
- Withholding is treated as paid evenly through the year by default, so extra December withholding can repair earlier short quarters in a way a late estimate cannot.
- Triage in early December: freelancers, landlords, investors with big gains and clients who had a bonus or sold property need a number, not a generic reminder.
- A reminder works when it states the amount, the deadline, the payment method and the reason, and arrives before the holiday slowdown.
When is the fourth-quarter estimated tax payment due?
For the 2026 tax year, the IRS splits estimated tax into four periods. The last one covers income received from September 1 to December 31, 2026, and the payment is due January 15, 2027. If a due date falls on a weekend or legal holiday, a payment made on the next business day counts as on time.
That date is the problem for most firms. It sits in the fortnight after the holidays, when staff are working through their tax season checklist and clients are not reading email. A reminder sent on January 10 is a reminder most clients see on January 16.
There is one notable exception. If at least two-thirds of a client's 2025 or 2026 gross income comes from farming or fishing, they can either pay by January 15, 2027 or skip estimates altogether by filing the 2026 return and paying in full by March 1, 2027. The IRS estimated tax FAQs set out both routes.
Which clients actually need a reminder?
Not every client needs one. The general rule is that an individual must pay estimated tax if they expect to owe at least $1,000 after withholding and refundable credits, and their withholding and credits will fall short of the safe harbor. A salaried client with normal W-2 withholding and nothing else usually does not need a January payment.
The clients who do need one tend to fall into a few groups:
- Self-employed and 1099 workers, who pay both income tax and self-employment tax with nothing withheld.
- Landlords with net rental income.
- Investors who realized large gains, received big year-end mutual fund distributions or sold crypto or property at a profit.
- Clients with a one-off event: a severance, a large bonus with too little withholding, a Roth conversion or a business sale.
- Retirees drawing IRA distributions or pensions without enough withheld.
A client who had no tax liability at all for 2025, was a US citizen or resident for the whole year and filed a return covering a full 12 months does not have to pay estimated tax for 2026, according to the IRS estimated taxes page.
How does the safe harbor work for a January payment?
The penalty is avoided if total withholding and estimated payments reach the smaller of two targets: 90% of the tax on the 2026 return, or 100% of the tax on the 2025 return. If the client's 2025 adjusted gross income was above $150,000 ($75,000 if married filing separately), the prior-year target becomes 110%. The prior-year return must cover all 12 months.
In practice, the prior-year target is the one firms can use in December, because it is a known number. The current-year figure is still a projection until January documents arrive.
Two quirks matter for the reminder:
- The penalty is figured installment by installment. A large January payment brings the year up to the safe harbor, but it does not undo a short April or June payment. The client may still owe a smaller penalty for earlier quarters.
- Withholding is treated as paid evenly through the year unless the taxpayer elects to use actual withholding dates on Form 2210. That means an extra December withholding from a W-2 job, or from a spouse's paycheck on a joint return, can be credited across all four due dates. For a client who missed earlier estimates, that can beat a January check.
A worked example: sizing one client's January payment
Take an illustrative client, Dana, a freelance designer filing jointly with a spouse who has a salaried job. These figures are made up to show the arithmetic.
- 2025 total tax on the return: $14,000
- 2025 AGI: $162,000, so the prior-year safe harbor is 110%: $15,400
- Spouse's federal withholding for 2026, projected to December 31: $6,200
- Estimated payments already made for April, June and September 2026: $2,500 each, $7,500 in total
Paid so far: $6,200 + $7,500 = $13,700. Shortfall against the safe harbor: $15,400 − $13,700 = $1,700.
The firm has two ways to cover the gap. Dana can send a $1,700 estimate by January 15, 2027, which reaches the annual target but leaves each earlier installment about $350 short of a quarter of $15,400 ($3,850, against $3,425 paid per quarter once withholding is spread evenly). Or the spouse can ask the employer to withhold an extra $1,700 from the last December paycheck. Because withholding is spread evenly by default, that extra amount lifts every quarter, and the earlier shortfalls disappear.
The reminder Dana receives should say exactly that: the amount, the two options, the deadline and which one the firm recommends. If Dana's 2026 income is clearly lower than 2025, the firm may instead run the 90% current-year figure or the annualized income installment method, which is covered in IRS Publication 505.
What should a December triage checklist include?
Run the triage in the first two weeks of December. Clients still have time to change December withholding, and the reminder lands before the holiday slowdown. Firms already sending year-end tax planning letters can fold this check into the same pass. Here is a simple decision table a firm can use for each client on the list.
| Client situation | Action | Reminder type |
|---|---|---|
| W-2 only, normal withholding, no other income | No action | None |
| Paid all three 2026 estimates at the safe-harbor level | Confirm the fourth installment amount | Short reminder with amount and link |
| Missed or underpaid one or more 2026 estimates | Compare a January estimate with extra December withholding | Personal email or call with both options |
| Large gain, bonus or property sale in Q4 | Project 2026 tax; check whether prior-year safe harbor still caps the bill | Personal note with figures |
| Income fell sharply in 2026 | Test the 90% current-year figure or annualized method | Advice note; may reduce or skip payment |
| Two-thirds of income from farming or fishing | Choose between January 15 payment and filing by March 1 | Option note |
Keep the list in one place with the client's name, the figure, the method and the date the reminder went out. In January, that list doubles as the follow-up queue.
How should the reminder be written and sent?
Generic newsletters about estimated taxes do not change behavior. A useful reminder has five parts:
- The amount, in dollars, for this client.
- The deadline: January 15, 2027.
- How to pay: IRS Online Account, IRS Direct Pay from a bank account, EFTPS, the IRS2Go app, a card through an approved processor, or a check with a Form 1040-ES voucher. A mailed payment is dated by its postmark.
- Why, in one sentence: for example, the September sale of the rental property.
- What to send back: a confirmation number or a screenshot so the firm can record the payment for the 2026 return.
Send it by the channel the client actually reads. For many clients that is a text message pointing to a secure portal, not a long email. Schedule a second nudge for around January 8 to anyone who has not confirmed, and a final one on January 13.
Record each confirmed payment as it comes in. In March, when the return is prepared, the firm will need the date and amount of every estimate, and asking the client again is wasted time on both sides.
What if a client misses the January 15 deadline?
Missing the date does not make things worse in a fixed jump. The underpayment penalty works like interest on the amount that was short, for the time it was short, so paying as soon as possible after January 15 still reduces it. The IRS FAQs make the same point about late payers.
The penalty is figured on Form 2210 when the return is filed, and the IRS can waive it in limited cases. These include a casualty, disaster or other unusual circumstance, or a client who retired after age 62 or became disabled during the year or the year before, where the underpayment had a reasonable cause. Whether a waiver applies depends on the facts, so treat it as a question for the return review, not a promise in the reminder.
Clients who miss every year need a different setup, not a louder reminder. Move them onto a quarterly reminder schedule from April, or help them raise W-4 withholding if they also have wage income.
How Zaquil handles fourth-quarter estimated tax reminders
The hard part of this workflow is not the arithmetic. It is getting the right figure to the right client before the holidays and recording the confirmation when it comes back. That is where Zaquil helps.
Recurring jobs and deadlines. The firm sets up a recurring quarterly job for each estimate client, with a January 15 deadline and a checklist: confirm the amount, send the reminder, record the confirmation. The jobs appear on a kanban board, so December triage becomes a filtered view rather than a spreadsheet.
One inbox across channels. The reminder goes out by email, SMS or WhatsApp from the firm's own number. Replies such as "paid, here's the confirmation" land in one inbox, attached to the client record.
Secure client portal. The client opens a branded portal and sees the amount, the due date and a short questionnaire asking for the payment date and confirmation number, with an upload slot for a screenshot. The firm sees which clients have answered and which have not.
Automations. A simple rule sends a follow-up on January 8 to anyone without a confirmation, then creates a call task on January 13. When the questionnaire comes back, the job moves to done.
AI reply drafts. When a client asks "do I really need to pay this?", Zaquil drafts a reply from the firm's own knowledge base for staff to check and send.
See how this fits a tax practice on the accounting and tax solution page, or compare Zaquil plans and pricing: plans start at $19 a month, with a 14-day free trial and no credit card to start.
Frequently asked questions
When is the fourth-quarter estimated tax payment due for 2026?
The final 2026 estimated tax payment is due January 15, 2027. It covers income received from September 1 through December 31, 2026. If a due date falls on a weekend or legal holiday, a payment made on the next business day is treated as on time. Farmers and fishers who meet the two-thirds income test have an alternative: file the 2026 return and pay in full by March 1, 2027.
Who has to make estimated tax payments?
Generally, an individual must pay estimated tax if they expect to owe at least $1,000 for the year after subtracting withholding and refundable credits, and their withholding and credits will be less than the safe-harbor amount. Self-employed people, landlords, investors with large gains and retirees with little withholding are the usual cases. A person with no tax liability for the prior full year who was a US citizen or resident all year generally does not need to pay.
What is the estimated tax safe harbor?
A taxpayer generally avoids the underpayment penalty if withholding and estimated payments equal at least the smaller of 90% of the current year's tax or 100% of the prior year's tax. If prior-year adjusted gross income was more than $150,000, or $75,000 for married filing separately, the prior-year figure rises to 110%. The prior-year return must cover a full 12 months for that test to apply.
Can a big January payment fix missed earlier estimates?
Only partly. The underpayment penalty is figured separately for each installment period, so a large January 15 payment brings the year up to the target but does not erase the shortfall for April, June or September. The client may still owe a smaller penalty for those earlier periods. Extra withholding from wages can work better, because withholding is treated as paid evenly through the year by default.
Why does extra December withholding help more than an estimate?
Federal income tax withheld from wages is treated as paid in equal amounts on each estimated tax due date unless the taxpayer elects to use the actual withholding dates. So an extra amount withheld from a December paycheck is spread back across all four quarters. A January estimate counts only from the date it is paid. For clients who missed earlier estimates and have wage income, raising final withholding can remove the earlier shortfalls.
How can clients pay the January estimated tax?
The IRS lists several options: the IRS Online Account, IRS Direct Pay from a checking or savings account, EFTPS, the IRS2Go app, a debit or credit card or digital wallet through an approved processor, cash at participating retailers, or a check mailed with a Form 1040-ES voucher. A mailed payment is dated by its US postmark. Ask clients to keep and share the confirmation number.
What happens if a client pays the fourth-quarter estimate late?
The underpayment penalty works like interest on the amount that was short, for the period it was short. Paying soon after January 15 therefore still reduces the penalty compared with waiting until April. The penalty is calculated on Form 2210 when the return is filed. Clients should pay as soon as they can rather than assume the opportunity has passed.
Can the estimated tax penalty be waived?
The IRS may waive the penalty if the underpayment was due to a casualty, disaster or other unusual circumstance where imposing it would be inequitable. It may also waive it for a taxpayer who retired after reaching age 62 or became disabled in the tax year or the prior year, if the underpayment had reasonable cause and was not due to willful neglect. Waivers depend on the facts and are requested with Form 2210.
Do farmers and fishers have to pay by January 15?
Not necessarily. If at least two-thirds of a taxpayer's 2025 or 2026 gross income comes from farming or fishing, a calendar-year filer can either pay all 2026 estimated tax by January 15, 2027, or file the 2026 Form 1040 or 1040-SR and pay the full tax due by March 1, 2027. Fiscal-year filers follow different dates set out by the IRS.
When should a tax firm send estimated tax reminders?
Early December is the practical window. Clients can still adjust December paycheck withholding, the firm has third-quarter figures in hand, and the reminder arrives before the holiday slowdown. A second reminder around January 8 and a final one on January 13 catch the stragglers. Sending the first reminder in mid-January means many clients will see it after the deadline has passed.
Should a firm use the 90% current-year or the prior-year safe harbor?
In December, the prior-year figure is usually easier because it is a known number from the filed return. The 90% current-year test can be better when income has fallen sharply, but it relies on a projection that may change once January documents arrive. For uneven income, the annualized income installment method in IRS Publication 505 can reduce or eliminate earlier-quarter penalties. The right choice depends on the client's facts.
What should a tax firm record after the client pays?
Record the payment date, the amount and the confirmation number for each estimate, and attach any screenshot or bank confirmation to the client file. These details go on the 2026 return and on Form 2210 if a penalty calculation is needed. Asking the client for them again in March wastes time, so capture them when the client confirms payment.
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