December 15 and January 15 Estimated Tax Deadlines

The last estimated tax payments of the year come in two waves. Calendar-year corporations make their fourth 2026 installment on December 15, 2026. Individuals, including owners of S corporations, partnerships, and sole proprietorships, make their fourth installment on January 15, 2027. Both are a chance to correct the year's payments before penalties lock in, and 2026 brings new federal deductions that California does not follow.

December 15, 2026: The Corporate Fourth Installment

The IRS fourth quarter tax calendar lists December 15, 2026 as the date corporations deposit the fourth installment of their 2026 estimated tax. Under Publication 542, each required installment is generally the smaller of:

• 25% of the current year's tax, or
• 25% of the prior year's tax, if the corporation filed a 12-month return for the prior year that showed a positive tax.

A large corporation, meaning one with taxable income of $1 million or more in any of the three preceding tax years, can use the prior-year method only for its first installment. Electronic payments through EFTPS must be scheduled by 8 p.m. Eastern time at least one calendar day before the due date.

California corporations. The FTB's 2026 Form 100-ES instructions use a different split: 30% of the year's estimated tax for the first installment, 40% for the second, nothing for the third, and 30% for the fourth. For a calendar-year corporation, the fourth California installment is also due December 15. S corporations compute California tax at 1.5%, and the minimum franchise tax must be paid with the first installment to avoid a penalty.

January 15, 2027: The Individual Fourth Installment

For Individuals, Publication 505 sets the fourth 2026 payment for January 15, 2027. You generally must pay estimated tax if you expect to owe at least $1,000 after withholding and credits, and you avoid the underpayment penalty if your withholding and timely estimates cover the smaller of:

• 90% of your 2026 tax, or
• 100% of your 2025 tax, increased to 110% if your 2025 adjusted gross income was more than $150,000 ($75,000 if married filing separately).

Publication 505 also allows you to skip the January 15 payment if you file your 2026 Form 1040 by January 31, 2027 and pay the full balance. If your income arrived unevenly, for example from a large sale late in the year, the annualized income installment method can reduce the required earlier payments.

California's 30/40/0/30 Schedule for Individuals

California does not split Individual estimates evenly. The FTB's estimated tax payments page lists the 2026 schedule as 30% by April 15, 2026, 40% by June 15, 2026, 0% on September 15, 2026, and 30% by January 15, 2027. Other California rules from the FTB and the 2026 Form 540-ES instructions:

• Estimates are generally required if you expect to owe at least $500 ($250 if married/RDP filing separately).
• The safe harbor is the smaller of 90% of 2026 tax or 100% of 2025 tax, rising to 110% of 2025 tax if 2025 California AGI was more than $150,000 ($75,000 if married/RDP filing separately).
• If 2026 California AGI is $1,000,000 or more ($500,000 if married/RDP filing separately), the prior-year safe harbor is not available and estimates must be based on 2026 tax.
• Filing your 2026 California return by January 31, 2027 and paying the entire balance replaces the fourth payment.

An Example

An S corporation owner's 2025 California tax was $20,000, and 2025 California AGI was $250,000. With no withholding, the 110% prior-year safe harbor requires $22,000 for 2026, paid as $6,600 in April, $8,800 in June, nothing in September, and $6,600 on January 15, 2027. If the owner paid only $10,000 by June 15 instead of the required $15,400, the shortfall began accruing a penalty, and a larger January payment stops the penalty from growing but does not erase what has already accrued. That is why recalculating in December, rather than in March, matters.

Recalculating Under the 2026 Rules

Publication 505 summarizes several federal changes that can lower 2026 federal tax, including a deduction for qualified tips (up to $25,000), a deduction for qualified overtime (up to $12,500, or $25,000 on a joint return), an added deduction of up to $6,000 per eligible taxpayer age 65 or older, and a charitable deduction of up to $1,000 ($2,000 joint) for people who do not itemize. Each has income limits.

California has not adopted these changes. The FTB has stated that its current conformity update does not include the One Big Beautiful Bill Act. A recalculation that lowers your federal estimate may therefore leave your California estimate unchanged, and the two should be computed separately.

Also look at withholding. S corporation owners who are behind can increase federal and California withholding on a December payroll, and we can show how that affects the penalty calculation. If you elected the California pass-through entity elective tax, that payment and its credit also belong in the projection. See our 2026 year-end tax planning checklist for related moves.

Key Takeaways

• Corporate fourth installments for 2026 are due December 15, 2026; Individual fourth installments are due January 15, 2027.
• Federal Individual safe harbor: 90% of 2026 tax or 100% of 2025 tax (110% above $150,000 of 2025 AGI).
• California uses 30/40/0/30 for both Individuals and corporations, and taxpayers with $1,000,000 or more of California AGI cannot use the prior-year safe harbor.
• New 2026 federal deductions do not apply in California, so recalculate each estimate on its own.

If you want your fourth-quarter payments recalculated for both the IRS and the FTB, schedule a consultation with SMB CPA Group, or contact us with your questions.

Disclaimer: The information presented on this website or any SMB CPA Group, PC online platform is for general information and illustrative purposes only. It should not be considered tax, legal, financial, or other professional advice, and it is not a substitute for advice from a licensed professional who knows your circumstances.