2026 estimated taxes: separate this year's payments from last year's return

One of the most frequent causes of IRS balance-due or refund-adjustment notices for self-employed individuals and small-business owners is mixing up which tax year an electronic payment was designated for. In a single calendar year, a business owner often pays the final quarterly installment for the prior tax year in January, a balance due or extension payment in April, and three or four quarterly estimated tax installments for the current tax year.

When all of those drafts come out of the same bank account labeled "IRS USA Tax Payment," it is easy to double-count or omit a payment when preparing the return.

1. Know the four regular 2026 Form 1040-ES installment periods

For calendar-year individual taxpayers (including sole proprietors, 1099 contractors, partners, and S-corporation shareholders), IRS Form 1040-ES divides estimated tax into four installment periods:

• 1st installment: April 15, 2026
• 2nd installment: June 15, 2026
• 3rd installment: September 15, 2026
• 4th installment: January 15, 2027 (adjusted when a due date falls on a weekend or legal holiday)

Notice that the fourth-quarter payment for tax year 2026 is paid in January 2027, while a January 2026 payment is normally the fourth-quarter payment for tax year 2025. Recording payments strictly by the calendar year they cleared the bank—rather than the tax year selected in IRS Direct Pay or EFTPS—creates a mismatch between your return and your IRS account transcript.

2. Track prior-year overpayments applied to 2026

If your 2025 Form 1040 showed an overpayment and you elected to apply part or all of that refund toward your 2026 estimated tax rather than receiving a direct deposit, that credit counts as a 2026 estimated tax payment even though no cash left your bank account in 2026.

Conversely, if you paid a 2025 balance due or an April extension payment (Form 4868) in spring 2026, that payment settled your 2025 tax liability and cannot be claimed again as a 2026 quarterly estimated payment.

3. Understand how safe-harbor rules work (Publication 505)

Under IRS Publication 505 (Tax Withholding and Estimated Tax), individuals generally avoid an estimated tax underpayment penalty if they owe less than $1,000 after subtracting withholding and refundable credits, or if their timely withholding and estimated payments equal at least:

• 90% of the tax shown on the current year's (2026) return, or
• 100% of the tax shown on the prior year's (2025) return (increasing to 110% if prior-year adjusted gross income exceeded $150,000, or $75,000 if married filing separately)

Remember that a prior-year safe harbor only protects against the underpayment penalty during the year—if your 2026 business profit grew substantially above 2025, you will still owe the remaining tax balance by the April filing deadline unless you update your 2026 year-end tax forecast.

Keep a one-page 2026 estimated tax payment log

For every IRS payment made by you or your spouse, save a single folder with:

1. The IRS Direct Pay or EFTPS confirmation number
2. The exact tax year (2025 vs. 2026) and form type (1040-ES, 1040 Balance Due, or 4868 Extension) selected
3. The Social Security number or ITIN of the primary taxpayer under which the payment was submitted (payments made under the spouse's SSN should be noted so the return credits them properly)
4. The date and exact dollar amount debited

Tax N Tips helps Houston business owners and 1099 professionals with quarterly tax planning, monthly bookkeeping, small-business payroll, individual and business tax preparation, and ITIN assistance across all services. Book a free consultation to reconcile your 2026 estimated tax payments before the fourth-quarter deadline.

Sources checked November 2, 2026: IRS About Form 1040-ES and IRS Publication 505 (Tax Withholding and Estimated Tax).