Does Paying Last Year's Tax in Full Satisfy the Estimated Tax Safe Harbor?

The safe harbor stops one addition to tax, not the tax

The phrase "safe harbor" is shorthand. It does not appear in section 6654 of the Internal Revenue Code, and it hides what the provision does. Section 6654(a) directs that where an individual underpays estimated tax, "there shall be added to the tax under chapter 1, the tax under chapter 2, and the tax under chapter 2A for the taxable year an amount determined by applying—(1) the underpayment rate established under section 6621, (2) to the amount of the underpayment, (3) for the period of the underpayment."

Everything that follows is machinery for deciding whether an underpayment exists at all. Meeting the test switches off that addition. It does not cancel the tax. A filer can sit inside the estimated tax safe harbor and still owe a five-figure balance on April 15.

So the title question has a shape. Paying an amount equal to last year's tax is one of two candidate figures. Whether it suffices depends on which candidate is smaller, on one adjusted gross income line, on two conditions attached to the prior-year branch, and on the calendar.

Two candidates, and the statute keeps the smaller one

Section 6654(d)(1)(A) sets the size of each payment: "Except as provided in paragraph (2), the amount of any required installment shall be 25 percent of the required annual payment." Subparagraph (B) defines that annual figure as "the lesser of—(i) 90 percent of the tax shown on the return for the taxable year (or, if no return is filed, 90 percent of the tax for such year), or (ii) 100 percent of the tax shown on the return of the individual for the preceding taxable year."

Two amounts are computed and the smaller governs. It is the drafting shape that also appears in wage attachment, where a payroll office runs two ceilings and applies whichever withholds less, a mechanism traced in Reading a Garnishment Order Means Running Two Caps and Taking the Smaller One. The consequence is that the prior-year branch works as a ceiling on what must be prepaid, not as a separate requirement.

Take a calendar-year individual whose 2025 return showed $30,000 of tax and who expects $40,000 of tax for 2026. Candidate one is 90 percent of $40,000, or $36,000. Candidate two is 100 percent of $30,000, or $30,000. The required annual payment is $30,000, and each of the four installments is 25 percent of that, or $7,500. The filer prepays $30,000, files owing roughly $10,000 more, and section 6654 has nothing to say about the shortfall.

One AGI line moves 100 percent to 110 percent

Subparagraph (C) modifies the prior-year branch. Clause (i) reads: "If the adjusted gross income shown on the return of the individual for the preceding taxable year exceeds $150,000, clause (ii) of subparagraph (B) shall be applied by substituting '110 percent' for '100 percent'." Clause (ii) halves that line for one filing status: for "a married individual (within the meaning of section 7703) who files a separate return for the taxable year for which the amount of the installment is being determined, clause (i) shall be applied by substituting '$75,000' for '$150,000'."

The test mixes two years. The dollar comparison runs against adjusted gross income shown on the preceding year's return; the married-filing-separately reduction turns on filing status for the current year, the one whose installment is being determined. IRS Form 1040-ES states it in the form's own words: "If your adjusted gross income (AGI) for 2025 was more than $150,000 ($75,000 if your filing status for 2026 is married filing separately), substitute 110% for 100%."

Neither figure carries an inflation adjustment. The $150,000 and $75,000 lines sit in the statute unindexed, so the share of filers on the far side drifts upward with nominal income rather than with any legislative decision. That is the static-threshold design examined in Combined Income: The Social Security Tax Formula and Its Fixed Thresholds.

In the running example, crossing the line changes candidate two from $30,000 to $33,000. The required annual payment becomes the lesser of $36,000 and $33,000, so $33,000, and each installment rises from $7,500 to $8,250. That is $750 a quarter, $3,000 for the year. The 90 percent branch still caps the result: had the 2025 tax been $50,000, the 110 percent figure would be $55,000, the lesser amount would remain $36,000, and the AGI line would change nothing.

Two conditions ride on the prior-year branch

The sentence following clause (ii) narrows it: "Clause (ii) shall not apply if the preceding taxable year was not a taxable year of 12 months or if the individual did not file a return for such preceding taxable year." Two disqualifiers joined by "or," and either one removes the prior-year option entirely. A short taxable year on one side, an unfiled prior-year return on the other, and the required annual payment collapses to the single remaining candidate, 90 percent of the current year's tax.

A filer on extension who had not yet filed the prior-year return would be reading clause (ii) against a document that does not exist. Clause (C)(i) depends on a filed return as well, since it reads adjusted gross income "shown on the return." The regulation at 26 CFR 1.6654-2, titled "Exceptions to imposition of the addition to the tax in the case of individuals," frames the general exception in the same lesser-of terms, but the operative percentages and dollar thresholds live in the statute.

The test runs four times, not once

Section 6654(c)(1) is blunt: "There shall be 4 required installments for each taxable year." The table in 6654(c)(2) fixes the dates as April 15, June 15, September 15, and January 15 of the following taxable year. For tax year 2026 that is April 15, June 15 and September 15, 2026, then January 15, 2027, and all four fall on weekdays, so no weekend shift applies.

Under 6654(b)(1), the underpayment "shall be the excess of—(A) the required installment, over (B) the amount (if any) of the installment paid on or before the due date." The comparison is period by period. A filer who pays nothing until December and then wires the full $33,000 has met the annual figure and still generated three underpayments, because the first three due dates passed with less than $8,250 credited to each.

The clock on each is set by 6654(b)(2): the period "shall run from the due date for the installment to whichever of the following dates is the earlier—(A) the 15th day of the 4th month following the close of the taxable year, or (B) with respect to any portion of the underpayment, the date on which such portion is paid." For a calendar-year 2026 filer the outer endpoint is April 15, 2027 — a date tied to the close of the taxable year, so an October extension does not lengthen it.

Withholding spreads in equal parts unless the taxpayer shows otherwise

Section 6654(g)(1) shows how a default differs from an option in drafting. It provides that "the amount of the credit allowed under section 31 for the taxable year shall be deemed a payment of estimated tax, and an equal part of such amount shall be deemed paid on each due date for such taxable year, unless the taxpayer establishes the dates on which all amounts were actually withheld, in which case the amounts so withheld shall be deemed payments of estimated tax on the dates on which such amounts were actually withheld."

Both halves use "shall." The equal-part allocation is not a concession granted on request; it is the operative rule, displaced only when one condition is met, and the statute names the taxpayer as the party who can establish those dates. The Instructions for Form 2210 restate the default from the filer's side: "For withheld federal income tax and excess social security or tier 1 RRTA, you are considered to have paid one-fourth of these amounts on each payment due date unless you can show otherwise."

Suppose $30,000 is withheld entirely in December 2026. Under the default, $7,500 is deemed paid on each of the four due dates, including the three that passed before a dollar was withheld. Had the actual dates been established instead, the first three periods would show nothing and the whole $30,000 would sit in the final period. That is why late-year withholding behaves differently from a late-year estimated payment.

The rate is a quarterly number, compounded daily

Section 6654(a)(1) borrows the rate from section 6621 rather than fixing one. As the IRS states it: "For taxpayers other than corporations, the overpayment and underpayment rate is the federal short-term rate plus 3 percentage points." The same release notes that "the rate of interest is determined on a quarterly basis." For the quarter beginning October 1, 2026 the agency reported 7 percent for individual underpayments. Its quarterly interest rates page adds that the rates "are compounded daily."

Because the rate resets every quarter, an underpayment surviving from April 2026 to April 2027 can be charged at more than one rate across its 365 days. Any single-rate estimate of the charge is therefore an approximation.

Annualized installments, and two exceptions that need no ruling

Section 6654(d)(2) offers a different route for income arriving unevenly. Subparagraph (A) applies "if the individual establishes that the annualized income installment is less than the amount determined under paragraph (1)," in which case that installment is reduced to the annualized figure and "any reduction in a required installment resulting from the application of this subparagraph shall be recaptured by increasing the amount of the next required installment determined under paragraph (1) by the amount of such reduction (and by increasing subsequent required installments to the extent that the reduction has not previously been recaptured under this clause)." The applicable percentages step up across the four installments: 22.5, 45, 67.5, and 90.

Two features are easy to miss. The method is conditioned on the individual establishing the smaller figure. And the recapture clause makes the reduction temporary: what comes off one installment is added back to the next, shifting timing rather than lowering the annual requirement.

Alongside that sit two exceptions that operate without any determination by the Secretary. Section 6654(e)(1) removes the addition "if the tax shown on the return for such taxable year (or, if no return is filed, the tax), reduced by the credit allowable under section 31, is less than $1,000." Section 6654(e)(2) removes it where "(A) the preceding taxable year was a taxable year of 12 months, (B) the individual did not have any liability for tax for the preceding taxable year, and (C) the individual was a citizen or resident of the United States throughout the preceding taxable year."

Contrast 6654(e)(3)(A), which removes the addition only "to the extent the Secretary determines that by reason of casualty, disaster, or other unusual circumstances the imposition of such addition to tax would be against equity and good conscience." Paragraphs (e)(1) and (e)(2) are self-executing conditions; paragraph (e)(3) is a discretionary waiver someone has to grant. The same framing governs 6654(e)(3)(B), for a taxpayer who "retired after having attained age 62, or ... became disabled" in the year estimated payments were required or the year before, where the underpayment "was due to reasonable cause and not to willful neglect."

Numbers to Re-check

The prior-year AGI figure, from the return as filed. Clause (C)(i) compares against adjusted gross income "shown on the return." An amended return can move that number across $150,000 after installments were already set.

Which candidate is actually smaller. The 90 percent branch caps the prior-year branch, so computing only the 110 percent figure can overstate what is required.

Filing status for the current year. The $75,000 reduction attaches to a married individual filing separately for the year the installment is being determined, not to the prior year's status.

The underpayment rate for each quarter touched. The 7 percent figure reported for the quarter beginning October 1, 2026 applies to that quarter; section 6621 resets the rate quarterly.

Whether the prior-year return exists and covered 12 months. Either failure removes clause (ii) outright, and there is no partial version of that branch.

The outer endpoint date. April 15, 2027 for a calendar-year 2026 filer comes from "the 15th day of the 4th month following the close of the taxable year," not from an extended filing date.

Where This Doesn't Apply

Farmers and fishermen. Section 6654(i)(1) replaces the four-installment structure with one: "there shall be only 1 required installment for the taxable year," due January 15 of the following year, computed by substituting "66⅔ percent" for "90 percent" and "without regard to subparagraph (C) of subsection (d)(1)." The $150,000 line is switched off for them. Eligibility turns on gross income from farming or fishing reaching at least 66⅔ percent of total gross income in the current or the preceding year.

Estates and trusts. Section 6654(l) applies the section to estates and trusts "except as otherwise provided in this subsection," and the carve-outs there are real.

Anyone below the $1,000 line. Where the 6654(e)(1) test is met, the installment machinery produces no addition regardless of how the payments were timed.

State estimated tax. States write their own percentages, thresholds and due dates. A filer inside the federal safe harbor can sit outside a state one, and the $150,000 line has no automatic state counterpart.

Disaster postponements and fiscal years. A federally declared disaster can postpone estimated tax due dates inside the affected window. And the dates in the 6654(c)(2) table are expressed for a calendar year; a fiscal-year filer reads the equivalent months of its own year, with the (b)(2)(A) endpoint shifting accordingly.

Sources

Statutory text was read at Office of the Law Revision Counsel, 26 U.S.C. § 6654 and cross-checked against GovInfo, United States Code Title 26 Section 6654. The regulation is eCFR, 26 CFR § 1.6654-2, Exceptions to imposition of the addition to the tax in the case of individuals. Form-level statements come from IRS 2026 Form 1040-ES, Estimated Tax for Individuals and the IRS Instructions for Form 2210, Underpayment of Estimated Tax by Individuals, Estates, and Trusts. Rate figures come from IRS news release IR-2026-98, Interest rates remain the same for the fourth quarter of 2026 and the IRS Quarterly interest rates page.