What the IRS Underpayment Penalty Actually Is

Section 6654 of the Internal Revenue Code calls it an "addition to tax," and the name matters, because it behaves like interest rather than like a fine. There is no flat dollar amount and no percentage of the balance owed at filing. Instead the IRS looks at each of the four estimated tax installments separately, asks how much that installment was short, and charges interest on that shortfall for exactly as long as it stayed unpaid.

That structure is why the charge surprises people who settle up in April. Paying the full year's tax on the filing deadline clears the balance but leaves the first three installments to have been short for months. It is also why the charge is often small: a $2,000 shortfall carried for a full year at 7% is about $140, not a headline number, but it is money that a differently timed payment would have kept.

The rate is not a permanent figure. Under section 6621 the underpayment rate for individuals equals the federal short-term rate plus three percentage points, and the IRS re-determines it every calendar quarter in a Revenue Ruling. A shortfall that spans several quarters accrues at whichever rate was in force on each day.

The published quarterly rates

These are the rates this calculator uses, taken from the IRS Revenue Rulings that announced them. Nothing here is estimated or interpolated.

The quarter beginning October 1, 2026 had not been announced when this page was written. Rather than invent a number, the calculator carries the last announced rate — 7%, from Rev. Rul. 2026-15 — forward across any unannounced days, and says so in the result whenever the calculation touches one. A 2026 tax-year result always touches at least one such day, because interest runs to April 15, 2027. A 2025 tax-year result never does: every day from April 15, 2025 to April 15, 2026 sits inside an announced quarter.

The four installment due dates

For tax year 2026 the installments are due April 15, 2026, June 15, 2026, September 15, 2026 and January 15, 2027 — all weekdays, so none of them move. For tax year 2025 the second installment shifted to June 16, 2025 because June 15 fell on a Sunday; the calculator uses that shifted date. Interest on any remaining shortfall runs until the filing deadline for the year: April 15, 2027 for a 2026 return, April 15, 2026 for a 2025 return.

The Three Ways Out

Section 6654 gives three separate exits. Reaching any one of them means no charge at all, regardless of what the arithmetic above would have produced.

1. The safe harbors

• 90% of this year's tax. Timely payments covering 90% of the tax on the return you are about to file.
• 100% of last year's tax. Timely payments covering the total tax shown on the prior-year return.
• 110% of last year's tax when prior-year adjusted gross income exceeded $150,000, or $75,000 for married filing separately.

You only have to clear the lower of the two, and that is the whole trick of the prior-year harbor: it is a known number in January, while this year's tax is a guess until December. A freelancer whose income doubles can still pay a quarter of last year's tax four times, owe a large balance in April, and owe no interest on it. The calculator names whichever harbor is cheaper for your numbers and what is left to pay to reach it.

2. The $1,000 threshold

No charge applies when the tax shown on the return, reduced by withholding, comes to less than $1,000. Note the wording: withholding counts against this test, estimated payments do not. Someone with a $900 balance after withholding is clear even having made no estimated payments at all.

3. No tax last year

No charge applies when the prior year showed zero tax, the prior year was a full twelve months, and you were a U.S. citizen or resident for the whole of it. First full year of freelancing after a year with no tax liability, and the entire question goes away.

What This Calculator Models, and What It Does Not

Everything here is the regular method: four equal required installments, each one a quarter of the safe harbor you have to clear, measured against what was credited by its due date. That is Form 2210 Part III as it applies to most people, and for steady income it is the right answer.

• The annualized income method is not modeled. If your income is lumpy — a book advance in November, a consulting project that closes in Q4 — the annualized income installment method on Form 2210 Schedule AI reshapes the required installments to follow when the income actually arrived, and it can reduce the charge substantially or eliminate it. This calculator uses the regular method, so for lumpy income treat its number as an upper bound and work Schedule AI or ask your preparer to.
• Waivers are not modeled. The section 6654(e)(3) relief described in the questions below is discretionary and fact-specific; no calculator can predict it.
• Withholding is spread evenly. Withholding is credited a quarter at each installment date, the default rule. You may instead elect to credit withholding on the dates it was actually withheld, which helps if most of it came late in the year and hurts if it came early.
• Farmers and fishermen are outside this. They have their own installment rule (one payment, and a 66⅓% harbor rather than 90%).
• State charges are separate. Most states run their own estimated tax interest at their own rates.
• Interest here is simple. The section 6654 charge is not compounded — the shortfall is multiplied by the rate and by days over 365. This differs from the section 6601 interest on an unpaid balance after filing, which does compound daily.

Nothing you enter leaves your browser. The whole calculation runs client-side.

Common Questions

What is the IRS underpayment penalty rate right now?

For the quarter beginning July 1, 2026 the underpayment rate for individuals is 7% per year, set by Rev. Rul. 2026-15 (Internal Revenue Bulletin 2026-22). It was 6% for the quarter beginning April 1, 2026 (Rev. Rul. 2026-5, IRB 2026-08) and 7% for the quarter beginning January 1, 2026 (Rev. Rul. 2025-22, IRB 2025-48). Under section 6621 the rate is the federal short-term rate plus 3 percentage points, reset every quarter. The rate for the quarter beginning October 1, 2026 has not been announced yet at the time of writing.

How is the estimated tax penalty calculated?

The section 6654 charge is interest on each late or short installment, not a flat fee. For every quarter the IRS takes the amount you were short, multiplies it by the interest rate in force, and prorates by the number of days the shortfall was outstanding: shortfall × rate × days / 365. It is simple interest, so it does not compound, and each quarter is measured separately from its own due date until you pay it or until the filing deadline for the year, whichever comes first.

What are the safe harbors that avoid the penalty?

You owe nothing if your timely payments cover the smaller of (a) 90% of the tax on this year's return or (b) 100% of the tax on last year's return, raised to 110% when last year's adjusted gross income was above $150,000 ($75,000 if married filing separately). Two further exceptions in section 6654(e) stand on their own: no charge applies when the tax on the return minus withholding is under $1,000, and none applies when the prior year showed zero tax on a full twelve-month return and you were a U.S. citizen or resident for that whole year.

Can the underpayment penalty be waived?

The IRS can waive the charge under section 6654(e)(3). One route covers a casualty, disaster or other unusual circumstance where imposing it would be inequitable; the other covers taxpayers who retired after reaching age 62 or became disabled, where the shortfall was due to reasonable cause and not willful neglect. A waiver is requested on Form 2210 Part II with a statement of the facts, and the IRS decides. This calculator does not model waivers, and its result does not predict whether one would be granted.

When are the 2026 estimated tax payments due?

For tax year 2026 the four installments are due April 15, 2026, June 15, 2026, September 15, 2026 and January 15, 2027. All four fall on weekdays, so none of them shift. Interest on any shortfall runs until the earlier of the day you pay it or April 15, 2027, the filing deadline for the 2026 return.

Keep Going

Self-employed and buying your own health coverage? Your marketplace subsidy is settled on the same return as these installments, and a mis-estimated income figure moves both. The ACA subsidy calculator works that side.

Methodology and Sources

Quarterly interest rates come from the IRS Revenue Rulings listed in the rate table above: Rev. Rul. 2025-7, 2025-11, 2025-18 and 2025-22 for the 2025 and Q1-2026 quarters, Rev. Rul. 2026-5 for the quarter beginning April 1, 2026, and Rev. Rul. 2026-15 for the quarter beginning July 1, 2026. Quarters that the IRS has not yet announced carry the last announced rate forward, and the result flags this whenever the calculation depends on it.

Installment mechanics follow section 6654 and Form 2210: four required installments of 25% each, the safe harbors and the $1,000 threshold as described above, withholding credited evenly under section 6654(g)(1), estimated payments credited on the dates entered, excess payments carried forward against later installments, and simple interest of shortfall × rate × days / 365 accrued per quarter until paid or until the filing deadline. The regular method only — see the section above for what is deliberately left out. This is an estimate for planning, not tax advice; the number on your Form 2210 is the one that counts.